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Influence of Surfactant-Mediated Interparticle Contacts on the Mechanical Stability of Supraparticles

  • Junwei Wang
    Junwei Wang
    Institute of Particle Technology, Friedrich-Alexander University Erlangen-Nürnberg, Cauerstrasse 4, 91058 Erlangen, Germany
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  • , 
  • Eunsoo Kang
    Eunsoo Kang
    Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
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    Orcidhttps://orcid.org/0000-0002-9894-0900
  • , 
  • Umair Sultan
    Umair Sultan
    Institute of Particle Technology, Friedrich-Alexander University Erlangen-Nürnberg, Cauerstrasse 4, 91058 Erlangen, Germany
    Institute of Chemical Reaction Engineering, Friedrich-Alexander University Erlangen-Nürnberg, Egerlandstrasse 3, 91058 Erlangen, Germany
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    Orcidhttps://orcid.org/0000-0003-3765-1061
  • , 
  • Benoit Merle
    Benoit Merle
    Materials Science and Engineering I and Interdisciplinary Center for Nanostructured Films (IZNF), Friedrich-Alexander University Erlangen-Nürnberg, 91058 Erlangen, Germany
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  • , 
  • Alexandra Inayat
    Alexandra Inayat
    Institute of Chemical Reaction Engineering, Friedrich-Alexander University Erlangen-Nürnberg, Egerlandstrasse 3, 91058 Erlangen, Germany
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  • , 
  • Bartlomiej Graczykowski
    Bartlomiej Graczykowski
    Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
    Faculty of Physics, Adam Mickiewicz University, Uniwersytetu Poznanskiego 2, Poznan 61-614, Poland
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  • , 
  • George Fytas*
    George Fytas
    Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
    *E-mail: [email protected]
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    Orcidhttps://orcid.org/0000-0003-2504-6374
  • , and 
  • Nicolas Vogel*
    Nicolas Vogel
    Institute of Particle Technology, Friedrich-Alexander University Erlangen-Nürnberg, Cauerstrasse 4, 91058 Erlangen, Germany
    *E-mail: [email protected]
    More by Nicolas Vogel
    Orcidhttps://orcid.org/0000-0002-9831-6905
Cite this: J. Phys. Chem. C 2021, 125, 42, 23445–23456
Publication Date (Web):October 18, 2021
https://doi.org/10.1021/acs.jpcc.1c06839

Copyright © 2021 The Authors. Published by American Chemical Society. This publication is licensed under

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Abstract

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Colloidal supraparticles are micron-scale spherical assemblies of uniform primary particles, which exhibit emergent properties of a colloidal crystal, yet exist as a dispersible powder. A prerequisite to utilize these emergent functionalities is that the supraparticles maintain their mechanical integrity upon the mechanical impacts that are likely to occur during processing. Understanding how the internal structure relates to the resultant mechanical properties of a supraparticle is therefore of general interest. Here, we take the example of supraparticles templated from water/fluorinated oil emulsions in droplet-based microfluidics and explore the effect of surfactants on their mechanical properties. Stable emulsions can be generated by nonionic block copolymers consisting of a hydrophilic and fluorophilic block and anionic fluorosurfactants widely available under the brand name Krytox. The supraparticles formed in the presence of both types of surfactants appear structurally similar, but differ greatly in their mechanical properties. While the nonionic surfactant induces superior mechanical stability and ductile fracture behavior, the anionic Krytox surfactant leads to weak supraparticles with brittle fracture. We complement this macroscopic picture with Brillouin light spectroscopy that is very sensitive to the interparticle contacts for subnanometer-thick adsorbed layers atop of the nanoparticle. While the anionic Krytox does not significantly affect the interparticle bonds, the amphiphilic nonionic surfactant drastically strengthens these bonds to the point that individual particle vibrations are not resolved in the experimental spectrum. Our results demonstrate that seemingly subtle changes in the physicochemical properties of supraparticles can drastically impact the resultant mechanical properties.

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Published as part of The Journal of Physical Chemistry virtual special issue “Marie-Paule Pileni Festschrift”.

1. Introduction

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Supraparticles are finite assemblies of primary colloidal particles with micron-scale dimensions fabricated by confined self-assembly processes, for example, within emulsion droplets. (1−4) If the primary particles are sufficiently monodispersed, such supraparticles exhibit a well-defined structural order determined by the interplay between the curvature imposed by the templating droplet, (5−7) the system size, (8,9) and the shape of the primary particle, (10−12) as well as the drying kinetics. (13,14) As inherently hierarchical materials, supraparticles bridge the microscopic and macroscopic scales and combine emergent properties arising from the collective behavior of the primary building blocks with characteristics of a macroscopic powder and finite properties that can be different from self-assembled materials in the bulk. The size of the primary colloidal particles can be chosen to match the wavelengths of visible light, (15,16) in which case the defined structural arrangement in the supraparticles produces tunable structural color with an isotropic or anisotropic appearance. (17−19) The latter depends on whether their internal structure is amorphous, (20) partially crystalline, (21−23) or completely crystalline. (24) Such supraparticles already find use in structural pigments, (25) displays, and sensors. (26)
The surface topography and composition of supraparticles can be controlled via mixtures of different primary particle sizes and materials, creating hierarchical porous (27) materials with gradients (13) for catalysis (28) and tissue engineering (29) or powders with enhanced flowability for additive manufacturing. (30) Incorporating functional primary particles encodes additional properties into such supraparticles, such as magnetic actuation, (31) efficient separation, (32) object identification, (33) antitampering, (34) lasing, (35) photothermal conversion, (36) and drug delivery. (37) Supraparticles also serve as model systems in soft condensed matter, providing insights for fundamental questions on structure formation, (6,7,38,39) crystallization pathways, (8,40) and the number–structure–property relationship (9,41) of confined finite systems.
For any application, a fundamental prerequisite is that the supraparticles must have sufficient mechanical stability to ensure structural integrity. This is especially important since the emergent properties arise from the defined internal arrangement of the primary particles and will therefore be lost upon breakage or disintegration. The mechanical properties of supraparticles, especially under compression, which is likely to occur in applications, need to take into account both the properties of primary particles and the strength of the interparticle bonds holding these particles together.
On the primary particle level, nanomechanical studies of submicron particles made from polystyrene, (42) silica, (43) gold, (44) or titania (45) provided detailed mechanical properties such as their Young’s moduli, yield strengths, or elastic-plastic loading indexes based on theoretical Hertz, Johnson–Kendell–Roberts (JKR), or Derjaguin–Muller–Toporov (DMT) models. (46) At the supraparticle level, pioneering work in the chemical engineering community has built the foundation to understand the breakage (47−49) and elastic-plastic properties (50−52) of supraparticles assisted by simulation methods. (53−55) However, typically produced in large-scale processes such as spray drying, (14) emulsification, (56) or wet granulation, the supraparticles in these investigations were inhomogeneous in size and consisted of ill-defined primary particles without a uniform shape and size, rendering a detailed correlation of their structure with the resultant mechanical properties challenging. With advancements in colloidal science, supraparticles with a well-defined internal structure, accurate spherical shape, and narrow size distributions can be fabricated, often by the consolidation of well-defined colloidal primary particles encapsulated in uniform emulsion droplets produced in microfluidics. (5,8,9,15,24,57−59) The precise control of the size and number of primary particles and, hence, the size and geometric feature of the supraparticles enables a detailed investigation of their size- and structure-dependent mechanical properties. (60,61)
From bulk studies of colloidal assemblies, it has been established that surprisingly attractive mechanical properties can result from a strengthening of the interparticle bonds, for example, by the addition of binders, (48) sintering, (62) or chemically interconnecting organic ligands on the surface of the materials. (63−66) Recently, we focused on the mechanical properties of supraparticles in the absence of such binders. Intuitively, it would be anticipated that such a material easily breaks upon compression due to the relatively weak interparticle contacts. (61) However, supraparticles made from polystyrene primary particles exhibit a surprisingly high deformation resistance and ductile fracture behavior, which was comparable to solid polystyrene microparticles at ambient conditions. (61) Based on an adapted Griffith fracture model, we established scaling relationships connecting fracture stress and strain to the dimensions of primary particles and supraparticles. In this context, the interplay between the physical interparticle bonds and the mechanical strength of the individual primary particles determines the complex fracture behavior of supraparticles.
The microfluidic fabrication of such defined supraparticles occurs by evaporating a droplet of aqueous colloidal dispersion in an continuous oil phase. Since no additive is added in the water phase, in the first approximation, the formed supraparticles should be considered a binderless granulate consisting only of the primary particles, held together by van der Waals forces and capillary bridges. However, the fabrication process necessitates the presence of a surfactant that stabilizes the individual droplets. (67−69) These surfactant molecules, albeit used in small concentration in the continuous oil phase, can remain at the contacts between the primary particles and therefore affect the mechanical properties of the supraparticles. Such influences have not been addressed in our previous study on supraparticle mechanics and, in general, can be easily overlooked in the design of experimental systems to prepare supraparticles from emulsion-based approaches.
Here we focus on the effect of these surfactants and show that even trace amounts of the surfactant molecules can significantly change the mechanical behavior of supraparticles. Depending on the nature of the surfactant and its interaction with the primary particles, the supraparticles can exhibit strongly varying mechanical stability, with a fracture behavior changing from ductile to brittle. We investigate the fracture mechanics of the supraparticles with a detailed study of the interparticle contacts with the different surfactant molecules. To shed light on changes in the interparticle contacts by the presence of surfactants, we employ the noncontact, noninvasive, and zero-strain Brillouin Light Spectroscopy (BLS) (70−76) to record the primary particle mechanical vibrations in the supraparticles. The vibration spectrum is highly sensitive to the interparticle contacts (77−80) and thus allows correlating the mechanical properties of the supraparticles under compression with the micromechanics of the interparticle contacts. This correlation helps to rationalize the changes observed in the supraparticle mechanical stability and showcases the possibility of mechanical property design for particulate materials via surfactants.

2. Methods

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2.1. Microfluidic Chip Fabrication

PDMS microfluidic devices were produced by a typical soft lithography method, as described in the literature. (81) In short, a silicon wafer was spin-coated with SU-8 negative photoresist, and a pattern mask was used to create microstructures on the coated water surface through UV light. The flow-focusing microfluidic channel microstructures were then hardened and coated with an antisticking layer (trichloro-(1H,1H,2H,2H-perfluorooctyl)silane, Sigma-Aldrich) to produce the master wafer. Polydimethylsiloxane (Sylgard 184 PDMS from Dow Corning) was mixed with a curing agent at a 10:1 weight ratio and degassed before pouring onto the master wafer to an approximately 1 cm thickness. The PDMS was cured at 80 °C in an oven overnight. The cured PDMS chip was cut off by a razor blade and peeled from the wafer. A biopsy punch (1.0 mm in diameter, Kai Group) was used to create the inlets and outlets in the PDMS chip. The remaining debris and dust on the PDMS chip surface were removed by Scotch adhesive tape. The chip was washed with ethanol and water (Milli-Q) and dried with compressed air. After cleaning, the PDMS chip and a clean glass slide were plasma-treated for 30 s in an oxygen environment at 30 W power (Diener electronic, Femto). After surface activation, the PDMS chip was bonded to the glass slide and put in the oven at 80 °C for 30 min to enhance bonding. Afterward, the microfluidics channels were immediately flushed with 2 wt % fluorosilane dissolved in HFE 7500 oil (3M) by injection through a 1 mL syringe of a blunt needle. After 1 h, the channels were flushed by compressed gas to remove the liquid and kept in the oven to dry.

2.2. Supraparticle Fabrication

Styrene, acrylic acid, and ammonium peroxodisulfate were purchased from Sigma-Aldrich and used as received. Polystyrene colloidal particles were synthesized using acrylic acid as comonomer and ammonium peroxodisulfate as initiator in surfactant-free emulsion polymerization following literature protocols. (82) The resultant aqueous polystyrene colloidal particle dispersion (1 wt %) was used as a dispersed phase in the flow-focusing microfluidic channel. Fluorinated oil HFE 7500 (3M) containing 0.1 wt % of fluorosurfactants was used as the continuous phase. Anionic surfactant, perfluoropolyether carboxylic acid (Krytox 157 FSH, DuPont,) was used as received. Nonionic PFPE-PEG-PFPE surfactant (perfluoropolypropylene glycol-block-poly(propylene glycol)-block-poly(ethylene glycol)-block-poly(propylene glycol)-perfluoropolypropylene glycol-block) was synthesized following literature protocols. (68) The anionic surfactant at the chosen concentration yielded an interfacial tension of approximately 20 mN/m, the nonionic surfactant further reduced the interfacial tension to ∼10 mN/m. The liquids were pumped by a precision pump (Cronus) into the microfluidic channel via medical grade PE tubes at flow rates of 200 and 500 μL/h for the disperse phase and the continuous phase, respectively. The emulsion droplets were collected in a 1.5 mL glass vial for storage and drying. Typically, 20 μL of the emulsion was collected in 200 μL of oil. For spherical supraparticles, the glass vial was stored at room temperature without a closing cap, which allows water to be removed overnight by evaporation and diffusion through the oil phase and thus yields consolidated supraparticles suspended in oil. For crystalline supraparticles, the glass vial was sealed with parafilm in which one hole was poked with a 0.4 mm needle and stored at 5 °C to slow down the evaporation. The drying process usually takes 4 weeks. (9) These supraparticles were deposited onto a solid substrate for scanning electron microscope characterization (Zeiss GeminiSEM 500), mechanical testing, and Brillouin light spectroscopy.

2.3. Compression of Supraparticles via Nanoindentation

Nanoindentation on the colloidal supraparticles was performed in a Nanoindenter G200 (KLA, U.S.A.) with 0.01 nm depth resolution and 50 nN load resolution in displacement-controlled mode. The diamond flat punch indenter had a diameter of 90 μm (Synton MDP). The diameters of the colloidal supraparticles were measured in SEM. For the indentation measurements, the samples were deposited onto a piece of silicon wafer directly from the fluorinated oil phase. The silicon wafer was glued to the sample holder with a minimal amount of superglue (Ultra gel, Pattex), and the samples were left for 24 h for the glue to cure. After mounting the sample holder into the indenter, at least 1 h was allowed for thermal stabilization before starting the compression measurements. The measurements were performed at ambient room condition of 50% relative humidity. The loading and unloading speeds were fixed at 50 nm/s, the maximum displacement set at 60% nominal strain, and the peak holding time to 10 s. A double-sided tape was attached next to the sample holder to facilitate the cleaning of the indenter tip. To this purpose, multiple indentations were performed into the tape after each particle measurement in order to remove the debris sticking to the tip. (61)

2.4. Brillouin Light Spectroscopy (BLS)

BLS is an optical technique based on the scattering of light by thermally excited hypersonic (GHz) phonons existing in all materials. For isotropic transparent material, a single acoustic phonon at frequency ω (=2πf) is probed at a given wave vector, ±q = ki – ks selected by the scattering geometry, where ±q is for Stokes and anti-Stokes components, ki and ks are wavevectors of the incident and scattered light. Using energy and momentum conservation laws, a doublet with frequency, ω = ±cq, relative to the central Rayleigh line (ω = 0) appears in the spectrum of the scattered light analyzed by a six-pass tandem-Fabry–Perot interferometer. (66,83) Here, c denotes the sound velocity (longitudinal or transverse) in the medium, and the momentum magnitude q depends on the scattering geometry. In the so-called transmission geometry, qtrans = (4π/λ)sin α, where α = θ/2, with θ being the scattering angle between ki and ks and λ (=532 nm) is the laser wavelength in vacuum. In a general case, q = (4πn/λ) sin(θ/2) depends on the material refractive index, n. (84) For localized phonons in space, for example, particle resonances, the frequencies are q-independent, as revealed by recording the BLS spectra at two different magnitudes of q.

3. Results and Discussion

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3.1. Colloidal Supraparticles Formed in Emulsion Droplets Stabilized by Surfactants

Colloidal supraparticles with a well-defined internal structure, spherical shape, and low size polydispersity are fabricated using emulsion droplets prepared by droplet-based microfluidics as confinements. (9,21) In the course of the self-assembled process, water is removed from the droplet by evaporation and diffusion into the oil phase until the colloidal primary particles eventually consolidate into compact supraparticles. The diameter of the supraparticles as well as the material, number, and diameter of the primary particles can be controlled via the droplet size and the concentrations of colloidal primary particles. (9,21) The kinetics of the drying process determine the internal structure of the supraparticles. At very slow drying conditions, crystalline supraparticles with defined geometrical features are formed. Depending on the system size, either truncated icosahedra, decahedra, or octahedra are formed as minimum energy structures. (8,9,24) A shorter drying time yields spherical supraparticles. These structures exhibit a well-ordered arrangement with close-packed particle layers at the surface and a more disordered inside (21) and are the most commonly observed type of colloidal supraparticles. (15,85)
As suggested from Figure 1a, the stability of the emulsion droplets during formation and self-assembly of primary particles depends on the presence of surfactant molecules to stabilize the droplet interface. In this study, we use a water/fluorinated oil system that is typically used in PDMS-based microfluidic devices because the fluorinated oil does not swell the PDMS and therefore ensures reliable operation. (86) We use two types of surfactant that efficiently stabilize the water/fluorinated oil interface and are often used in such microfluidic systems. (67,68) The nonionic surfactant, illustrated in Figure 1b, is a triblock copolymer formed by a water-soluble middle part and two perfluoropolyether polymers to induce steric repulsion in the continuous fluorinated oil phase. The hydrophilic block (blue) itself is based on a PPO-PEO-PPO triblock copolymer (polyethylene glycol-block-polypropylene glycol-block-polyethylene glycol) with a molecular weight MW = 900 g/mol and is commercially available under the trade name of “Jeffamine”. The perfluoropolyether is based on a commercially available fluorinated oil under the name Krytox (Krytox 157 FSH, MW = 7000 g/mol). These Krytox molecules can also directly function as an anionic surfactant. As illustrated in Figure 1c, Krytox is a perfluoropolyether with a carboxylic acid termination, which is deprotonated in an aqueous environment and thus negatively charged. Note that while conventional charged surfactants stabilize emulsions via electrostatic repulsion, in this case, the charged moiety sits in the dispersed phase, while the surfactant exposes the fluorophilic polymer to the continuous phase and therefore induces steric stabilization.

Figure 1

Figure 1. Emulsion-templated fabrication of supraparticles. (a) Schematic illustration of the supraparticle self-assembly in an emulsion droplet and the role of surfactants. (b) Nonionic perfluoropolyether-block- (polyethylene glycol-block-polypropylene glycol-block-polyethylene glycol)-block perfluoropolyether surfactant (PPP) used in supraparticle fabrication. (c) Anionic Krytox surfactant used in supraparticle fabrication. (d) Monodispersed water in oil droplets (gray) encapsulating polystyrene colloidal particles produced by the microfluidic device. (e, f) Colloidal supraparticles with spherical shape and ordered surface structure, made with PPP and Krytox surfactant, respectively. (g) Colloidal supraparticle under a nanoindenter tip.

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At a concentration as low as 0.1 wt %, both surfactants can efficiently stabilize the water-in-fluorinated oil emulsion (Figure 1d). The uniformity of the droplet results in supraparticles with near-monodispersed sizes, which allows careful investigation of their mechanical properties. Both surfactants repel the negatively charged polystyrene particles (PPP surfactant by steric and Krytox by electrostatic repulsion) from the droplet interface and allow the formation of supraparticles with spherical outline and ordered surface without discernible structural differences (Figure 1e,f). However, as we will see, the mechanical stability of both types of supraparticles is heavily influenced by the type of the surfactants.

3.2. Supraparticles with High Mechanical Stability

We first demonstrate that supraparticles formed with nonionic surfactant (abbreviated as “PPP”) exhibit high mechanical stability under compression. We use supraparticles with a diameter of 7 μm consisting of 244 nm polystyrene primary particles. We use an indentation tip of 90 μm to exclude any edge effects and ensure that the supraparticle is compressed between two parallel plates. The force and displacement of the indenter tip is recorded from first contact to the supraparticle until the tip advances up to a nominal strain of 60% (i.e., 4 μm into the sample) during deformation. Since the diameter of the supraparticle during compression changes dynamically during the compression experiment, we define the nominal stress by normalizing the force by the initial supraparticle cross-sectional area and we define the nominal strain by normalizing the displacement by the initial supraparticle diameter. We define the slope in the linear region of the nominal stress–strain curve before fracture as the deformation resistance of the supraparticle. We define the first pop-in (or jump in displacement) in the nominal stress–strain curve as the first fracture event, and the area under the curve up to fracture as the work of deformation. (61)
The nominal stress–strain curve of three typical supraparticles is shown in Figure 2a. All three curves show a linear region leading up to a pop-in at about 15 MPa at 10% nominal strain, indicating a first fracture event. Several consecutive plateaus indicate multiple fractures that occur in the course of compression. Noteworthily, these plateaus also demonstrate that the supraparticle deforms in a ductile manner and remains in one piece after fracture. The almost vertical unloading curves suggest predominantly plastic deformation. An average nominal stress–strain curve of seven other samples with the same diameter is shown in the inset by the solid yellow line. The standard deviation is represented by the color band (Figure 2a, inset) and shows the good uniformity of the mechanical properties, which arises from the uniform character of our supraparticles.

Figure 2

Figure 2. Supraparticles with nonionic PPP surfactant exhibit ductile mechanical properties and high stability from favorable surfactant-particle interactions. (a) Supraparticles formed by polystyrene primary particles and the nonionic PPP surfactant show a reproducible compression behavior with ductile fracture. Multiple, defined plateaus indicate multiple crack formation events without breakage of the supraparticle. (b–d) SEM investigation of the supraparticle surface and morphology. (b) Supraparticle after compression shows several large cracks at the periphery but remains as a single piece. (c) Close up of a deformed supraparticle. Individual primary particles are heavily deformed and solid material bridging neighboring particles is seen. (d) Rig-like residues at the primary particle surface indicate the formation of solid deposits at the contact points between primary particles.

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In a simplified view, the supraparticles are granular assemblies of primary particles held together only by van der Waals and capillary forces acting between their contact points. For such a system, the supraparticles exhibit a surprisingly high ductility and mechanical stability of 15 MPa fracture stress. We examine the supraparticle morphology before and after compression. Figure 2b shows a deformed supraparticle after compression, exhibiting several large cracks extending from the periphery to the center. We hypothesize that these cracks correspond to the multiple plateaus in the nominal stress–strain curve. Evidently, the supraparticle shows ductile deformation and remains as one single piece after fracture. In Figure 2c, clear deformation of primary particles can be observed. At the top surface, where the supraparticle was in contact with the flat indenter, the spherical primary particles increasingly flatten into a disk shape with increasing diameter and reduced interstitial volume. Importantly, solid bridges (blue arrows) connecting neighboring primary particles are observed within the small cracks, likely resulting from a viscous polymer material residing at the contact points between the primary particles. In Figure 2d, a similar solid residue is evident in a ring-like structure at the surface of primary particles (blue arrows) in a rare broken fragment of a supraparticle after compression (supraparticles consisting of 1 μm primary particles were used to illustrate the residue morphology). The location of these solid features at the surface gives evidence that it was formed in the contact points between neighboring primary particles within the supraparticle. Since the supraparticles are suspended in the fluorinated oil phase before deposition to the solid substrate and mechanical investigations, it can be inferred that the surfactant molecules at least partially infiltrate the porous supraparticle structure. Capillary bridges forming during drying will subsequently drag them toward the contact points where they accumulate. Seemingly, even at the low surfactant concentration of 0.1 wt % used in our fabrication process, the nonionic surfactant molecules precipitate around the contact points of the primary particles inside the supraparticle. Estimation of the dimensions of the formed rings from SEM images gives an approximate volume ratio between the residue surfactant to the particle, which we estimate to be around 0.02%. This value is significantly lower than the typical binder content up to 10% in industrial agglomeration, (87) yet it effectively stabilizes the supraparticles.
Interestingly, contrary to the common belief that reducing the void volume or increasing the number of contacts increases the mechanical strength of particulate materials, (48,88) we do not observe an increase of the deformation resistance or fracture stress in crystalline supraparticles, (9,24,41) which minimizes void volume and maximize contacts (Figure S1a). We distinguish the crystalline supraparticle by its faceted surface, indicating a complete ordered interior structure, (9,24,41) in contrast to the disordered interior region of spherical supraparticles with only an ordered surface. (21) In addition, the crystalline supraparticles do not exhibit a tendency to break along crystal planes in the compression experiment at ambient conditions, as may be expected for an atomistic system. (89) Noteworthily, this behavior changes when the supraparticles are subjected to mechanical stress directly in the oil phase. In this case, the surfactant may reside at the supraparticle surface but is not assumed to infiltrate and manipulate the contact points. A treatment of 30 s ultrasonication to induce mechanical stress in a supraparticle suspension shows that crystalline supraparticles tend to fracture into defined pieces along their (111) crystal planes, while spherical supraparticles with an amorphous core survive without fracture (Figure S1b), retaining the similarity to the expected behavior of atomic materials. These differences in the behavior of crystalline supraparticles further suggest that the mechanical stability in ambient conditions is not entirely controlled by van der Waals and capillary forces, as is the case for typical granular materials, but mediated by the infiltrated surfactants. It also suggests that the PPP surfactant residues may strengthen the contacts, hence, the unexpected ductile deformation of the supraparticles, as we will argue below.

3.3. Supraparticles with Low Mechanical Stability

We change the surfactant from nonionic PPP to anionic Krytox and observe a completely different mechanical behavior. Three individual nominal stress–strain curves for these supraparticles are shown in Figure 3a. Both the supraparticle diameter (7 μm) and the primary PS particle diameter (244 nm) remain the same, but their fracture stress drops significantly from 15 MPa (orange, PPP surfactant, inset) to about 1 MPa (black, Krytox surfactant). In addition, the first fracture occurs at a smaller strain at about 6%, compared to 12% for supraparticles made with PPP surfactant. Noteworthily, after the initiation of a pop-in fracture event, the stress decreases (instead of forming a plateau in the case of PPP surfactant), meaning that the indenter tip experiences hardly any resistance any longer as it advances after the supraparticle fractures. This suggests brittle and catastrophic fracture of the supraparticles where the structure of the deformation-resisting supraparticle changes abruptly and the contact between the indenter and the supraparticle may be reduced or lost. This is confirmed by SEM images of compressed supraparticles (Figure. 3b). Contrary to ductile supraparticles fabricated with nonionic PPP surfactant (Figure 2b), only pieces of supraparticles fabricated in anionic Krytox remain on the substrate. The primary particles remain spherical and undeformed. This, together with the reduced deformation resistance, as well as the ductile to brittle transition, agrees with change of supraparticle mechanical properties induced by weakened interparticle adhesion. (61) The anionic surfactants do not form bridges that connect neighboring primary particles, as seen in Figure 2c, but are observed to be tiny spherical dots at some primary particle surfaces (Figure 3c), suggesting dewetting from the negatively charged polystyrene surface. Note that despite the observed dewetting from the particle surfaces and contact points of the formed supraparticles, the Krytox surfactant is clearly able to stabilize the water in oil emulsion and allows the formation of supraparticles. The dewetting therefore seems to occur only at the last stages of the drying process.

Figure 3

Figure 3. Supraparticles with Krytox surfactant exhibit brittle mechanical properties and low stability from unfavorable surfactant-particle interactions. (a) Supraparticles formed by primary polystyrene particles and the anionic Krytox surfactant show brittle fracture with reduced fracture stress and strain. (b) Supraparticles fracture in a brittle way without visible deformation of the individual primary particles. (c) Surfactant traces reside on primary particle surface as tiny spherical dots.

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Table 1 compares the values of typical mechanical properties extracted from the compression curves of supraparticles made with PPP and Krytox surfactant. Although the surfactant residue takes up as little as approximately 0.02% volume, it significantly influences the mechanical behavior of the entire supraparticles.
Table 1. Typical Mechanical Properties of Supraparticles
 fracture stress (MPa)fracture strain (%)deformation resistance (MPa)work of deformation (pJ)
with PPP1512125320
with Krytox161640

3.4. Molecular Interpretation

The contrasting different mechanical properties of the supraparticle containing minute nonionic or anionic surfactant residues suggests a unique effect of the surfactant molecules at the primary particle contacts. As illustrated in Figure 4, we postulate that the different molecular properties of the two types of surfactants modify the adhesion between neighboring primary particles, which results in the widely different behavior of supraparticles. For the nonionic PPP surfactant, the amphiphilic PPO-PEO-PPO block is known to adsorb and cover the polystyrene particle surface. (90−92) The remaining PFPE blocks form a viscous material, a natural state of this type of polymer at room temperature. Together, these properties allow the surfactant to increase the adhesion between neighboring primary particles (Figure 5a). In addition, the hydrophilic nature of the polymer may be beneficial to bind water and thus strengthen capillary bridges. Separating these two particles requires not only overcoming the van der Waals and capillary forces, but also energy as mechanical work to deform and stretch the viscous polymer surfactant. With PPP surfactant, the mechanical stability of the supraparticles is therefore enhanced. In contrast, the carboxylic head of the Krytox surfactant carries a negative charge and is therefore repelled from the negatively charged particle surface due to the molecular layer of water in ambient conditions (Figure 4b). The PFPE block (which accounts for more than 99% of the surfactant) does not wet the particle surface and hence does not provide any additional adhesive force between neighboring particles. The hydrophobic PFPE polymer may even reduce the formation of capillary water bridges and act as a lubricant between neighboring particles, reducing the mechanical stability of supraparticles. According to this hypothesis, the nonionic PPP surfactant strongly binds neighboring primary particles, connecting all polymeric particles in the supraparticle through polymer bridges into effectively a large porous polymer material. In contrast, the Krytox surfactant does not enhance the adhesion or may even weaken the supraparticles. Visual inspection of broken supraparticles supports this hypothesis. While supraparticles fabricated with the PPP surfactant display large ring-like residues around the contact points (Figure 2d), Krytox surfactant seemingly leaves dot-like residues (Figure 3c) that do not seem to interconnect primary particles.

Figure 4

Figure 4. Hypothesis on the influence of surfactant in supraparticle mechanical stability. (a) Nonionic PPP surfactant has a PPO-PEO-PPO block that adsorbs to polystyrene surface due to hydrophobic interactions, the polymer chains in the PFPE blocks from different surfactant molecules are entangled. The PFPE polymer chains are viscous at room temperature and promote adhesion between neighboring particles. (b) Anionic Krytox surfactant has a carboxyl group that deprotonates in ambient humidity to carry a negative charge, which causes an electrostatic repulsion with the negatively charged particle surface. The PFPE block also dewets the particle surface. The surfactant molecule resides at the surface without providing extra adhesive forces. The sketch is not drawn to scale.

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Figure 5

Figure 5. Brillouin light spectroscopy (BLS) spectra (circles) of a colloidal crystal of polystyrene primary particles in (a) normal I(f) and (b) normalized I(f)·f2 presentations. The spectra show the as prepared, dry colloidal crystal film (gray), as well as the film infiltrated with HFE oil containing 0.1% nonionic PPP surfactant (red), pure HFE oil (green), and HFE oil containing 0.1% anionic Krytox surfactant (purple). Dashed lines show for Gaussian line shape fits. Inset to (a): The single peak BLS spectrum recorded in a backscattering geometry shows the same film infiltrated only with PPP. (c) SEM images of the colloidal crystal film as fabricated. Scale bars in (c) are 5 μm (upper) and 200 nm (lower panel).

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We use Brillouin light spectroscopy (BLS) to test the hypothesis. To this end, we record the vibration spectrum of both supraparticles and individual primary particles with and without the different surfactants and investigate their effect on the particle contacts via the changes to the characteristic vibrations. In the absence of any interparticle interactions, the individual primary particles exhibit the characteristic spheroidal Lamb (n, l) modes with n and l denoting the radial and angular momentum numbers, respectively. In this case, the lowest peak of the vibration spectrum is the (1,2) mode at the frequency f(1,2) = Act/d, where ct is the transverse sound velocity in the primary particle, d is diameter, and A (≈0.84 for polystyrene) is a constant. The vibration spectrum is very sensitive to interparticle interactions, manifested by the appearance of a new low-frequency mode, termed interaction mode, and a split and concurrently blue shift of the (1,2) mode. The low-frequency mode is assigned to a longitudinal phonon in a cluster of particles. It corresponds to the (1,1) translational mode, which is of zero frequency in independent particles, that is, no particle–particle interaction. (66,83,93,94) The strength and number of contacts of individual particles in a cluster influence their vibration eigenmodes. Thus, if primary particles in the supraparticles strongly adhere to their neighbors, their individual vibration modes cannot be experimentally resolved. Both the split of the (1,2) mode and the emergence of the interaction induced (1,1) mode allow for a subnanometer thickness sensitivity for adsorbed layers atop of nanoparticles. (95,96)
Figure 5a displays the experimental BLS spectra for a flat colloidal crystal film (black) of the same polystyrene primary particles with d = 236 nm, formed by the deposition and ordering of colloidal particles in a drying droplet of colloid dispersion on a solid substrate. We use this colloidal crystal film with a well-ordered arrangement of the primary particles in a face-centered cubic lattice (Figure 5c) as a reference material to probe how the BLS spectra change upon exposure to surfactants that occur in the supraparticle formation. To this end, we first infiltrate the colloidal crystal with the following liquid by dropcasting the liquid into the colloidal crystal: pure HFE oil (green); HFE oil with nonionic 0.1% PPP surfactant (red); HFE oil with 0.1% anionic Krtyox surfactant (purple). A total of 5 μL of the liquid is dropcasted above the center of the colloidal crystal film, which subsequently spreads and is immersed in the film. BLS spectra are recorded after the liquid dried under ambient room conditions, which left the surfactants inside the colloidal crystal film.
The strong wing at about 1–3 GHz indicates the presence of the interaction-induced (1,1) mode. This spectral feature is better resolved in the normalized spectrum I(f)·f2 displayed in Figure 5b. Here, the prefactor f2 accounts for the population of the low frequency thermally excited phonons. (83) A distinct discernible broad (1,1) peak at about 2.5 GHz is resolved for all infiltrating liquids (pure HFE oil, oil with PPP surfactant, and with Krytox surfactant). Similarly, the particle interactions cause a split of the (1,2) mode at about 5 GHz. The split of the (1,2) mode can be represented by two Gaussian lineshapes (Figure S2) centered at f1 and f2 > f1. The frequency fL = 2f1 – f2 accounts for the presence of the interparticle interactions, that is, it is equal to the frequency of (1,2) in the case of a single primary particle. While the position, f1, and f2 and their relative intensities of the two peaks vary among the different infiltrated polystyrene colloidal crystal films, the frequency, fL = 2f1 – f2 that accounts for the presence of interparticle interactions fL = 3.82 ± 0.07 GHz remains unaffected by the infiltrated fluids. The frequency fL (=0.84ct/d) represents the shear modulus, G = ρct2 (ρ is the mass density) of the polystyrene primary particles. The value of G = 1.1 GPa (for d = 236 nm) is experimentally the value of bulk polystyrene. The robust vibration spectrum indicates that the vibration spectra of the pure polystyrene colloidal crystal can be recovered upon infiltration with surfactants used in the supraparticle formation. Note, however, that the treatment in both protocols cannot be strictly identical: while a supraparticle is fully surrounded by surfactants, in the case of the flat film, the distribution of surfactant is likely less uniform. To probe how a spectrum of a fully infiltrated colloidal crystal would appear, we added pure PPP surfactant to the colloidal crystal. The resulting spectra shown as an inset to Figure 5a do not show any particle vibrations and only display a peak at about 11 GHz. This spectrum was recorded in a backscattering geometry and corresponds to the effective medium longitudinal sound velocity (∼1900 m/s). In this case, the PPP clearly induces sufficient interparticle adhesion to prevent individual particle vibrations entirely.
Next, we focus on the vibration spectra of individual supraparticles prepared from the two different surfactants. To investigate individual supraparticle, we employ micro-BLS that provides a spot size of the probe beam of about 1 μm. (93)Figure 6 displays the BLS spectra of crystalline and disordered supraparticles (Figure S1), both fabricated in HFE oil containing 0.1 wt % of the anionic Krytox surfactant. In this case, elastic vibrations of the constituent primary PS particles are observed. Similar to the flat colloidal crystal film, the single quadrupolar mode (1,2), observed for noninteracting spherical colloidal particles, is split in both ordered and disordered supraparticles, as shown in Figure 6. The two supraparticles reveal minor differences in the (1,2) peak shape characterized by different relative intensities of the two spectral contributions (dashed lines in the lower panel of Figure 6) and f1, f2 values. The different packing in crystalline and disordered particles can cause different contacts between the primary polystyrene particles sensitively reflected in the shape of the (1,2) mode split. The comparison of the intensity ratios f2/f1 between the crystalline and the disordered supraparticle suggests stronger interactions for the crystalline supraparticle, which is expected due to the larger number of neighbors. Notably, the spectra of the crystalline supraparticle is very similar to the colloidal crystal film, underlining the high degree of crystallinity in this supraparticle (Figure S3). From the computed fL (=2f1 – f2) it follows that the shear modulus has the same value (1.2 GPa) for the two supraparticles (Figure 6), which agrees with the value of the primary PS particles (Figure 5).

Figure 6

Figure 6. BLS spectra of supraparticles fabricated in two different surfactants. The red and purple curves correspond to crystalline and disordered supraparticles using Krytox surfactant. The interaction-induced split of the (1,2) mode is represented by two Gaussian peaks (dashed lines). The green curve corresponds to the supraparticle using the nonionic PPP surfactant. Notably, no particle vibrations are resolved, and the BLS spectrum shows signatures characteristic of two propagating acoustic phonons.

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In contrast, the BLS spectrum of a supraparticle fabricated in HFE oil containing 0.1 wt % PPP surfactant drastically changes the BLS spectrum. No individual particle vibrations are resolved in the experimental spectrum. Instead, the BLS spectrum shows longitudinal acoustic phonons propagating in the effective medium. While a single effective medium phonon is observed in the crystalline films of the polystyrene primary particles (inset of Figure 5a), the spectrum of the supraparticle (upper spectrum in Figure 6) displays two acoustic phonons at about 5.8 and 11 GHz. The latter frequency is the same in both systems phonon and represents the effective medium acoustic phonon, whereas the lower frequency mode in PPP coincides with the longitudinal phonon in the neat PPP (Figure S4). The presence of this phonon implies either an inhomogeneous medium with submicron PPP microregions exceeding the phonon wavelength (∼200 nm) or a thin, continuous surface PPP layer. We hypothesize that such pockets may form as residues at the contact points, while a surface layer may form upon drying of the supraparticle as the surfactant stabilizing the shell interaction with the outermost surface of the PS primary particles via hydrophobic interactions of the propylene glycol groups, as discussed in Figure 4 above. The BLS investigation of the interparticle contacts clearly indicates a strong increase in contact strength in the presence of PPP surfactant, corroborating with the increased resistance to mechanical compression shown above. In contrast, using Krytox as a surfactant did not alter the particle vibrations, corroborating with the hypothesis that this type of surfactant does not re-enforce the interparticle contacts. As a consequence, supraparticles prepared with Krytox exhibit lower mechanical properties and break in a brittle manner. Note that while the affinity of the surfactant to the particles enhance their mechanical stability, it needs to be carefully balanced with the primary role of the surfactant, which is to ensure emulsion stability during the supraparticle formation process. In particular, using strong attraction, such as the anionic Krytox surfactant with positively charged polystyrene colloidal particles, we observe that individual primary particles leave the emulsion droplet and thus do not form supraparticles in our emulsion-based process.

4. Conclusions

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We show that the mechanical stability of colloidal supraparticles is mediated by the inevitable surfactant residue from the emulsion fabrication step. Using the example of a microfluidic production process with perfluorinated oil as the continuous phase, we probe this difference by two types of commonly used surfactants. Even at a trace amount, using the nonionic block copolymer-based surfactant PPP results in mechanically stable supraparticles that exhibit high deformation resistance and ductile fracture behavior. The anionic surfactant Krytox, in contrast, yields supraparticles with mechanical properties that are reduced by about 1 order of magnitude from 15 MPa fracture stress to 1 MPa. In addition, the fracture behavior with this type of surfactant changes from ductile to brittle. We complement the macroscopic mechanical investigation with a microscopic monitoring of the interparticle contacts via Brillouin light spectroscopy. These investigations corroborate the macroscopic interpretation that the nonionic surfactant strongly strengthens the interparticle contacts to the point that no individual vibrations can be resolved. Our work sheds light on the critical role of surfactants in tailoring the mechanical properties of colloidal supraparticles and provides guidelines for the fabrication of mechanically robust supraparticles, which are the prerequisite for any targeted applications of such materials.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpcc.1c06839.

  • Details of nominal stress–strain curves of crystalline and spherical supraparticles, BLS spectrum of supraparticles and crystalline films, and backscattered spectra of two surfactants (PDF)

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Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

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  • Corresponding Authors
  • Authors
    • Junwei Wang - Institute of Particle Technology, Friedrich-Alexander University Erlangen-Nürnberg, Cauerstrasse 4, 91058 Erlangen, Germany
    • Eunsoo Kang - Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany;  Orcidhttps://orcid.org/0000-0002-9894-0900
    • Umair Sultan - Institute of Particle Technology, Friedrich-Alexander University Erlangen-Nürnberg, Cauerstrasse 4, 91058 Erlangen, Germany;  Institute of Chemical Reaction Engineering, Friedrich-Alexander University Erlangen-Nürnberg, Egerlandstrasse 3, 91058 Erlangen, Germany;  Orcidhttps://orcid.org/0000-0003-3765-1061
    • Benoit Merle - Materials Science and Engineering I and Interdisciplinary Center for Nanostructured Films (IZNF), Friedrich-Alexander University Erlangen-Nürnberg, 91058 Erlangen, Germany
    • Alexandra Inayat - Institute of Chemical Reaction Engineering, Friedrich-Alexander University Erlangen-Nürnberg, Egerlandstrasse 3, 91058 Erlangen, Germany
    • Bartlomiej Graczykowski - Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany;  Faculty of Physics, Adam Mickiewicz University, Uniwersytetu Poznanskiego 2, Poznan 61-614, Poland
  • Funding

    Open access funded by Max Planck Society.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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We thank A. Stroebel and A. Krapf for their help with nanoindentation. J.W., U.S., A.I., and N.V. acknowledge financial support by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation, Project-ID 416229255, SFB 1411). E.K. and G.F. acknowledge financial support by ERC AdG SmartPhon (Grant No. 694977). B.M.’s contribution to this project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 Research and Innovation Programme (Grant Agreement No. 949626). B.G. acknowledges the financial support from Foundation for Polish Science (POIR.04.04.00-00-5D1B/18).

References

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    Nature Communications (2018), 9 (1), 5259CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
    Clusters in systems as diverse as metal atoms, virus proteins, noble gases, and nucleons have properties that depend sensitively on the no. of constituent particles. Certain nos. are termed magic because they grant the system with closed shells and exceptional stability. To this point, magic no. clusters have been exclusively found with attractive interactions as present between atoms. Here we show that magic no. clusters exist in a confined soft matter system with negligible interactions. Colloidal particles in an emulsion droplet spontaneously organize into a series of clusters with precisely defined shell structures. Crucially, free energy calcns. demonstrate that colloidal clusters with magic nos. possess higher thermodn. stability than those off magic nos. A complex kinetic pathway is responsible for the efficiency of this system in finding its min. free energy configuration. Targeting similar magic no. states is a strategy towards unique configurations in finite self-organizing systems across the scales.
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  10. 10
    Wang, D.; Hermes, M.; Kotni, R.; Wu, Y.; Tasios, N.; Liu, Y.; de Nijs, B.; van der Wee, E. B.; Murray, C. B.; Dijkstra, M.; van Blaaderen, A. Interplay between Spherical Confinement and Particle Shape on the Self-Assembly of Rounded Cubes. Nat. Commun. 2018, 9, 2228,  DOI: 10.1038/s41467-018-04644-4
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    10
    Interplay between spherical confinement and particle shape on the self-assembly of rounded cubes
    Wang Da; Hermes Michiel; Kotni Ramakrishna; Tasios Nikos; Liu Yang; de Nijs Bart; van der Wee Ernest B; Dijkstra Marjolein; van Blaaderen Alfons; Wu Yaoting; Murray Christopher B; Liu Yang; Murray Christopher B
    Nature communications (2018), 9 (1), 2228 ISSN:.
    Self-assembly of nanoparticles (NPs) inside drying emulsion droplets provides a general strategy for hierarchical structuring of matter at different length scales. The local orientation of neighboring crystalline NPs can be crucial to optimize for instance the optical and electronic properties of the self-assembled superstructures. By integrating experiments and computer simulations, we demonstrate that the orientational correlations of cubic NPs inside drying emulsion droplets are significantly determined by their flat faces. We analyze the rich interplay of positional and orientational order as the particle shape changes from a sharp cube to a rounded cube. Sharp cubes strongly align to form simple-cubic superstructures whereas rounded cubes assemble into icosahedral clusters with additionally strong local orientational correlations. This demonstrates that the interplay between packing, confinement and shape can be utilized to develop new materials with novel properties.
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  11. 11
    Wang, T.; Zhuang, J.; Lynch, J.; Chen, O.; Wang, Z.; Wang, X.; LaMontagne, D.; Wu, H.; Wang, Z.; Cao, Y. C. Self-Assembled Colloidal Superparticles from Nanorods. Science 2012, 338, 358,  DOI: 10.1126/science.1224221
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    11
    Self-Assembled Colloidal Superparticles from Nanorods
    Wang, Tie; Zhuang, Jiaqi; Lynch, Jared; Chen, Ou; Wang, Zhongliang; Wang, Xirui; LaMontagne, Derek; Wu, Huimeng; Wang, Zhongwu; Cao, Y. Charles
    Science (Washington, DC, United States) (2012), 338 (6105), 358-363CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Colloidal superparticles are nanoparticle assemblies in the form of colloidal particles. The assembly of nanoscopic objects into mesoscopic or macroscopic complex architectures allows bottom-up fabrication of functional materials. We report that the self-assembly of cadmium selenide-cadmium sulfide (CdSe-CdS) core-shell semiconductor nanorods, mediated by shape and structural anisotropy, produces mesoscopic colloidal superparticles having multiple well-defined supercryst. domains. Moreover, functionality-based anisotropic interactions between these CdSe-CdS nanorods can be kinetically introduced during the self-assembly and, in turn, yield single-domain, needle-like superparticles with parallel alignment of constituent nanorods. Unidirectional patterning of these mesoscopic needle-like superparticles gives rise to the lateral alignment of CdSe-CdS nanorods into macroscopic, uniform, freestanding polymer films that exhibit strong photoluminescence with a striking anisotropy, enabling their use as downconversion phosphors to create polarized light-emitting diodes.
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  12. 12
    Deng, K.; Luo, Z.; Tan, L.; Quan, Z. Self-Assembly of Anisotropic Nanoparticles into Functional Superstructures. Chem. Soc. Rev. 2020, 49, 6002– 6038,  DOI: 10.1039/D0CS00541J
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    12
    Self-assembly of anisotropic nanoparticles into functional superstructures
    Deng, Kerong; Luo, Zhishan; Tan, Li; Quan, Zewei
    Chemical Society Reviews (2020), 49 (16), 6002-6038CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    A review. Self-assembly of colloidal nanoparticles (NPs) into superstructures offers a flexible and promising pathway to manipulate the nanometer-sized particles and thus make full use of their unique properties. This bottom-up strategy builds a bridge between the NP regime and a new class of transformative materials across multiple length scales for technol. applications. In this field, anisotropic NPs with size- and shape-dependent phys. properties as self-assembly building blocks have long fascinated scientists. Self-assembly of anisotropic NPs not only opens up exciting opportunities to engineer a variety of intriguing and complex superlattice architectures, but also provides access to discover emergent collective properties that stem from their ordered arrangement. Thus, this has stimulated enormous research interests in both fundamental science and technol. applications. This present review comprehensively summarizes the latest advances in this area, and highlights their rich packing behaviors from the viewpoint of NP shape. We provide the basics of the exptl. techniques to produce NP superstructures and structural characterization tools, and detail the delicate assembled structures. Then the current understanding of the assembly dynamics is discussed with the assistance of in situ studies, followed by emergent collective properties from these NP assemblies. Finally, we end this article with the remaining challenges and outlook, hoping to encourage further research in this field.
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  13. 13
    Liu, W.; Midya, J.; Kappl, M.; Butt, H.-J.; Nikoubashman, A. Segregation in Drying Binary Colloidal Droplets. ACS Nano 2019, 13, 4972– 4979,  DOI: 10.1021/acsnano.9b00459
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    13
    Segregation in Drying Binary Colloidal Droplets
    Liu, Wendong; Midya, Jiarul; Kappl, Michael; Butt, Hans-Juergen; Nikoubashman, Arash
    ACS Nano (2019), 13 (5), 4972-4979CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    When a colloidal suspension droplet evaps. from a solid surface, it leaves a characteristic deposit in the contact region. These deposits are common and important for many applications in printing, coating, or washing. By the use of superamphiphobic surfaces as a substrate, the contact area can be reduced so that evapn. is almost radially sym. While drying, the droplets maintain a nearly perfect spherical shape. Here, we exploit this phenomenon to fabricate supraparticles from bidisperse colloidal aq. suspensions. The supraparticles have a core-shell morphol. The outer region is predominantly occupied by small colloids, forming a close-packed cryst. structure. Toward the center, the no. of large colloids increases and they are packed amorphously. The extent of this stratification decreases with decreasing the evapn. rate. Complementary simulations indicate that evapn. leads to a local increase in d., which, in turn, exerts stronger inward forces on the larger colloids. A comparison between expts. and simulations suggest that hydrodynamic interactions between the suspended colloids reduce the extent of stratification. Our findings are relevant for the fabrication of supraparticles for applications in the fields of chromatog., catalysis, drug delivery, photonics, and a better understanding of spray-drying.
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  14. 14
    Zellmer, S.; Garnweitner, G.; Breinlinger, T.; Kraft, T.; Schilde, C. Hierarchical Structure Formation of Nanoparticulate Spray-Dried Composite Aggregates. ACS Nano 2015, 9, 10749– 10757,  DOI: 10.1021/acsnano.5b05220
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    14
    Hierarchical Structure Formation of Nanoparticulate Spray-Dried Composite Aggregates
    Zellmer, Sabrina; Garnweitner, Georg; Breinlinger, Thomas; Kraft, Torsten; Schilde, Carsten
    ACS Nano (2015), 9 (11), 10749-10757CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    The design of hierarchically structured nano- and microparticles of different sizes, porosities, surface areas, compns., and internal structures from nanoparticle building blocks is important for new or enhanced application properties of high-quality products in a variety of industries. Spray-drying processes are well-suited for the design of hierarchical structures of multicomponent products. This structure design using various nanoparticles as building blocks is one of the most important challenges for the future to create products with optimized or completely new properties. Furthermore, the transfer of designed nanomaterials to large-scale products with favorable handling and processing can be achieved. The resultant aggregate structure depends on the utilized nanoparticle building blocks as well as on a large no. of process and formulation parameters. In this study, structure formation and segregation phenomena during the spray drying process were investigated to enable the synthesis of tailor-made nanostructures with defined properties. Moreover, a theor. model of this segregation and structure formation in nanosuspensions is presented using a discrete element method simulation.
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  15. 15
    Zhao, Y.; Shang, L.; Cheng, Y.; Gu, Z. Spherical Colloidal Photonic Crystals. Acc. Chem. Res. 2014, 47, 3632– 3642,  DOI: 10.1021/ar500317s
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    15
    Spherical Colloidal Photonic Crystals
    Zhao, Yuanjin; Shang, Luoran; Cheng, Yao; Gu, Zhongze
    Accounts of Chemical Research (2014), 47 (12), 3632-3642CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
    A review. The authors outline various types of spherical colloidal PhCs, such as close-packed, nonclose-packed, inverse opal, biphasic or multiphasic Janus structured, and core-shell structured geometries. Based on their unique optical properties, applications of the spherical colloidal PhCs for displays, sensors, barcodes, and cell culture microcarriers are presented. Future developments of the spherical colloidal PhC materials are also envisioned.
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  16. 16
    Goerlitzer, E. S. A.; Klupp Taylor, R. N.; Vogel, N. Bioinspired Photonic Pigments from Colloidal Self-Assembly. Adv. Mater. 2018, 30, 1706654,  DOI: 10.1002/adma.201706654
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    Xiao, M.; Hu, Z.; Wang, Z.; Li, Y.; Tormo, A. D.; Le Thomas, N.; Wang, B.; Gianneschi, N. C.; Shawkey, M. D.; Dhinojwala, A. Bioinspired Bright Noniridescent Photonic Melanin Supraballs. Sci. Adv. 2017, 3, e1701151,  DOI: 10.1126/sciadv.1701151
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    Bioinspired bright noniridescent photonic melanin supraballs
    Xiao, Ming; Hu, Ziying; Wang, Zhao; Li, Yiwen; Tormo, Alejandro Diaz; Le Thomas, Nicolas; Wang, Boxiang; Gianneschi, Nathan C.; Shawkey, Matthew D.; Dhinojwala, Ali
    Science Advances (2017), 3 (9), e1701151/1-e1701151/7CODEN: SACDAF; ISSN:2375-2548. (American Association for the Advancement of Science)
    Structural colors enable the creation of a spectrum of nonfading colors without pigments, potentially replacing toxic metal oxides and conjugated org. pigments. However, significant challenges remain to achieve the contrast needed for a complete gamut of colors and a scalable process for industrial application. We demonstrate a feasible soln. for producing structural colors inspired by bird feathers. We have designed core-shell nanoparticles using high-refractive index (RI) (∼1.74)melanin cores and low-RI (∼1.45) silica shells. The design of these nanoparticles was guided by finite-difference time-domain simulations. These nanoparticles were self-assembled using a one-pot reverse emulsion process, which resulted in bright and noniridescent supraballs. With the combination of only two ingredients, synthetic melanin and silica, we can generate a full spectrum of colors. These supraballs could be directly added to paints, plastics, and coatings and also used as UV-resistant inks or cosmetics.
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  18. 18
    Song, D. P.; Zhao, T. H.; Guidetti, G.; Vignolini, S.; Parker, R. M. Hierarchical Photonic Pigments via the Confined Self-Assembly of Bottlebrush Block Copolymers. ACS Nano 2019, 13, 1764– 1771,  DOI: 10.1021/acsnano.8b07845
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    18
    Hierarchical Photonic Pigments via the Confined Self-Assembly of Bottlebrush Block Copolymers
    Song, Dong-Po; Zhao, Tianheng H.; Guidetti, Giulia; Vignolini, Silvia; Parker, Richard M.
    ACS Nano (2019), 13 (2), 1764-1771CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Hierarchical, structurally colored materials offer a wide variety of visual effects that cannot be achieved with std. pigments or dyes. However, their fabrication requires simultaneous control over multiple length-scales. Here we introduce a robust strategy for the fabrication of hierarchical photonic pigments via the confined self-assembly of bottlebrush block copolymers within emulsified microdroplets. The bottlebrush block copolymer self-assembles into highly ordered concentric lamellae, giving rise to a near perfect photonic multilayer in the solid state, with reflectivity up to 100%. The reflected color can be readily tuned across the whole visible spectrum by either altering the mol. wt. or by blending the bottlebrush block copolymers. Furthermore, the developed photonic pigments are responsive, with a selective and reversible color change obsd. upon swelling in different solvents. Our system is particularly suited for the scalable prodn. of photonic pigments, arising from their rapid self-assembly mechanism and size-independent color.
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  19. 19
    Choi, T. M.; Lee, G. H.; Kim, Y. S.; Park, J. G.; Hwang, H.; Kim, S. H. Photonic Microcapsules Containing Single-Crystal Colloidal Arrays with Optical Anisotropy. Adv. Mater. 2019, 31, 1900693,  DOI: 10.1002/adma.201900693
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    Kim, S.-H.; Park, J.-G.; Choi, T. M.; Manoharan, V. N.; Weitz, D. A. Osmotic-Pressure-Controlled Concentration of Colloidal Particles in Thin-Shelled Capsules. Nat. Commun. 2014, 5, 3068,  DOI: 10.1038/ncomms4068
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    20
    Osmotic-pressure-controlled concentration of colloidal particles in thin-shelled capsules
    Kim Shin-Hyun; Park Jin-Gyu; Manoharan Vinothan N; Weitz David A; Choi Tae Min
    Nature communications (2014), 5 (), 3068 ISSN:.
    Colloidal crystals are promising structures for photonic applications requiring dynamic control over optical properties. However, for ease of processing and reconfigurability, the crystals should be encapsulated to form 'ink' capsules rather than confined in a thin film. Here we demonstrate a class of encapsulated colloidal photonic structures whose optical properties can be controlled through osmotic pressure. The ordering and separation of the particles within the microfluidically created capsules can be tuned by changing the colloidal concentration through osmotic pressure-induced control of the size of the individual capsules, modulating photonic stop band. The rubber capsules exhibit a reversible change in the diffracted colour, depending on osmotic pressure, a property we call osmochromaticity. The high encapsulation efficiency and capsule uniformity of this microfluidic approach, combined with the highly reconfigurable shapes and the broad control over photonic properties, make this class of structures particularly suitable for photonic applications such as electronic inks and reflective displays.
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  21. 21
    Vogel, N.; Utech, S.; England, G. T.; Shirman, T.; Phillips, K. R.; Koay, N.; Burgess, I. B.; Kolle, M.; Weitz, D. A.; Aizenberg, J. Color from Hierarchy: Diverse Optical Properties of Micron-Sized Spherical Colloidal Assemblies. Proc. Natl. Acad. Sci. U. S. A. 2015, 112, 10845– 10850,  DOI: 10.1073/pnas.1506272112
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    21
    Color from hierarchy: Diverse optical properties of micron-sized spherical colloidal assemblies
    Vogel, Nicolas; Utech, Stefanie; England, Grant T.; Shirman, Tanya; Phillips, Katherine R.; Koay, Natalie; Burgess, Ian B.; Kolle, Mathias; Weitz, David A.; Aizenberg, Joanna
    Proceedings of the National Academy of Sciences of the United States of America (2015), 112 (35), 10845-10850CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    Materials are characterized by structural order over multiple length scales have evolved for max. performance and multifunctionality, and are often produced by self-assembly processes. A striking example of this design principle is structural coloration, where interference, diffraction, and absorption effects result in vivid colors. Mimicking this emergence of complex effects from simple building blocks is a key challenge for man-made materials. Here, a simple confined self-assembly process leads to a complex hierarchical geometry that displays a variety of optical effects. Colloidal crystn. in an emulsion droplet creates micron-sized superstructures, termed photonic balls. The curvature imposed by the emulsion droplet leads to frustrated crystn. The authors observe spherical colloidal crystals with ordered, cryst. layers and a disordered core. This geometry produces multiple optical effects. The ordered layers give rise to structural color from Bragg diffraction with limited angular dependence and unusual transmission due to the curved nature of the individual crystals. The disordered core contributes nonresonant scattering that induces a macroscopically whitish appearance, which the authors mitigate by incorporating absorbing Au nanoparticles that suppress scattering and macroscopically purify the color. With increasing size of the constituent colloidal particles, grating diffraction effects dominate, which result from order along the crystal's curved surface and induce a vivid polychromatic appearance. The control of multiple optical effects induced by the hierarchical morphol. in photonic balls paves the way to use them as building blocks for complex optical assemblies-potentially as more efficient mimics of structural color as it occurs.
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  22. 22
    Park, J. G.; Kim, S. H.; Magkiriadou, S.; Choi, T. M.; Kim, Y. S.; Manoharan, V. N. Full-Spectrum Photonic Pigments with Non-Iridescent Structural Colors through Colloidal Assembly. Angew. Chem., Int. Ed. 2014, 53, 2899– 2903,  DOI: 10.1002/anie.201309306
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    22
    Full-Spectrum Photonic Pigments with Non-iridescent Structural Colors through Colloidal Assembly
    Park, Jin-Gyu; Kim, Shin-Hyun; Magkiriadou, Sofia; Choi, Tae Min; Kim, Young-Seok; Manoharan, Vinothan N.
    Angewandte Chemie, International Edition (2014), 53 (11), 2899-2903CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Structurally colored materials could potentially replace dyes and pigments in many applications, but it is challenging to fabricate structural colors that mimic the appearance of absorbing pigments. The authors demonstrate the microfluidic fabrication of photonic pigments consisting of microcapsules contg. dense amorphous packings of core-shell colloidal particles. These microcapsules show noniridescent structural colors that are independent of viewing angle, a crit. requirement for applications such as displays or coatings. The design of the microcapsules facilitates the suppression of incoherent and multiple scattering, enabling the fabrication of photonic pigments with colors spanning the visible spectrum. The authors' findings should provide new insights into the design and synthesis of materials with structural colors.
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  23. 23
    Klein, S. M.; Manoharan, V. N.; Pine, D. J.; Lange, F. F. Synthesis of Spherical Polymer and Titania Photonic Crystallites. Langmuir 2005, 21, 6669– 6674,  DOI: 10.1021/la0469957
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    Synthesis of Spherical Polymer and Titania Photonic Crystallites
    Klein, Sascha M.; Manoharan, Vinothan N.; Pine, David J.; Lange, Fred F.
    Langmuir (2005), 21 (15), 6669-6674CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    The fabrication of small structured spherical particles that are essentially small photonic crystals is described. The particles are 1-50 μm in diam. and are porous with nearly close-packed monodisperse pores whose size is comparable to the wavelength of light. The solid matrix of the particles is titania, which provides a large refractive index contrast between the particle matrix and pores. The particles are made by encapsulating polymer colloidal particles in emulsion droplets of hexanes in which a titanium alkoxide precursor is dissolved. Subsequent osmotic removal of the hexanes from the droplets and condensation of the alkoxide precursor leads to spherical aggregates of polymer spheres with titania filling the spaces between the polymer spheres. The polymer particles are then burned out leaving behind the desired porous titania particles. The size and structure of the pores and high refractive index of the titania matrix are expected to produce particles that are very efficient scatterers of light, making them useful as pigments.
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    Wang, J.; Sultan, U.; Goerlitzer, E. S. A.; Mbah, C. F.; Engel, M.; Vogel, N. Structural Color of Colloidal Clusters as a Tool to Investigate Structure and Dynamics. Adv. Funct. Mater. 2020, 30, 1907730,  DOI: 10.1002/adfm.201907730
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    24
    Structural Color of Colloidal Clusters as a Tool to Investigate Structure and Dynamics
    Wang, Junwei; Sultan, Umair; Goerlitzer, Eric S. A.; Mbah, Chrameh Fru; Engel, Michael; Vogel, Nicolas
    Advanced Functional Materials (2020), 30 (26), 1907730CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Colloidal assemblies have applications as photonic crystals and templates for functional porous materials. While there has been significant progress in controlling colloidal assemblies into defined structures, their 3D order remains difficult to characterize. Simple, low-cost techniques are sought that characterize colloidal structures and assist optimization of process parameters. Here, structural color is presented to image the structure and dynamics of colloidal clusters prepd. by a confined self-assembly process in emulsion droplets. It is shown that characteristic anisotropic structural color motifs such as circles, stripes, triangles, or bowties arise from the defined interior grain geometry of such colloidal clusters. The optical detection of these motifs reliably distinguishes icosahedral, decahedral, and face-centered cubic colloidal clusters and thus enables a simple yet precise characterization of their internal structure. In addn., the rotational motion and dynamics of such micrometer-scale clusters suspended in a liq. can be followed in real time via their anisotropic coloration. Finally, monitoring the evolution of structural color provides real-time information about the crystn. pathway within the confining emulsion droplet. Together, this work demonstrates that structural color is a simple and versatile tool to characterize the structure and dynamic properties of colloidal clusters.
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    Clough, J. M.; Guimard, E.; Rivet, C.; Sprakel, J.; Kodger, T. E. Photonic Paints: Structural Pigments Combined with Water-Based Polymeric Film-Formers for Structurally Colored Coatings. Adv. Opt. Mater. 2019, 7, 1900218,  DOI: 10.1002/adom.201900218
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    There is no corresponding record for this reference.
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    Zheng, F.; Cheng, Y.; Wang, J.; Lu, J.; Zhang, B.; Zhao, Y.; Gu, Z. Aptamer-Functionalized Barcode Particles for the Capture and Detection of Multiple Types of Circulating Tumor Cells. Adv. Mater. 2014, 26, 7333– 7338,  DOI: 10.1002/adma.201403530
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    26
    Aptamer-Functionalized Barcode Particles for the Capture and Detection of Multiple Types of Circulating Tumor Cells
    Zheng, Fuyin; Cheng, Yao; Wang, Jie; Lu, Jie; Zhang, Bin; Zhao, Yuanjin; Gu, Zhongze
    Advanced Materials (Weinheim, Germany) (2014), 26 (43), 7333-7338CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    We developed a novel barcode-particle technol. that can simultaneously capture, detect and release multiple types of CTCs from a complex sample.
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    Liu, W.; Kappl, M.; Butt, H. J. Tuning the Porosity of Supraparticles. ACS Nano 2019, 13, 13949– 13956,  DOI: 10.1021/acsnano.9b05673
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    27
    Tuning the Porosity of Supraparticles
    Liu, Wendong; Kappl, Michael; Butt, Hans-Juergen
    ACS Nano (2019), 13 (12), 13949-13956CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Supraparticles consisting of nano- or microparticles have potential applications as, for example, photonic crystals, drug carriers, or heterogeneous catalysts. To avoid the use of solvent or processing liq., one can make supraparticles by evapg. droplets of aq. suspensions from super-liq.-repellent surfaces. Herein, a method to adjust the porosity of supraparticles is described; a high porosity is desired, for example, in catalysis. To prep. highly porous TiO2 supraparticles, polymer nanoparticles are co-dispersed in the suspension. Supraparticles are formed through evapn. of aq. suspension droplets on superamphiphobic surfaces followed by calcination of the sacrificial polymer particles. The increase of porosity of up to 92% resulted in enhanced photocatalytic activity while maintaining sufficient mech. stability.
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    Hou, K.; Han, J.; Tang, Z. Formation of Supraparticles and Their Application in Catalysis. ACS Mater. Lett. 2020, 2, 95– 106,  DOI: 10.1021/acsmaterialslett.9b00446
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    Formation of Supraparticles and Their Application in Catalysis
    Hou, Ke; Han, Jianyu; Tang, Zhiyong
    ACS Materials Letters (2020), 2 (1), 95-106CODEN: AMLCEF; ISSN:2639-4979. (American Chemical Society)
    A review. Monodisperse supraparticles has been an emerging hot research topic in the fields of chem., materials science and nanotechnol. in the past several years. Great attention has been paid to construct monodisperse supraparticles with different sizes, morphologies, and compns. Their distinctive hierarchical structures and collective properties from various building blocks offer huge benefits towards catalyzing the reactions. In this review, the authors firstly outline the recent progress on the formation of supraparticles. Afterwards, their applications in photocatalysis, electrocatalysis and thermal catalysis are summarized. At last, the prospect in this research area is discussed.
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  29. 29
    Egly, S.; Fröhlich, C.; Vogel, S.; Gruenewald, A.; Wang, J.; Detsch, R.; Boccaccini, A. R.; Vogel, N. Bottom-Up Assembly of Silica and Bioactive Glass Supraparticles with Tunable Hierarchical Porosity. Langmuir 2018, 34, 2063– 2072,  DOI: 10.1021/acs.langmuir.7b03904
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    29
    Bottom-Up Assembly of Silica and Bioactive Glass Supraparticles with Tunable Hierarchical Porosity
    Egly, Steffen; Froehlich, Christina; Vogel, Stefanie; Gruenewald, Alina; Wang, Junwei; Detsch, Rainer; Boccaccini, Aldo R.; Vogel, Nicolas
    Langmuir (2018), 34 (5), 2063-2072CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    The authors study the formation of spherical supraparticles with controlled and tunable porosity on the nanometer and micrometer scales using the self-organization of a binary mixt. of small (nanometer scale) oxidic particles with large (micrometer scale) polystyrene particles in the confinement of an emulsion droplet. The external confinement dets. the final, spherical structure of the hybrid assembly, while the small particles form the matrix material. The large particles act as templating porogens to create micropores after combustion at elevated temps. The authors control the pore sizes on the micrometer scale by varying the size of the coassembled polystyrene microspheres and produce supraparticles from both SiO2- and Ca-contg. CaO/SiO2 particles. Although porous supraparticles are obtained in both cases, the presence of Ca ions substantially complicated the fabrication process since the increased ionic strength of the dispersion compromises the colloidal stability during the assembly process. The authors minimized these stability issues via the addn. of a steric stabilizing agent and by mixing bioactive and SiO2 colloidal particles. The authors studied the interaction of the porous particles with bone marrow stromal cells and found an increase in cell attachment with increasing pore size of the self-assembled supraparticles.
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  30. 30
    Canziani, H.; Chiera, S.; Schuffenhauer, T.; Kopp, S.; Metzger, F.; Bück, A.; Schmidt, M.; Vogel, N. Bottom-Up Design of Composite Supraparticles for Powder-Based Additive Manufacturing. Small 2020, 16, 2002076,  DOI: 10.1002/smll.202002076
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    30
    Bottom-Up Design of Composite Supraparticles for Powder-Based Additive Manufacturing
    Canziani, Herbert; Chiera, Salvatore; Schuffenhauer, Thomas; Kopp, Sebastian-Paul; Metzger, Florian; Bueck, Andreas; Schmidt, Michael; Vogel, Nicolas
    Small (2020), 16 (30), 2002076CODEN: SMALBC; ISSN:1613-6810. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Additive manufg. promises high flexibility and customized product design. Powder bed fusion processes use a laser to melt a polymer powder at predefined locations and iterate the scheme to build 3D objects. The design of flowable powders is a crit. parameter for a successful fabrication process that currently limits the choice of available materials. Here, a bottom-up process is introduced to fabricate tailored polymer- and composite supraparticles for powder-based additive manufg. processes by controlled aggregation of colloidal primary particles. These supraparticles exhibit a near-spherical shape and tailored compn., morphol., and surface roughness. These parameters can be precisely controlled by the mixing and size ratio of the primary particles. Polystyrene/silica composite particles are chosen as a model system to establish structure-property relations connecting shape, morphol., and surface roughness to the adhesion within the powder, which is accessed by tensile strength measurements. The adhesive properties are then connected to powder flowability and it is shown that the resulting powders allow the formation of dense powder films with uniform coverage. Finally, successful powder bed fusion is demonstrated by producing macroscopic single layer specimens with uniform distribution of nanoscale silica additives.
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  31. 31
    Nam, S. K.; Kim, J. B.; Han, S. H.; Kim, S.-H. Photonic Janus Balls with Controlled Magnetic Moment and Density Asymmetry. ACS Nano 2020, 14, 15714– 15722,  DOI: 10.1021/acsnano.0c06672
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    31
    Photonic Janus Balls with Controlled Magnetic Moment and Density Asymmetry
    Nam, Seong Kyeong; Kim, Jong Bin; Han, Sang Hoon; Kim, Shin-Hyun
    ACS Nano (2020), 14 (11), 15714-15722CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Colloidal crystals show structural colors through wavelength-selective diffraction at photonic stopbands. Here, we design photonic Janus balls with a controlled magnetic moment for programmable structural color switching. The Janus balls are produced from microfluidically produced paired drops of two distinct photocurable resins. The lighter resin contains magnetic nanoparticles and carbon black, whereas heavier one contains silica particles at a high vol. fraction. The paired drops spontaneously align vertically due to the d. asymmetry. The magnetic moment is assigned in the vertically aligned drops by aligning magnetic nanoparticles with an external field and capturing them through photopolymn. Silica particles in the heavier compartment spontaneously form cryst. arrays due to interparticle repulsion, developing structural colors. The resulting photonic Janus balls vertically align without an external field, like a roly-poly toy, so that carbon-black-laden compartments face upward. With an external magnetic field, the Janus balls align their magnetic moment to the field and display structural colors. Importantly, the direction of the magnetic moment is set by the direction of the external field during photopolymn., which enables the simultaneous manipulation of orientations of distinct photonic Janus balls in a programmed manner. These photonic Janus balls are potentially useful as active color inks for anti-counterfeiting tags.
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  32. 32
    Schneider, M.; Tschöpe, A.; Hanselmann, D.; Ballweg, T.; Gellermann, C.; Franzreb, M.; Mandel, K. Adsorber Particles with Magnetically-Supported Improved Electrochemical Conversion Behavior for Waste Water Treatment Processes. Part. Part. Syst. Charact. 2020, 37, 1900487,  DOI: 10.1002/ppsc.201900487
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    32
    Adsorber Particles with Magnetically-Supported Improved Electrochemical Conversion Behavior for Waste Water Treatment Processes
    Schneider, Michael; Tschoepe, Andre; Hanselmann, Doris; Ballweg, Thomas; Gellermann, Carsten; Franzreb, Matthias; Mandel, Karl
    Particle & Particle Systems Characterization (2020), 37 (2), 1900487CODEN: PPCHEZ; ISSN:1521-4117. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Micron-sized supraparticles, consisting of a plurality of discrete nano- and microscale functional units, are assembled and fused by means of a droplet extrusion process. By combining nano magnetite, activated carbon, and conductive carbon with a polymeric binder matrix, particles are obtained which unite good magnetic properties, elec. cond., and adsorber activity through the high accessible surface area of the incorporated activated carbon of about 570 m2 g-1, thereby enabling a new approach toward sustainable water treatment processes. Due to the interplay of the components, it is possible to adsorb target substances, dissolved in the water which is demonstrated by the adsorption of the model dye methylene blue. A very fast adsorption kinetic and an adsorption capacity of about 400 mg g-1 is detd. By using the developed composite particles, it is also possible to electrochem. alter substances flowing through a magnetically-stabilized fluidized-bed reactor by electrochem. charging/discharging, significantly supported by the magnetic field enabling alternatingly optimum mobility/adsorption phases with contact/charging intervals. The electrochem. conversion can be increased up to 151% depending on the applied flow-rate and elec. voltage. By applying an external magnetic field, a further increase of electrochem. conversion of up to 70% can be obsd.
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  33. 33
    Miller, F.; Wintzheimer, S.; Prieschl, J.; Strauss, V.; Mandel, K. A Supraparticle-Based Five-Level-Identification Tag That Switches Information Upon Readout. Adv. Opt. Mater. 2021, 9, 2001972,  DOI: 10.1002/adom.202001972
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    33
    A Supraparticle-Based Five-Level-Identification Tag That Switches Information Upon Readout
    Miller, Franziska; Wintzheimer, Susanne; Prieschl, Johannes; Strauss, Volker; Mandel, Karl
    Advanced Optical Materials (2021), 9 (4), 2001972CODEN: AOMDAX; ISSN:2195-1071. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Product identification tags are of great importance in a globalized world with increasingly complex trading routes and networks. Beyond currently used coding strategies, such as QR codes, higher data d., flexible application as well as miniaturization and readout indication are longed for in the next generation of security tags. In this work, micron-sized supraparticles (SPs) with encoded information (ID) are produced that not only exhibit multiple initially covert identification levels but are also irreversibly marked as "read" upon readout. To achieve this, lanthanide doped CaF2 nanoparticles are assembled in various quantity-weighted ratios via spray-drying in presence of a broad-spectrum stealth fluorophore (StFl), yielding covert spectrally encoded ID-SPs. Using these as pigments, QR codes, initially dominated by the green fluorescence of the StFl, could be generated. Upon thermal energy input, these particle-based tags irreversibly switch to an activated state revealing not only multiple luminescent colors but also spectral IDs. This strategy provides the next generation of material-based security tags with a high data d. and security level that switch information upon readout and can be, therefore, used as seal of quality.
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  34. 34
    Müssig, S.; Granath, T.; Schembri, T.; Fidler, F.; Haddad, D.; Hiller, K.-H.; Wintzheimer, S.; Mandel, K. Anisotropic Magnetic Supraparticles with a Magnetic Particle Spectroscopy Fingerprint as Indicators for Cold-Chain Breach. ACS Appl. Nano Mater. 2019, 2, 4698– 4702,  DOI: 10.1021/acsanm.9b00977
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    34
    Anisotropic Magnetic Supraparticles with a Magnetic Particle Spectroscopy Fingerprint as Indicators for Cold-Chain Breach
    Muessig, Stephan; Granath, Tim; Schembri, Tim; Fidler, Florian; Haddad, Daniel; Hiller, Karl-Heinz; Wintzheimer, Susanne; Mandel, Karl
    ACS Applied Nano Materials (2019), 2 (8), 4698-4702CODEN: AANMF6; ISSN:2574-0970. (American Chemical Society)
    Magnetic particle spectroscopy (MPS) is used in this work to obtain a magnetic fingerprint signal from anisotropic supraparticles, i.e., microrods assembled from superparamagnetic iron oxide nanoparticles. Exceeding its intended purpose of nanoparticle characterization for biomedical magnetic particle imaging, it is shown that MPS is capable of resolving structural differences between the anisotropic alignment of individual nanoparticles and its isotropic counterpart. Addnl., orientation-dependent MPS signal variations of anisotropic supraparticles are identifiable. This finding enables the detection of cold-chain breaches (for instance, during delivery of a product that needs to be cooled all of the time) by recording the initial and final MPS signals of microrod samples integrated into the container of a frozen product.
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  35. 35
    Montanarella, F.; Urbonas, D.; Chadwick, L.; Moerman, P. G.; Baesjou, P. J.; Mahrt, R. F.; Van Blaaderen, A.; Stöferle, T.; Vanmaekelbergh, D. Lasing Supraparticles Self-Assembled from Nanocrystals. ACS Nano 2018, 12, 12788– 12794,  DOI: 10.1021/acsnano.8b07896
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    Lasing Supraparticles Self-Assembled from Nanocrystals
    Montanarella, Federico; Urbonas, Darius; Chadwick, Luke; Moerman, Pepijn G.; Baesjou, Patrick J.; Mahrt, Rainer F.; van Blaaderen, Alfons; Stoeferle, Thilo; Vanmaekelbergh, Daniel
    ACS Nano (2018), 12 (12), 12788-12794CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    One of the most attractive com. applications of semiconductor nanocrystals (NCs) is their use in lasers. Thanks to their high quantum yield, tunable optical properties, photostability, and wet-chem. processability, NCs have arisen as promising gain materials. Most of these applications, however, rely on incorporation of NCs in lasing cavities sep. produced using sophisticated fabrication methods and often difficult to manipulate. Here, we present whispering gallery mode lasing in supraparticles (SPs) of self-assembled NCs. The SPs composed of NCs act as both lasing medium and cavity. Moreover, the synthesis of the SPs, based on an in-flow microfluidic device, allows precise control of the dimensions of the SPs, i.e. the size of the cavity, in the micrometer range with polydispersity as low as several percent. The SPs presented here show whispering gallery mode resonances with quality factors up to 320. Whispering gallery mode lasing is evidenced by a clear threshold behavior, coherent emission, and emission lifetime shortening due to the stimulation process.
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  36. 36
    Mazzanti, A.; Yang, Z.; Silva, M. G.; Yang, N.; Rizza, G.; Coulon, P.-E.; Manzoni, C.; de Paula, A. M.; Cerullo, G.; Della Valle, G. Light–Heat Conversion Dynamics in Highly Diversified Water-Dispersed Hydrophobic Nanocrystal Assemblies. Proc. Natl. Acad. Sci. U. S. A. 2019, 116, 8161– 8166,  DOI: 10.1073/pnas.1817850116
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    Light-heat conversion dynamics in highly diversified water-dispersed hydrophobic nanocrystal assemblies
    Mazzanti, Andrea; Yang, Zhijie; Silva, Mychel G.; Yang, Nailiang; Rizza, Giancarlo; Coulon, Pierre-EugA ne; Manzoni, Cristian; de Paula, Ana Maria; Cerullo, Giulio; Della Valle, Giuseppe; Pileni, Marie-Paule
    Proceedings of the National Academy of Sciences of the United States of America (2019), 116 (17), 8161-8166CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    The authors study, with a combination of ultrafast optical spectroscopy and semiclassical modeling, the photothermal properties of various water-sol. nanocrystal assemblies. Broadband pump-probe expts. with ~100 fs time resoln. in the visible and near IR reveal a complex scenario for their transient optical response that is dictated by their hybrid compn. at the nanoscale, comprising metallic (Au) or semiconducting (Fe3O4) nanostructures and a matrix of org. ligands. The authors track the whole chain of energy flow that starts from light absorption by the individual nanocrystals and subsequent excitation of out-of-equil. carriers followed by the electron-phonon equilibration, occurring in a few picoseconds, and then by the heat release to the matrix on the 100-ps timescale. Two-dimensional finite-element method electromagnetic simulations of the composite nanostructure and multi-temp. modeling of the energy flow dynamics enable the authors to identify the key mechanism presiding over the light-heat conversion in these kinds of nanomaterials. Hybrid (org.-inorg.) nanocrystal assemblies can operate as efficient nanoheaters by exploiting the high absorption from the individual nanocrystals, enabled by the diln. of the inorg. phase that is followed by a relatively fast heating of the embedding org. matrix, occurring on the 100-ps timescale.
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  37. 37
    Nicolas-Boluda, A.; Yang, Z.; Guilbert, T.; Fouassier, L.; Carn, F.; Gazeau, F.; Pileni, M. P. Self-Assemblies of Fe 3 O 4 Nanocrystals: Toward Nanoscale Precision of Photothermal Effects in the Tumor Microenvironment. Adv. Funct. Mater. 2021, 31, 2006824,  DOI: 10.1002/adfm.202006824
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    Self-Assemblies of Fe3O4 Nanocrystals: Toward Nanoscale Precision of Photothermal Effects in the Tumor Microenvironment
    Nicolas-Boluda, Alba; Yang, Zhijie; Guilbert, Thomas; Fouassier, Laura; Carn, Florent; Gazeau, Florence; Pileni, Marie Paule
    Advanced Functional Materials (2021), 31 (4), 2006824CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Fe3O4 nanocrystals are self-assembled into two different conformations: colloidosome and supraball that confer them with distinct properties detg. their photo-induced heating capacities. These self-assemblies are assessed for photothermal therapy, an adjuvant strategy for tumor therapy. The tumor microenvironment is a heterogeneous ecosystem including immune cells and the extracellular matrix. The interactions between photothermal therapy agents and the different components of the tumor microenvironment det. the outcome of this therapy. In this study, the fate of both colloidosomes and supraballs within the tumor microenvironment in comparison to their Fe3O4 nanocrystal building blocks is revealed. This study highlights how these two hybrid self-assemblies target different compartments of the tumor microenvironment and trigger local photothermal damages that are inaccessible for isolated nanocrystals and not predicted by global temp. measurements.
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  38. 38
    Donaldson, J. G.; Schall, P.; Rossi, L. Magnetic Coupling in Colloidal Clusters for Hierarchical Self-Assembly. ACS Nano 2021, 15, 4989– 4999,  DOI: 10.1021/acsnano.0c09952
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    Magnetic Coupling in Colloidal Clusters for Hierarchical Self-Assembly
    Donaldson, Joe G.; Schall, Peter; Rossi, Laura
    ACS Nano (2021), 15 (3), 4989-4999CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Manipulating the way in which colloidal particles self-organize is a central challenge in the design of functional soft materials. Meeting this challenge requires the use of building blocks that interact with one another in a highly specific manner. Their fabrication, however, is limited by the complexity of the available synthesis procedures. Here, we demonstrate that, starting from exptl. available magnetic colloids, we can create a variety of complex building blocks suitable for hierarchical self-organization through a simple scalable process. Using computer simulations, we compress spherical and cubic magnetic colloids in spherical confinement, and investigate their suitability to form small clusters with reproducible structural and magnetic properties. We find that, while the structure of these clusters is highly reproducible, their magnetic character depends on the particle shape. Only spherical particles have the rotational degrees of freedom to produce consistent magnetic configurations, whereas cubic particles frustrate the minimization of the cluster energy, resulting in various magnetic configurations. To highlight their potential for self-assembly, we demonstrate that already clusters of three magnetic particles form highly nontrivial Archimedean lattices, namely, staggered kagome, bounce, and honeycomb, when focusing on different aspects of the same monolayer structure. The work presented here offers a conceptually different way to design materials by utilizing preassembled magnetic building blocks that can readily self-organize into complex structures.
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    Yang, Y.; Wang, B.; Shen, X.; Yao, L.; Wang, L.; Chen, X.; Xie, S.; Li, T.; Hu, J.; Yang, D. Scalable Assembly of Crystalline Binary Nanocrystal Superparticles and Their Enhanced Magnetic and Electrochemical Properties. J. Am. Chem. Soc. 2018, 140, 15038– 15047,  DOI: 10.1021/jacs.8b09779
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    Scalable Assembly of Crystalline Binary Nanocrystal Superparticles and Their Enhanced Magnetic and Electrochemical Properties
    Yang, Yuchi; Wang, Biwei; Shen, Xiudi; Yao, Luyin; Wang, Lei; Chen, Xiao; Xie, Songhai; Li, Tongtao; Hu, Jianhua; Yang, Dong; Dong, Angang
    Journal of the American Chemical Society (2018), 140 (44), 15038-15047CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Self-assembled binary nanocrystal superlattices (BNSLs) represent an important class of solid-state materials with potentially designed properties. In pursuit of widening the range of applications for binary superlattice materials, it is desirable to develop scalable assembly methods that enable high-quality BNSLs with tailored compns., structures, and morphologies. We report the gram-scale assembly of cryst. binary nanocrystal superparticles with high phase purity through an emulsion-based process. The structure of the resulting BNSL colloids can be tuned in a wide range (AB13, AlB2, MgZn2, NaCl, and CaCu5) by varying the size and(or) no. ratios of the 2 nanocrystal components. Access to large-scale, phase-pure BNSL colloids offers vast opportunities for investigating their physiochem. properties, as exemplified by AB13-type CoFe2O4-Fe3O4 binary superparticles. CoFe2O4-Fe3O4 binary superparticles not only display enhanced magnetic coupling but also exhibit superior lithium-storage properties. The nonclosed-packed NC packing arrangements of AB13-type binary superparticles are found to play a key role in facilitating lithiation/delithiation kinetics and maintaining structural integrity during repeated cycling. Our work establishes the scalable assembly of high-quality BNSL colloids, which is beneficial for accelerating the exploration of multicomponent nanocrystal superlattices toward various applications.
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    Wang, D.; Dasgupta, T.; van der Wee, E. B.; Zanaga, D.; Altantzis, T.; Wu, Y.; Coli, G. M.; Murray, C. B.; Bals, S.; Dijkstra, M. Binary Icosahedral Clusters of Hard Spheres in Spherical Confinement. Nat. Phys. 2021, 17, 128– 134,  DOI: 10.1038/s41567-020-1003-9
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    Binary icosahedral clusters of hard spheres in spherical confinement
    Wang, Da; Dasgupta, Tonnishtha; van der Wee, Ernest B.; Zanaga, Daniele; Altantzis, Thomas; Wu, Yaoting; Coli, Gabriele M.; Murray, Christopher B.; Bals, Sara; Dijkstra, Marjolein; van Blaaderen, Alfons
    Nature Physics (2021), 17 (1), 128-134CODEN: NPAHAX; ISSN:1745-2473. (Nature Research)
    Abstr.: The influence of geometry on the local and global packing of particles is important to many fundamental and applied research themes, such as the structure and stability of liqs., crystals and glasses. Here we show by expts. and simulations that a binary mixt. of hard-sphere-like nanoparticles crystg. into a MgZn2 Laves phase in bulk spontaneously forms icosahedral clusters in slowly drying droplets. Using advanced electron tomog., we are able to obtain the real-space coordinates of all the spheres in the icosahedral clusters of up to about 10,000 particles. The local structure of 70-80% of the particles became similar to that of the MgCu2 Laves phase. These observations are important for photonic applications. In addn., we obsd. in simulations that the icosahedral clusters nucleated away from the spherical boundary, which is distinctly different from that of the single species clusters. Our findings open the way for particle-level studies of nucleation and growth of icosahedral clusters, and of binary crystn.
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    Wang, J.; Mbah, C. F.; Przybilla, T.; Englisch, S.; Spiecker, E.; Engel, M.; Vogel, N. Free Energy Landscape of Colloidal Clusters in Spherical Confinement. ACS Nano 2019, 13, 9005– 9015,  DOI: 10.1021/acsnano.9b03039
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    Free Energy Landscape of Colloidal Clusters in Spherical Confinement
    Wang, Junwei; Mbah, Chrameh Fru; Przybilla, Thomas; Englisch, Silvan; Spiecker, Erdmann; Engel, Michael; Vogel, Nicolas
    ACS Nano (2019), 13 (8), 9005-9015CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    The structure of finite self-assembling systems depends sensitively on the no. of constituent building blocks. Recently, it was demonstrated that hard sphere-like colloidal particles show a magic no. effect when confined in emulsion droplets. Geometric construction rules permit a few dozen magic nos. that correspond to a discrete series of completely filled concentric icosahedral shells. Here, we investigate the free energy landscape of these colloidal clusters as a function of the no. of their constituent building blocks for system sizes up to several thousand particles. We find that min. in the free energy landscape, arising from the presence of filled, concentric shells, are significantly broadened, compared to their at. analogs. Colloidal clusters in spherical confinement can flexibly accommodate excess particles by ordering icosahedrally in the cluster center while changing the structure near the cluster surface. In between these magic no. regions, the building blocks cannot arrange into filled shells. Instead, we observe that defects accumulate in a single wedge and therefore only affect a few tetrahedral grains of the cluster. We predict the existence of this wedge by simulation and confirm its presence in expt. using electron tomog. The introduction of the wedge minimizes the free energy penalty by confining defects to small regions within the cluster. In addn., the remaining ordered tetrahedral grains can relax internal strain by breaking icosahedral symmetry. Our findings demonstrate how multiple defect mechanisms collude to form the complex free energy landscape of colloidal clusters.
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    Wang, X. D.; Chen, B.; Wang, H. F.; Wang, Z. S. Adhesion between Submicrometer Polystyrene Spheres. Powder Technol. 2011, 214, 447– 450,  DOI: 10.1016/j.powtec.2011.08.047
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    Adhesion between submicrometer polystyrene spheres
    Wang, Xiao-Dong; Chen, Bo; Wang, Hai-Feng; Wang, Zhan-Shan
    Powder Technology (2011), 214 (3), 447-450CODEN: POTEBX; ISSN:0032-5910. (Elsevier B.V.)
    The sphere-substrate contact method was usually used to study adhesion theory because it is rather difficult to make two micrometer or submicrometer spheres contact precisely. Here, we used sphere-sphere contact method by a novel, simple process to investigate deformations of spheres. The polystyrene particles size ranges from 60 nm to 600 nm. We found that the polystyrene particles underwent plastic deformations due to van der Waals interaction. The contact radii were obsd. by the scanning electron microscope (SEM).
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    Paul, J.; Romeis, S.; Mačković, M.; Marthala, V. R. R.; Herre, P.; Przybilla, T.; Hartmann, M.; Spiecker, E.; Schmidt, J.; Peukert, W. In Situ Cracking of Silica Beads in the SEM and TEM - Effect of Particle Size on Structure-Property Correlations. Powder Technol. 2015, 270, 337– 347,  DOI: 10.1016/j.powtec.2014.10.026
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    In situ cracking of silica beads in the SEM and TEM - Effect of particle size on structure-property correlations
    Paul, J.; Romeis, S.; Mackovic, M.; Marthala, V. R. R.; Herre, P.; Przybilla, T.; Hartmann, M.; Spiecker, E.; Schmidt, J.; Peukert, W.
    Powder Technology (2015), 270 (Part_A), 337-347CODEN: POTEBX; ISSN:0032-5910. (Elsevier B.V.)
    St.ovrddot.ober Fink Bohn (SFB) silica is a well-known colloidal system: monodisperse spherical particles are readily obtained by this wet chem. synthesis route. Despite the frequent application of SFB silica, its mech. properties are not comprehensively understood. Within this account, size dependent mech. properties of single SFB particles are systematically investigated using in situ SEM and TEM techniques: particle properties are addressed in the size range of 1.5 μm down to 200 nm. Both, untreated particles and heat treated particles densified at 1000°C are investigated. Structural characterization by vibrational spectroscopy, by solid state NMR spectroscopy, by SEM and by focused ion beam machining allows correlating mech. properties and the internal particle structure. For untreated particles comprised of a weakly cross-linked silica network a high degree of plasticity assocd. with low Young's moduli and hardness values is found. At large strains only cracking without full fracture of the particles occurs, whereby the crack path is clearly linked to the synthesis conditions: for larger particles obtained from a multistep growth process cracks propagate along weak intraparticle interfaces and are deflected around an inner core structure. After the thermal treatment crosslinking of the silica network is significantly enhanced and Young's moduli comparable to vitreous silica accompanied by increased hardness values are found. Only the smallest particles exhibit cracking; fracture into two or more pieces is distinguished for particles above 500 nm. It is found by SEM imaging that the relative occurrence of cracking into several pieces is increasing with size - a behavior assocd. with the brittle to ductile transition of silica. By testing a large quantity of particles, statistical significance is ensured.
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    Paul, J.; Romeis, S.; Herre, P.; Peukert, W. Deformation Behavior of Micron-Sized Polycrystalline Gold Particles Studied by in Situ Compression Experiments and Frictional Finite Element Simulation. Powder Technol. 2015, 286, 706– 715,  DOI: 10.1016/j.powtec.2015.09.020
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    Deformation behavior of micron-sized polycrystalline gold particles studied by in situ compression experiments and frictional finite element simulation
    Paul, Jonas; Romeis, Stefan; Herre, Patrick; Peukert, Wolfgang
    Powder Technology (2015), 286 (), 706-715CODEN: POTEBX; ISSN:0032-5910. (Elsevier B.V.)
    We present a combined exptl. and finite element study on the deformation behavior of micron-sized polycryst. gold particles. This study enables detailed insights into the underlying deformation mechanisms of the particles. Scanning electron microscope supported in situ uniaxial compression expts. of the single spherical polycryst. gold particles were performed in the size range of 1 μm by using a custom built manipulation device. By testing a large no. of particles stress-strain data and information on the particle morphol. were obtained with statistical significance. The exptl. obsd. stress-strain behavior and the geometric shape of the stressed particles were found to be in excellent agreement with the elastic-perfectly plastic finite element model accounting for frictional effects at the contact interfaces. A significantly increased yield strength compared to bulk gold was found - grain size strengthening according to the Hall-Petch relation was identified as the main hardening mechanism. Hardness was found to vary with strain - an effect related to the altering geometric shape of the particles during compression. Comparison to a frictionless finite element model revealed the necessity of considering the effect of friction. These findings are not restricted to gold particles, but should be applicable to a wide range of elastic-perfectly plastic materials.
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    Herre, P.; Romeis, S.; Mačković, M.; Przybilla, T.; Paul, J.; Schwenger, J.; Torun, B.; Grundmeier, G.; Spiecker, E.; Peukert, W. Deformation Behavior of Nanocrystalline Titania Particles Accessed by Complementary in Situ Electron Microscopy Techniques. J. Am. Ceram. Soc. 2017, 100, 5709– 5722,  DOI: 10.1111/jace.15072
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    Deformation behavior of nanocrystalline titania particles accessed by complementary in situ electron microscopy techniques
    Herre, Patrick; Romeis, Stefan; Mackovic, Mirza; Przybilla, Thomas; Paul, Jonas; Schwenger, Jan; Torun, Boray; Grundmeier, Guido; Spiecker, Erdmann; Peukert, Wolfgang
    Journal of the American Ceramic Society (2017), 100 (12), 5709-5722CODEN: JACTAW; ISSN:0002-7820. (Wiley-Blackwell)
    The mech. behavior of nanostructured spherical submicrometer titania particles was studied by in situ uniaxial compression expts. in the scanning and transmission electron microscope (SEM and TEM). Mesoporous and amorphous titania particles were prepd. by a wet chem. sol-gel approach. To obtain nanocryst. (nc) single-phase anatase and rutile particles the amorphous particles were crystd. by high-temp. annealing. For each sample the deformation behavior of ≥50 particles was investigated by in situ compression expts. in the SEM. In all cases an elastic - predominantly plastic deformation behavior accompanied by crack initiation at exceptionally high engineering strain values of several percent were obsd. Crack propagation presumably along grain boundaries and a Weibull distributed fracture stress was shown for all nc particles. Complementary in situ TEM expts. and ex situ anal. of focused ion beam prepd. particle cross-sections were carried out to identify the underlying deformation mechanisms. Grain rotations and grain sliding are obsd. for nc anatase particles during in situ compression and are further identified to be linked to a densification of the mesoporous particle structure. Our dedicated prepn. and quant. in situ characterization methodol. provides an excellent basis for a better understanding of the mech. behavior of advanced ceramics.
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    Paul, J.; Romeis, S.; Tomas, J.; Peukert, W. A Review of Models for Single Particle Compression and Their Application to Silica Microspheres. Adv. Powder Technol. 2014, 25, 136– 153,  DOI: 10.1016/j.apt.2013.09.009
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    A review of models for single particle compression and their application to silica microspheres
    Paul, Jonas; Romeis, Stefan; Tomas, Jurgen; Peukert, Wolfgang
    Advanced Powder Technology (2014), 25 (1), 136-153CODEN: APTEEE; ISSN:0921-8831. (Elsevier B.V.)
    A review. We report on the deformation behavior of single silica microspheres. For the first time a detailed discussion on the quant. evaluation of force-deformation data in the elastic and plastic regime is given. The microspheres are compressed between two flat plates of a custom built manipulation device supported by a scanning electron microscope. The device allows a high sample throughput enabling full statistical evaluation of force-deformation data of single microspheres. Existing theories describing the deformation behavior of single spheres are discussed and applied to the elastic and plastic deformation regime of silica microspheres. The results obtained from the theories are compared between each other and values reported in literature. The silica microspheres exhibit a significantly different deformation behavior than expected from bulk fused silica, i.e. a distinct plastic deformation behavior. Furthermore, a significant decrease in Young's modulus and hardness was obsd. caused by high porosity and reduced crosslinking of silicon atoms, resp., that is inherent to silica produced by Stoeber synthesis. However, the calcd. contact pressures exceed the compressive strength of bulk fused silica by at least a factor of two as an effect of a reduced amt. of material flaws in microstructures.
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    Cheong, Y. S.; Adams, M. J.; Routh, A. F.; Hounslow, M. J.; Salman, A. D. The Production of Binderless Granules and Their Mechanical Characteristics. Chem. Eng. Sci. 2005, 60, 4045– 4053,  DOI: 10.1016/j.ces.2005.02.033
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    The production of binderless granules and their mechanical characteristics
    Cheong, Y. S.; Adams, M. J.; Routh, A. F.; Hounslow, M. J.; Salman, A. D.
    Chemical Engineering Science (2005), 60 (14), 4045-4053CODEN: CESCAC; ISSN:0009-2509. (Elsevier Ltd.)
    A granulation procedure is described for prepg. model binderless granules from spherical polystyrene colloids. The deformation and breakage behavior of the granules was also studied. Impact and slow diametrical compression expts. were used to simulate the mech. response of the granules at high and low strain rates, resp. They were found to deform elasto-plastically before fracturing in a semi-brittle manner. Densification or rearrangement of particle packing in the deformed region was concluded to be the main mechanism for energy dissipation under both impact and diametrical compression. In addn., the surface chem. of the constituent particles within the granules was one of the factors that govern the strength.
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    Reynolds, G. K.; Fu, J. S.; Cheong, Y. S.; Hounslow, M. J.; Salman, A. D. Breakage in Granulation: A Review. Chem. Eng. Sci. 2005, 60, 3969– 3992,  DOI: 10.1016/j.ces.2005.02.029
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    Breakage in granulation: A review
    Reynolds, G. K.; Fu, J. S.; Cheong, Y. S.; Hounslow, M. J.; Salman, A. D.
    Chemical Engineering Science (2005), 60 (14), 3969-3992CODEN: CESCAC; ISSN:0009-2509. (Elsevier Ltd.)
    A review of breakage from the process scale down to the single granule and sub-granule scale, discussing largely exptl. results complemented with some modeling results. The study of breakage in granulation is important from a process and from a product quality perspective. Breakage is considered an important rate process in granulation, and plays roles in granule homogeneity and strength. Understanding this rate process has important implications in the design and control of the granulation process. From a product perspective, the study of breakage has important implications for the subsequent processing, transport, handling and final use of granular products. Breakage behavior of granules can be a strong signature of the consistency of properties between nominally identical granular products.
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    Antonyuk, S.; Tomas, J.; Heinrich, S.; Mörl, L. Breakage Behaviour of Spherical Granulates by Compression. Chem. Eng. Sci. 2005, 60, 4031– 4044,  DOI: 10.1016/j.ces.2005.02.038
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    Breakage behaviour of spherical granulates by compression
    Antonyuk, Sergiy; Tomas, Juergen; Heinrich, Stefan; Moerl, Lothar
    Chemical Engineering Science (2005), 60 (14), 4031-4044CODEN: CESCAC; ISSN:0009-2509. (Elsevier Ltd.)
    This paper describes the deformation and breakage behavior of granulates in single particle compression test. Three industrial spherical granulates-γ-Al2O3, the synthetic zeolite Koestrolith, and sodium benzoate (C6H5COONa) were used as model materials to study the mech. behavior from elastic to plastic range. The elastic compression behavior of granulates is described by means of force-displacement curves, by application of Hertz-Huber contact theory and continuum mechanics. An elastic-plastic contact model was proposed to describe the deformation behavior of elastic-plastic granules. The effects of granulate size and stressing velocity on the breakage force and contact stiffness during elastic and elastic-plastic displacement are examd. It is shown that the zeolite granulates with elastic-plastic behavior have viscous properties as well. Breakage mechanisms of granulates during elastic, elastic-plastic, and plastic deformation are also explained. The breakage probability is approximated by Weibull distribution function. The behavior of the granulate during compression under the repeated loading-unloading conditions was investigated.
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    Schilde, C.; Westphal, B.; Kwade, A. Effect of the Primary Particle Morphology on the Micromechanical Properties of Nanostructured Alumina Agglomerates. J. Nanopart. Res. 2012, 14, 745,  DOI: 10.1007/s11051-012-0745-4
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    Effect of the primary particle morphology on the micromechanical properties of nanostructured alumina agglomerates
    Schilde, Carsten; Westphal, Bastian; Kwade, Arno
    Journal of Nanoparticle Research (2012), 14 (3), 745/1-745/11CODEN: JNARFA; ISSN:1388-0764. (Springer)
    Depending on the application of nanoparticles, certain characteristics of the product quality such as size, morphol., abrasion resistance, sp. surface, dispersibility and tendency to agglomeration are important. These characteristics are a function of the physicochem. properties, i.e. the micromech. properties of the nanostructured material. The micromech. properties of these nanostructured agglomerates such as the max. indentation force, the plastic and elastic deformation energy and the strength give information on the product properties, e.g. the efficiency of a dispersion process of the agglomerates, and can be measured by nanoindentation. In this study a Berkovich indenter tip was used for the characterization of model aggregates out of sol-gel produced silica and pptd. alumina agglomerates with different primary particle morphologies (dimension of 15-40 nm). In general, the effect of the primary particle morphol. and the presence or absence of solid bonds can be characterized by the measurement of the micromech. properties via nanoindentation. The micromech. behavior of aggregates contg. solid bonds is strongly affected by the elastic-plastic deformation behavior of the solid bonds and the breakage of solid bonds. Moreover, varying the primary particle morphol. for similar particle material and approx. isotropic agglomerate behavior the particle-particle interactions within the agglomerates can be described by the elemental breaking stress according to the formula of Rumpf.
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    Sekido, T.; Wooh, S.; Fuchs, R.; Kappl, M.; Nakamura, Y.; Butt, H. J.; Fujii, S. Controlling the Structure of Supraballs by PH-Responsive Particle Assembly. Langmuir 2017, 33, 1995– 2002,  DOI: 10.1021/acs.langmuir.6b04648
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    Controlling the Structure of Supraballs by pH-Responsive Particle Assembly
    Sekido, Takafumi; Wooh, Sanghyuk; Fuchs, Regina; Kappl, Michael; Nakamura, Yoshinobu; Butt, Hans-Jurgen; Fujii, Syuji
    Langmuir (2017), 33 (8), 1995-2002CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    Supraballs of various sizes and compns. can be fabricated via drying of drops of aq. colloidal dispersions on super-liq.-repellent surfaces with no chem. waste and energy consumption. A "supraball" is a particle composed of colloids. Many properties, such as mech. strength and porosity, are detd. by the ordering of a colloidal assembly. To tune such properties, a colloidal assembly needs to be controlled when supraballs are formed during drying. Here, we introduce a method to control a colloidal assembly of supraballs by adjusting the dispersity of the colloids. Supraballs are fabricated on superamphiphobic surfaces from colloidal aq. dispersions of polystyrene microparticles carrying pH-responsive poly[2-(diethylamino)ethyl methacrylate]. Drying of dispersion drops at pH 3 on superamphiphobic surfaces leads to the formation of spherical supraballs with densely packed colloids. The pH 10 supraballs are more oblate and consist of more disordered colloids than the pH 3 supraballs, caused by particle aggregates with random sizes and shapes in the pH 10 dispersion. Thus, the shape, crystallinity, porosity, and mech. properties could be controlled by pH, which allows broader uses of supraballs.
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    Zellmer, S.; Lindenau, M.; Michel, S.; Garnweitner, G.; Schilde, C. Influence of Surface Modification on Structure Formation and Micromechanical Properties of Spray-Dried Silica Aggregates. J. Colloid Interface Sci. 2016, 464, 183– 190,  DOI: 10.1016/j.jcis.2015.11.028
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    Influence of surface modification on structure formation and micromechanical properties of spray-dried silica aggregates
    Zellmer, Sabrina; Lindenau, Maylin; Michel, Stephanie; Garnweitner, Georg; Schilde, Carsten
    Journal of Colloid and Interface Science (2016), 464 (), 183-190CODEN: JCISA5; ISSN:0021-9797. (Elsevier B.V.)
    Spray drying processes were utilized for the prodn. of hierarchical materials with defined structures. The structure formation during the spray drying process and the micromech. properties of the obtained aggregates depend on the particle-particle interactions, the primary particle size and morphol. as well as the process parameters of the spray drying process. Hence, the effect of different primary particle systems prepd. as stable dispersions with various surface modifications were investigated on the colloidal structure formation and the micromech. properties of silica particles as model aggregates and compared to theor. considerations. The obtained results show that the structure formation of aggregates during the spray drying process for stable suspensions is almost independent on the functional groups present at the particle surface. Further, the mech. properties of these aggregates differ considerably with the content of the bound ligand. This allows the defined adjustment of the aggregate properties, such as the strength and surface properties, as well as the formation of defined hierarchical aggregate structures.
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    Russell, A.; Müller, P.; Tomas, J. Quasi-Static Diametrical Compression of Characteristic Elastic-Plastic Granules: Energetic Aspects at Contact. Chem. Eng. Sci. 2014, 114, 70– 84,  DOI: 10.1016/j.ces.2014.04.016
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    Quasi-static diametrical compression of characteristic elastic-plastic granules: Energetic aspects at contact
    Russell, Alexander; Mueller, Peter; Tomas, Juergen
    Chemical Engineering Science (2014), 114 (), 70-84CODEN: CESCAC; ISSN:0009-2509. (Elsevier Ltd.)
    Comminution processes have traditionally been considered as empirical endeavors, with great difficulties in analyzing, predicting and controlling the obsd. behavior. Process industries have faced serious problems, ranging from poor product quality control up to managing product quantity losses due to unexpected attrition and breakage events. After a decade of ardent research, there has been a notable advancement in our understanding of granular behavior under mech. stresses. However, far less is known about the energetic aspects occurring at such conditions. In this article, we present crit. information from an energetic perspective concerning the deformation and breakage behavior of characteristic elastic-plastic granules under quasi-static compressive forces, studied using single granule diametrical compression tests at a const. strain controlled loading velocity. The force-displacement behavior has been approximated using the Hertz model (Hertz, 1882) for elastic loading, the Tomas model (Tomas, 2007a) for elastic-plastic loading and the Stronge-Antonyuk correlations (Stronge, 2000; Antonyuk, 2006) incorporated Hertz model (Hertz, 1882) for elastic unloading with addnl. displacements due to apparent viscous effects. The rate independent specific energy characteristics have been studied and a corresponding energetic coeff. of restitution has been derived. A comparison of the energetic characteristics at primary breakage of fresh and pre-loaded granules has been presented using breakage probability functions. The phenomenon of strain hardening during localized repetitive compressive loadings has also been analyzed. Furthermore, the influences of granule size, moisture content and stressing intensity have been considered in each of the investigated entities.
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    Thornton, C.; Ciomocos, M. T.; Adams, M. J. Numerical Simulations of Diametrical Compression Tests on Agglomerates. Powder Technol. 2004, 140, 258– 267,  DOI: 10.1016/j.powtec.2004.01.022
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    Numerical simulations of diametrical compression tests on agglomerates
    Thornton, C.; Ciomocos, M. T.; Adams, M. J.
    Powder Technology (2004), 140 (3), 258-267CODEN: POTEBX; ISSN:0032-5910. (Elsevier Science B.V.)
    The paper reports discrete element simulations of the diametrical compression test applied to two spherical agglomerates: one a dense agglomerate and the other a loosely packed agglomerate. The results obtained for the dense agglomerate show that the agglomerate fractures along a slightly inclined, approx. diametrical plane. Outwardly, the agglomerate shows all the characteristics of brittle fracture but half of the final no. of broken bonds was progressively broken during loading. In the simulation on the loose agglomerate, significant flattening occurred at the platens and the agglomerate failed by crushing.
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    Kozhar, S.; Dosta, M.; Antonyuk, S.; Heinrich, S.; Bröckel, U. DEM Simulations of Amorphous Irregular Shaped Micrometer-Sized Titania Agglomerates at Compression. Adv. Powder Technol. 2015, 26, 767– 777,  DOI: 10.1016/j.apt.2015.05.005
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    DEM simulations of amorphous irregular shaped micrometer-sized titania agglomerates at compression
    Kozhar, S.; Dosta, M.; Antonyuk, S.; Heinrich, S.; Broeckel, U.
    Advanced Powder Technology (2015), 26 (3), 767-777CODEN: APTEEE; ISSN:0921-8831. (Elsevier B.V.)
    Bulk solids are exposed to time-dependent mech. stressing due to particle-particle and particle-app. contact interactions during various manufg. processes and transportation steps. These interactions can be described by discrete element method (DEM) based on the contact models of particles. Usually in DEM simulations the particles are assumed to be spheres and as a consequence shape effects are neglected. However, most bulk solids processed in industry consist of irregular shaped particles. Therefore, in order to improve accuracy of numerical simulation the real shapes of particles must be considered in DEM. In this work, amorphous irregular shaped micrometer-sized titanium dioxide agglomerates were investigated. The force-displacement curves at compression were obtained with the help of a self-designed exptl. setup. Based on the exptl. data, several material parameters were detd. and implemented in viscoelastic and elastic-plastic contact models. To consider the shape effect in the estn. of contact parameters the DEM simulation of studied agglomerates was performed by multi-sphere approach and bonded-particle model. The shape and position of the agglomerates on the loading pin were obtained by X-ray computer tomog. and used in DEM simulations. From the obtained results it was pointed out that the bonded-particle model based on Maxwell viscoelastic model gives the best agreement with exptl. data from compression tests with titania agglomerates.
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    Plunkett, A.; Eldridge, C.; Schneider, G. A.; Domènech, B. Controlling the Large-Scale Fabrication of Supraparticles. J. Phys. Chem. B 2020, 124, 11263– 11272,  DOI: 10.1021/acs.jpcb.0c07306
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    Controlling the Large-Scale Fabrication of Supraparticles
    Plunkett, Alexander; Eldridge, Catriona; Schneider, Gerold A.; Domenech, Berta
    Journal of Physical Chemistry B (2020), 124 (49), 11263-11272CODEN: JPCBFK; ISSN:1520-5207. (American Chemical Society)
    Controlling the nanoscale interactions of colloidal building blocks is a key step for the transition from single nanoparticles to tailor-made, architected morphologies and their further integration into functional materials. Solvent evapn.-induced self-assembly within emulsion droplets emerges as a fast, versatile, and low-cost approach to obtain spherical, complex structures, such as supraparticles. Nevertheless, some process-structure relationships able to describe the effects of emulsion conditions on the synthesis outcomes still remain to be understood. Here, we explore the effect of different physicochem. parameters of emulsion-templated self-assembly (ETSA) on supraparticles' formation. Supraparticle size, size dispersity, microporosity, and sample homogeneity are rationalized based on the used surfactant formulation, stabilization mechanism, and viscosity of the emulsion. We further demonstrate the significance of the parameters found by optimizing a transferable, large-scale (gram-size) ETSA setup for the controlled synthesis of spherical supraparticles in a range of defined sizes (from 0.1-10μm). Ultimately, our results provide new key synthetic parameters able to control the process, promoting the development of supraparticle-based, functional nanomaterials for a wide range of applications.
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    Shang, L.; Cheng, Y.; Zhao, Y. Emerging Droplet Microfluidics. Chem. Rev. 2017, 117, 7964– 8040,  DOI: 10.1021/acs.chemrev.6b00848
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    Emerging Droplet Microfluidics
    Shang, Luoran; Cheng, Yao; Zhao, Yuanjin
    Chemical Reviews (Washington, DC, United States) (2017), 117 (12), 7964-8040CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review. Droplet microfluidics generates and manipulates discrete droplets through immiscible multiphase flows inside microchannels. Due to its remarkable advantages, droplet microfluidics bears significant value in an extremely wide range of area. In this review, a comprehensive and in-depth insight is provided into droplet microfluidics, covering fundamental research from microfluidic chip fabrication and droplet generation to the applications of droplets in bio(chem.) anal. and materials generation. The purpose of the review is to convey the fundamentals of droplet microfluidics, a crit. anal. on its current status and challenges, and opinions on its future development. The review will promote communications among biol., chem., physics, and materials science.
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    Park, S.; Hwang, H.; Kim, M.; Moon, J. H.; Kim, S.-H. Colloidal Assembly in Droplets: Structures and Optical Properties. Nanoscale 2020, 12, 18576– 18594,  DOI: 10.1039/D0NR04608F
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    Colloidal assembly in droplets: structures and optical properties
    Park, Sanghyuk; Hwang, Hyerim; Kim, Minjung; Moon, Jun Hyuk; Kim, Shin-Hyun
    Nanoscale (2020), 12 (36), 18576-18594CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    A review. Colloidal assembly in emulsion drops provides fundamental tools for studying optimum particle arrangement under spherical confinement and practical means for producing photonic microparticles. Recent progress has revealed that energetically favored cluster configurations are different from conventional supraballs, which could enhance optical performance. This paper reviews state-of-the-art emulsion-templated colloidal clusters, and particularly focuses on recently reported novel structures such as icosahedral, decahedral, and single-cryst. face-centered cubic (fcc) clusters. We classify the clusters according to the no. of component particles as small (N < O(102)), medium (O(102) ≤ N ≤ O(104)), and large (N ≥ O(105)). For each size of clusters, we discuss the detailed structures, mechanisms of cluster formation, and optical properties and potential applications. Finally, we outline current challenges and questions that require further investigation.
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    Kim, S. H.; Lee, S. Y.; Yi, G. R.; Pine, D. J.; Yang, S. M. Microwave-Assisted Self-Organization of Colloidal Particles in Confining Aqueous Droplets. J. Am. Chem. Soc. 2006, 128, 10897– 10904,  DOI: 10.1021/ja063528y
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    Microwave-Assisted Self-Organization of Colloidal Particles in Confining Aqueous Droplets
    Kim, Shin-Hyun; Lee, Su Yeon; Yi, Gi-Ra; Pine, David J.; Yang, Seung-Man
    Journal of the American Chemical Society (2006), 128 (33), 10897-10904CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Monodisperse aq. emulsion droplets encapsulating colloidal particles were produced in the oil phase, and controlled microwave irradn. of the aq. drop phase created spherical colloidal crystals by so-called evapn.-induced self-organization of the colloidal particles. Unlike usual colloidal crystals, colloidal crystals in spherical symmetry (or photonic balls) possessed photonic band gaps for the normal incident light independent of the position all over the spherical surface. While the consolidation of colloidal particles in emulsion droplets in an oven took several hours, the present microwave-assisted evapn. could reduce the time for complete evapn. to a few tens of minutes. Under the microwave irradn., the aq. phase in emulsions was superheated selectively and the evapn. rate of water could be controlled easily by adjusting the microwave intensity. The result showed that the packing quality of colloidal crystals obtained by the microwave-assisted self-organization was good enough to show photonic band gap characteristics. The reflectance of our photonic balls responded precisely to any change in phys. properties including the size of colloidal particles, refractive index mismatch, and angle of the incident beam. In particular, for polymeric particles, the photonic band gap could be tuned by the intensity of microwave irradn., and the reflection color was red-shifted with stronger microwave irradn. Finally, for better photonic band gap properties, inverted photonic balls were prepd. by using the spherical colloidal crystals as sacrificial templates.
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    Pileni, M.-P. Mechanical Properties of Supracrystals. EPL (Europhysics Letters). 2017, 119, 37002,  DOI: 10.1209/0295-5075/119/37002
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    Junwei, W.; Jan, S.; Andreas, S.; Patrick, F.; Patrick, H.; Stefan, R.; Wolfgang, P.; Benoit, M.; Nicolas, V. Mechanics of Colloidal Supraparticles Under Compression. Sci. Adv. 2021,  DOI: 10.1126/sciadv.abj0954
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    Kim, Y. R.; Lee, T. W.; Park, S.; Jang, J.; Ahn, C. W.; Choi, J. J.; Hahn, B. D.; Choi, J. H.; Yoon, W. H.; Bae, S. H. Supraparticle Engineering for Highly Dense Microspheres: Yttria-Stabilized Zirconia with Adjustable Micromechanical Properties. ACS Nano 2021, 15, 10264– 10274,  DOI: 10.1021/acsnano.1c02408
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    Supraparticle Engineering for Highly Dense Microspheres: Yttria-Stabilized Zirconia with Adjustable Micromechanical Properties
    Kim Young-Rok; Lee Tae Won; Park Seonhwa; Jang Jongmoon; Ahn Cheol-Woo; Choi Jong-Jin; Hahn Byung-Dong; Choi Joon-Hwan; Yoon Woon-Ha; Min Yuho; Kim Young-Rok; Bae Sung-Hwan
    ACS nano (2021), 15 (6), 10264-10274 ISSN:.
    Various supraparticles have been extensively studied owing to their excellent catalytic properties that are attributed to their inherent porous structure; however, their mechanical properties have not garnered attention owing to their less dense structure. We demonstrate a rational approach for fabricating assembled supraparticles and, subsequently, highly dense microspheres. In addition, 3 mol % yttria-stabilized zirconia (3YSZ) and alumina particles were selected as building blocks and assembled into higher-order architectures using a droplet-based template method (spray drying) for validation with proof-of-concept. Moreover, structural features such as density, size, sphericity, and morphology of supraparticles were controlled by adjusting the competing kinetics occurring between the assembly of building blocks and evaporation of the solvent in the droplets. The preparatory aqueous suspension and process parameters were optimized as well. A detailed understanding of the formation mechanism facilitated the yield of tailor-made supraparticles and, thereafter, highly dense microspheres (approximate relative density = 99%) with excellent sphericity (>98%) via heat treatment. The microspheres displayed highest hardness (26.77 GPa) and superior elastic modulus (210.19 GPa) compared with the mechanical properties of the 3YSZ samples reported to date. Ultimately, the proposed supraparticle engineering provided insight for controlling the structural features and resultant micromechanical properties, which widely extends the applicability of supraparticle-based functional materials for practical purposes that require materials with high density and excellent mechanical properties.
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    Giuntini, D.; Zhao, S.; Krekeler, T.; Li, M.; Blankenburg, M.; Bor, B.; Schaan, G.; Domenech, B.; Muller, M.; Scheider, I.; Ritter, M.; Schneider, G. A. Defects and Plasticity in Ultrastrong Supercrystalline Nanocomposites. Sci. Adv. 2021, 7, 1– 11,  DOI: 10.1126/sciadv.abb6063
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    Dreyer, A.; Feld, A.; Kornowski, A.; Yilmaz, E. D.; Noei, H.; Meyer, A.; Krekeler, T.; Jiao, C.; Stierle, A.; Abetz, V. Organically Linked Iron Oxide Nanoparticle Supercrystals with Exceptional Isotropic Mechanical Properties. Nat. Mater. 2016, 15, 522– 528,  DOI: 10.1038/nmat4553
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    Organically linked iron oxide nanoparticle supercrystals with exceptional isotropic mechanical properties
    Dreyer, Axel; Feld, Artur; Kornowski, Andreas; Yilmaz, Ezgi D.; Noei, Heshmat; Meyer, Andreas; Krekeler, Tobias; Jiao, Chengge; Stierle, Andreas; Abetz, Volker; Weller, Horst; Schneider, Gerold A.
    Nature Materials (2016), 15 (5), 522-528CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
    The authors show self-assembly of nearly spherical iron oxide nanoparticles in supercrystals linked together by a thermally induced crosslinking reaction of oleic acid mols. leads to a nanocomposite with exceptional bending modulus of 114 GPa, hardness of up to 4 GPa and strength of up to 630 MPa. By using a nanomech. model, the authors detd. that these exceptional mech. properties are dominated by the covalent backbone of the linked org. mols. Oleic acid has been broadly used as nanoparticle ligand, our crosslinking approach should be applicable to a large variety of nanoparticle systems.
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    Ayouch, A.; Dieudonné, X.; Vaudel, G.; Piombini, H.; Vallé, K.; Gusev, V.; Belleville, P.; Ruello, P. Elasticity of an Assembly of Disordered Nanoparticles Interacting via either Van der Waals-Bonded or Covalent-Bonded Coating Layers. ACS Nano 2012, 6, 10614– 10621,  DOI: 10.1021/nn303631d
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    Elasticity of an Assembly of Disordered Nanoparticles Interacting via Either van der Waals-Bonded or Covalent-Bonded Coating Layers
    Ayouch, Adil; Dieudonne, Xavier; Vaudel, Gwenaelle; Piombini, Herve; Valle, Karine; Gusev, Vitalyi; Belleville, Philippe; Ruello, Pascal
    ACS Nano (2012), 6 (12), 10614-10621CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Tailoring phys. and chem. properties at the nanoscale by assembling nanoparticles currently paves the way for new functional materials. Obtaining the desired macroscopic properties is usually detd. by a perfect control of the contact between nanoparticles. Therefore, the physics and chem. of nanocontacts are one of the central issues for the design of the nanocomposites. Since the birth of at. force microscopy, crucial advances were achieved in the quant. evaluation of van der Waals and Casimir forces in nanostructures and of adhesion between the nanoparticles. The authors present here a study, by a noncontact method, of the elasticity of an assembly of nanoparticles interacting via either van der Waals-bonded or covalent-bonded coating layers. The authors demonstrate indeed that the ultrafast opto-acoustic technique, based on the generation and detection of hypersound by femtosecond laser pulses, is very sensitive to probe the properties of the nanocontacts. In particular, the authors observe and evaluate how much the subnanometric mols. present at nanocontacts influence the coherent acoustic phonon propagation along the network of the interconnected SiO2 nanoparticles. Finally, this ultrafast opto-acoustic technique provides quant. ests. of the rigidity/stiffness of the nanocontacts.
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    Cang, Y.; Liu, B.; Das, S.; Xu, X.; Xie, J.; Deng, X.; Fytas, G. Surface Contacts Strongly Influence the Elasticity and Thermal Conductivity of Silica Nanoparticle Fibers. Phys. Chem. Chem. Phys. 2021, 23, 3707– 3715,  DOI: 10.1039/D0CP05377E
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    Surface contacts strongly influence elasticity and thermal conductivity of silica nanoparticle fibers
    Cang, Yu; Liu, Bohai; Das, Sudatta; Xu, Xiangfan; Xie, Jingli; Deng, Xu; Fytas, George
    Physical Chemistry Chemical Physics (2021), 23 (6), 3707-3715CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    Granular materials are often encountered in science and engineering disciplines, in which controlling the particle contacts is one of the crit. issues for the design, engineering, and utilization of their desired properties. The achievable rapid fabrication of nanoparticles with tunable phys. and chem. properties facilitates tailoring the macroscopic properties of particle assemblies through contacts at the nanoscale. Models have been developed to predict the mech. properties of macroscopic granular materials; however, their predicted power in the case of nanoparticle assemblies is still uncertain. Here, we investigate the influence of nanocontacts on the elasticity and thermal cond. of a granular fiber comprised of close-packed silica nanoparticles. A complete elastic moduli characterization was realized by non-contact and non-destructive Brillouin light spectroscopy, which also allowed resolving the stiffness of the constituent particles in situ. In the framework of effective medium models, the strong enhancement of the elastic moduli is attributed to the formation of adhesive nanocontacts with phys. and/or chem. bondings. The nanoparticle contacts are also responsible for the increase in the fiber thermal cond. that emphasizes the role of interface thermal resistance, which tends to be ignored in most porosity models. This insight into the fundamental understanding of structure-property relationships advances knowledge on the manipulation of granular systems at the nanoscale.
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    Holtze, C.; Rowat, A. C.; Agresti, J. J.; Hutchison, J. B.; Angile, F. E.; Schmitz, C. H. J.; Koster, S.; Duan, H.; Humphry, K. J.; Scanga, R. A. Biocompatible Surfactants for Water-in-Fluorocarbon Emulsions. Lab Chip 2008, 8, 1632– 1639,  DOI: 10.1039/b806706f
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    Biocompatible surfactants for water-in-fluorocarbon emulsions
    Holtze, C.; Rowat, A. C.; Agresti, J. J.; Hutchison, J. B.; Angile, F. E.; Schmitz, C. H. J.; Koster, S.; Duan, H.; Humphry, K. J.; Scanga, R. A.; Johnson, J. S.; Pisignano, D.; Weitz, D. A.
    Lab on a Chip (2008), 8 (10), 1632-1639CODEN: LCAHAM; ISSN:1473-0197. (Royal Society of Chemistry)
    Drops of water-in-fluorocarbon emulsions have great potential for compartmentalizing both in vitro and in vivo biol. systems; however, surfactants to stabilize such emulsions are scarce. Here we present a novel class of fluorosurfactants that we synthesize by coupling oligomeric perfluorinated polyethers (PFPE) with polyethyleneglycol (PEG). We demonstrate that these block copolymer surfactants stabilize water-in-fluorocarbon oil emulsions during all necessary steps of a drop-based expt. including drop formation, incubation, and reinjection into a second microfluidic device. Furthermore, we show that aq. drops stabilized with these surfactants can be used for in vitro translation (IVT), as well as encapsulation and incubation of single cells. The compatibility of this emulsion system with both biol. systems and polydimethylsiloxane (PDMS) microfluidic devices makes these surfactants ideal for a broad range of high-throughput, drop-based applications.
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    Etienne, G.; Kessler, M.; Amstad, E. Influence of Fluorinated Surfactant Composition on the Stability of Emulsion Drops. Macromol. Chem. Phys. 2017, 218, 1600365,  DOI: 10.1002/macp.201600365
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    Seemann, R.; Brinkmann, M.; Pfohl, T.; Herminghaus, S. Droplet Based Microfluidics. Rep. Prog. Phys. 2012, 75, 016601,  DOI: 10.1088/0034-4885/75/1/016601
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    Droplet based microfluidics
    Seemann, Ralf; Brinkmann, Martin; Pfohl, Thomas; Herminghaus, Stephan
    Reports on Progress in Physics (2012), 75 (1), 016601/1-016601/41CODEN: RPPHAG; ISSN:0034-4885. (Institute of Physics Publishing)
    A review. Droplet based microfluidics is a rapidly growing interdisciplinary field of research combining soft matter physics, biochem., and microsystems engineering. Its applications range from fast anal. systems or the synthesis of advanced materials to protein crystn., and biol. assays for living cells. Precise control of droplet vols. and reliable manipulation of individual droplets such as coalescence, mixing of their contents, and sorting in combination with fast anal. tools allow to perform chem. reactions inside the droplets under defined conditions. In this paper, reviews available drop generation and manipulation techniques. The main focus of the review is not to be comprehensive and explain all techniques in great detail but to identify and shed light on similarities and underlying phys. principles. Since geometry and wetting properties of the microfluidic channels are crucial factors for droplet generation, typical device fabrication methods in droplet based microfluidics are also briefly described. Examples of applications and reaction schemes which rely on the discussed manipulation techniques are also presented, such as the fabrication of special materials and biophys. expts.
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    Penciu, R. S.; Fytas, G.; Economou, E. N.; Steffen, W.; Yannopoulos, S. N. Acoustic Excitations in Suspensions of Soft Colloids. Phys. Rev. Lett. 2000, 85, 4622– 4625,  DOI: 10.1103/PhysRevLett.85.4622
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    Acoustic Excitations in Suspensions of Soft Colloids
    Penciu, R. S.; Fytas, G.; Economou, E. N.; Steffen, W.; Yannopoulos, S. N.
    Physical Review Letters (2000), 85 (21), 4622-4625CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    Vibrational modes in suspensions of soft colloids in a fluid can be detected exptl. by Brillouin light scattering. Besides the usual acoustic mode, being essentially the longitudinal phonon of the liq. matrix, an optic-like mode is obsd. in giant starlike micelles at low vol. fractions. Probably this optic-like mode is due mainly to the internal vibration of each hairy particle.
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    Kuok, M. H.; Lim, H. S.; Ng, S. C.; Liu, N. N.; Wang, Z. K. Brillouin Study of the Quantization of Acoustic Modes in Nanospheres. Phys. Rev. Lett. 2003, 90, 4,  DOI: 10.1103/PhysRevLett.90.255502
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    Courty, A.; Mermet, A.; Albouy, P. A.; Duval, E.; Pileni, M. P. Vibrational Coherence of Self-Organized Silver Nanocrystals in f.c.c. Supra-Crystals. Nat. Mater. 2005, 4, 395– 398,  DOI: 10.1038/nmat1366
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    Vibrational coherence of self-organized silver nanocrystals in f.c.c. supra-crystals
    Courty, A.; Mermet, A.; Albouy, P. A.; Duval, E.; Pileni, M. P.
    Nature Materials (2005), 4 (5), 395-398CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
    Fabrication of devices from inorg. nanocrystals normally requires that they are self-organized into ordered structures. It has now been demonstrated that nanocrystals are able to self-organize in a 'supra'-crystal with a face-centered cubic (f.c.c.) structure. The phys. properties of nanocrystals self-organized into compact arrays are quite different from those of both isolated nanocrystals and the bulk phase. The collective optical and magnetic properties of these nanocrystal assemblies are governed mainly by dipolar interactions. Here, we show that nanocrystals vibrate coherently when they are self-organized in f.c.c. supra-crystals. Hence, a phase relation exists between the vibrations of all of the nanocrystals in a supra-crystal. This vibrational coherence can be obsd. by a substantial change of the quadrupolar low-frequency Raman scattering peak. Although a change in electronic transport properties has previously been obsd. on self-organization of silver nanocrystals, vibrational coherence represents the first intrinsic property of f.c.c. supra-crystals.
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    Cheng, W.; Wang, J.; Jonas, U.; Fytas, G.; Stefanou, N. Observation and Tuning of Hypersonic Bandgaps in Colloidal Crystals. Nat. Mater. 2006, 5, 830– 836,  DOI: 10.1038/nmat1727
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    Observation and tuning of hypersonic bandgaps in colloidal crystals
    Cheng, Wei; Wang, Jianjun; Jonas, Ulrich; Fytas, George; Stefanou, Nikolaos
    Nature Materials (2006), 5 (10), 830-836CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
    Composite materials with periodic variations of d. and/or sound velocities, so-called phononic crystals, can exhibit bandgaps where propagation of acoustic waves is forbidden. Phononic crystals are the elastic analog of the well-established photonic crystals and show potential for manipulating the flow of elastic energy. So far, the exptl. realization of phononic crystals was restricted to macroscopic systems with sonic or ultrasonic bandgaps in the sub-MHz frequency range. Using high-resoln. Brillouin spectroscopy the authors report the 1st observation of a hypersonic bandgap in fcc. colloidal crystals formed by self-assembly of polystyrene nanoparticles with subsequent fluid infiltration. Depending on the particle size and the sound velocity in the infiltrated fluid, the frequency and the width of the gap can be tuned. Promising technol. applications of hypersonic crystals, ranging from tunable filters and heat management to acoustooptical devices, are anticipated.
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    Still, T.; Sainidou, R.; Retsch, M.; Jonas, U.; Spahn, P.; Hellmann, G. P.; Fytas, G. The “Music” of Core-Shell Spheres and Hollow Capsules: Influence of the Architecture on the Mechanical Properties at the Nanoscale. Nano Lett. 2008, 8, 3194– 3199,  DOI: 10.1021/nl801500n
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    The "Music" of Core-Shell Spheres and Hollow Capsules: Influence of the Architecture on the Mechanical Properties at the Nanoscale
    Still, T.; Sainidou, R.; Retsch, M.; Jonas, U.; Spahn, P.; Hellmann, G. P.; Fytas, G.
    Nano Letters (2008), 8 (10), 3194-3199CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    The elastic vibrational modes were measured in core-shell spheres (silica-poly(Me methacrylate), SiO2-PMMA) and corresponding spherical hollow capsules (PMMA) with different particle size and shell thickness using Brillouin light scattering, supported by numerical calcns. These localized modes allow access to the mech. moduli down to a few tens of nanometers. Reduced mech. strength of the porous silica core and for the core-shell spheres a striking increase of the moduli in both the SiO2 core and the PMMA shell were obsd. The peculiar behavior of the vibrational modes in the hollow capsules is attributed to antagonistic dependence on overall size and layer thickness in agreement with theor. predictions.
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    Still, T.; Mattarelli, M.; Kiefer, D.; Fytas, G.; Montagna, M. Eigenvibrations of Submicrometer Colloidal Spheres. J. Phys. Chem. Lett. 2010, 1, 2440– 2444,  DOI: 10.1021/jz100774b
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    Eigenvibrations of Submicrometer Colloidal Spheres
    Still, T.; Mattarelli, M.; Kiefer, D.; Fytas, G.; Montagna, M.
    Journal of Physical Chemistry Letters (2010), 1 (16), 2440-2444CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Brillouin light scattering (BLS) can record particle resonances, which allow access to thermomech. properties on a submicrometer scale. The full theor. representation of BLS eigenmode spectra of polymer and SiO2 colloids resolves a longstanding discussion showing that both even and odd l spheroidal modes are active. The BLS spectra of transparent SiO2 suspensions fully support the predicted dependence of the strength of the individual resonance modes on the probing wavevector. It is this dependence that selects which spheroidal modes will be revealed exptl.
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    Schneider, D.; Beltramo, P. J.; Mattarelli, M.; Pfleiderer, P.; Vermant, J.; Crespy, D.; Montagna, M.; Furst, E. M.; Fytas, G. Elongated Polystyrene Spheres as Resonant Building Blocks in Anisotropic Colloidal Crystals. Soft Matter 2013, 9, 9129– 9136,  DOI: 10.1039/c3sm50959a
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    Elongated polystyrene spheres as resonant building blocks in anisotropic colloidal crystals
    Schneider, Dirk; Beltramo, Peter J.; Mattarelli, Maurizio; Pfleiderer, Patrick; Vermant, Jan; Crespy, Daniel; Montagna, Maurizio; Furst, Eric M.; Fytas, George
    Soft Matter (2013), 9 (38), 9129-9136CODEN: SMOABF; ISSN:1744-683X. (Royal Society of Chemistry)
    Colloidal crystals have gained increasing importance due to their fascinating ability to mold the flow of light and sound (heat). The characteristics of these ordered assemblies of particles are strongly detd. by the resp. building blocks, which require complete understanding of their phys. properties. In this study the mech. properties of stretched polystyrene colloids (spheroids) are addressed. The non-invasive technique of Brillouin light scattering captures the vibrational spectra at hypersonic (GHz) frequencies. Resolved eigenmodes are considered fingerprints of the particles' shape, size and composing materials. A single particle model is used to simulate the exptl. data by calcn. of all active modes and subsequent evaluation of their contribution to the spectrum. Compared to spheres (high symmetry) more modes contribute to the spectra that limit the resoln. at very high frequencies, due to the lifted mode degeneracy. Knowing the nature of the principal modes of spheroids is a precondition to understand the phononic dispersion in the resp. colloidal crystals, in particular those responsible for anticrossing interactions with the effective medium acoustic phonon.
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    Tchebotareva, A. L.; Van Dijk, M. A.; Ruijgrok, P. V.; Fokkema, V.; Hesselberth, M. H. S.; Lippitz, M.; Orrit, M. Acoustic and Optical Modes of Single Dumbbells of Gold Nanoparticles. ChemPhysChem 2009, 10, 111– 114,  DOI: 10.1002/cphc.200800289
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    Acoustic and optical modes of single dumbbells of gold nanoparticles
    Tchebotareva, Anna L.; van Dijk, Meindert A.; Ruijgrok, Paul V.; Fokkema, Vincent; Hesselberth, Marcel H. S.; Lippitz, Markus; Orrit, Michel
    ChemPhysChem (2009), 10 (1), 111-114CODEN: CPCHFT; ISSN:1439-4235. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Gold vibrations: A new elastic (stretching) mode, appearing in individual dumbbells of gold nanospheres at 5-7 GHz (see figure), is a function of the contact area. This can be used to est. the contact area between the particles, which plays an important role in the local enhancement of electromagnetic fields in such nanoantenna structures.
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    Mattarelli, M.; Montagna, M.; Still, T.; Schneider, D.; Fytas, G. Vibration Spectroscopy of Weakly Interacting Mesoscopic Colloids. Soft Matter 2012, 8, 4235– 4243,  DOI: 10.1039/c2sm07034k
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    Vibration spectroscopy of weakly interacting mesoscopic colloids
    Mattarelli, Maurizio; Montagna, Maurizio; Still, Tim; Schneider, Dirk; Fytas, George
    Soft Matter (2012), 8 (15), 4235-4243CODEN: SMOABF; ISSN:1744-683X. (Royal Society of Chemistry)
    Brillouin spectroscopy has been used to study the vibrational dynamics of clusters of spherical polystyrene (PS) particles with different size. In a first approxn., the spectra can be described by a single particle model within the continuum approxn. of the Lamb theory. The model yields excellent results for particles with diam. d ⪆ 500 nm, but fails in accounting for the lineshapes of the obsd. lowest frequency signals in the vibration spectrum of smaller particles due to a broadening and shift of the lines of confined vibrations. The model also fails to predict an addnl. very low frequency broad band in the case of small particles (d .ltorsim. 400nm). This band is attributed to phonon propagation in multiple-particle clusters governed by the interactions among particles. These interactions also produce extended modes in close relation to the Lamb modes of the single sphere. A simple model for the particle interaction allows to represent the new spectral features and est. the strength of the interactions and the long wavelength longitudinal velocity in the colloidal clusters. Their thermal annealing near the glass transition of PS enhances the interactions which are manifested in the low frequency particle vibration spectrum and the phonon bands assocd. with the vibrations of the individual spheres. The emerging particle vibration spectroscopy becomes a sensitive tool of the colloids' thermo-mech. properties as well their interactions.
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  79. 79
    Girard, A.; Gehan, H.; Mermet, A.; Bonnet, C.; Lermé, J.; Berthelot, A.; Cottancin, E.; Crut, A.; Margueritat, J. Acoustic Mode Hybridization in a Single Dimer of Gold Nanoparticles. Nano Lett. 2018, 18, 3800– 3806,  DOI: 10.1021/acs.nanolett.8b01072
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    79
    Acoustic Mode Hybridization in a Single Dimer of Gold Nanoparticles
    Girard, Adrien; Gehan, Helene; Mermet, Alain; Bonnet, Christophe; Lerme, Jean; Berthelot, Alice; Cottancin, Emmanuel; Crut, Aurelien; Margueritat, Jeremie
    Nano Letters (2018), 18 (6), 3800-3806CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    The acoustic vibrations of single monomers and dimers of Au nanoparticles were studied by measuring for the 1st time their ultralow-frequency micro-Raman scattering. This expt. provides access not only to the frequency of the detected vibrational modes but also to their damping rate, which is obscured by inhomogeneous effects in measurements on ensembles of nano-objects. This allows a detailed anal. of the mech. coupling occurring between 2 close nanoparticles (mediated by the polymer surrounding them) in the dimer case. Such coupling induces the hybridization of the vibrational modes of each nanoparticle, leading to the appearance in the Raman spectra of 2 ultralow-frequency modes corresponding to the out-of-phase longitudinal and transverse (with respect to the dimer axis) quasi-translations of the nanoparticles. Addnl., it is also shown to shift the frequency of the quadrupolar modes of the nanoparticles. Exptl. results are interpreted using finite-element simulations, which enable the unambiguous identification of the detected modes and despite the simplifications made lead to a reasonable reprodn. of their measured frequencies and quality factors. The demonstrated feasibility of low-frequency Raman scattering expts. on single nano-objects opens up new possibilities to improve the understanding of nanoscale vibrations with this technique being complementary with single nano-object time-resolved spectroscopy as it gives access to different vibrational modes.
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  80. 80
    Noual, A.; Kang, E.; Maji, T.; Gkikas, M.; Djafari-Rouhani, B.; Fytas, G. Optomechanic Coupling in Ag Polymer Nanocomposite Films. J. Phys. Chem. C 2021, 125, 14854– 14864,  DOI: 10.1021/acs.jpcc.1c04549
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    80
    Optomechanic Coupling in Ag Polymer Nanocomposite Films
    Noual, Adnane; Kang, Eunsoo; Maji, Tanmoy; Gkikas, Manos; Djafari-Rouhani, Bahram; Fytas, George
    Journal of Physical Chemistry C (2021), 125 (27), 14854-14864CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Particle vibrational spectroscopy has emerged as a new tool for the measurement of elasticity, glass transition, and interactions at a nanoscale. For colloid-based materials, the weakly localized particle resonances in a fluid or solid medium renders their detection difficult. The strong amplification of the inelastic light scattering near surface plasmon resonance of metallic nanoparticles (NPs) allowed not only the detection of single NP eigenvibrations but also the interparticle interaction effects on the acoustic vibrations of NPs mediated by strong optomech. coupling. The rattling and quadrupolar modes of Ag/polymer and polymer-grafted Ag NPs with different diams. in their assemblies are probed by Brillouin light spectroscopy (BLS). The authors present thorough theor. 3D calcns. for anisotropic Ag elasticity to quantify the frequency and intensity of the rattling mode and hence its BLS activity for different interparticle sepns. and matrix rigidity. Theor., a liq.-like environment, e.g., poly(isobutylene) (PIB) does not support rattling vibration of Ag dimers but unexpectedly hardening of the extremely confined graft melt renders both activation of the former and a frequency blue shift of the fundamental quadrupolar mode in the grafted nanoparticle Ag@PIB film.
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    Xia, Y.; Whitesides, G. M. Soft Lithography. Angew. Chem., Int. Ed. 1998, 37, 550– 575,  DOI: 10.1002/(SICI)1521-3773(19980316)37:5<550::AID-ANIE550>3.0.CO;2-G
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    81
    Soft lithography
    Xia, Younan; Whitesides, George M.
    Angewandte Chemie, International Edition (1998), 37 (5), 550-575CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH)
    A review with 197 refs. is given. Microfabrication, the generation of small structures, is essential to much of modern science and technol.; it supports information technol. and permeates society through its role in microelectronics and optoelectronics. The patterning required in microfabrication is usually carried out with photolithog. Although it is difficult to find another technol. with more dominant influence, photolithog. nonetheless has disadvantages. The sizes of the features it can produce are limited by optical diffraction, and the high-energy radiation needed for small features requires complex facilities and technologies. Photolithog. is expensive; it cannot be easily applied to nonplanar surfaces; it tolerates little variation in the materials that can be used; and it provides almost no control over the chem. of patterned surfaces, esp. when complex org. functional groups of the sorts needed in chem., biochem., and biol. are involved. We wished to develop alternative, non-photolithog. microfabrication methods that would complement photolithog. These techniques would ideally circumvent the diffraction limits of photolithog., provide access to three-dimensional structures, tolerate a wide range of materials and surface chemistries, and be inexpensive, exptl. convenient, and accessible to mol. scientists. We have developed a set of such methods that we call "soft lithog.", since all members share the common feature that they use a patterned elastomer as the mask, stamp, or mold. We describe here soft lithog., and survey its ability to provide routes to high-quality patterns and structures with lateral dimensions of about 30 nm to 500 μm in systems presenting problems in topol., materials, or mol.-level definition that cannot (or at least not easily) be solved by photolithog.
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    Goodwin, J. W.; Hearn, J.; Ho, C. C.; Ottewill, R. H. Studies on the Preparation and Characterisation of Monodisperse Polystyrene Latices. Colloid Polym. Sci. 1974, 252, 464– 471,  DOI: 10.1007/BF01554752
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    Preparation and characterization of monodisperse polystyrene latexes. III. Preparation without added surface active agents
    Goodwin, J. W.; Hearn, J.; Ho, C. C.; Ottewill, R. H.
    Colloid and Polymer Science (1974), 252 (6), 464-71CODEN: CPMSB6; ISSN:0303-402X.
    By adjusting the ionic strength, initiator concn., and polymn. temp., monodisperse polystyrene [9003-53-6] latexes with particle sizes 0.1-1.0 μm were prepd. by single-stage polymns. in the absence of surfactants. In a typical polymn., 73 g styrene was added to 670 g H2O contg. NaCl under N with stirring at 60-95°, K2S2O8 initiator was added, and the dispersion was stirred for 24 hr. The total initial ionic strength largely detd. the particle size in the latexes.
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    Kim, H.; Cang, Y.; Kang, E.; Graczykowski, B.; Secchi, M.; Montagna, M.; Priestley, R. D.; Furst, E. M.; Fytas, G. Direct Observation of Polymer Surface Mobility via Nanoparticle Vibrations. Nat. Commun. 2018, 9, 1– 11,  DOI: 10.1038/s41467-018-04854-w
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    GABAergic signaling linked to autophagy enhances host protection against intracellular bacterial infections
    Kim, Jin Kyung; Kim, Yi Sak; Lee, Hye-Mi; Jin, Hyo Sun; Neupane, Chiranjivi; Kim, Sup; Lee, Sang-Hee; Min, Jung-Joon; Sasai, Miwa; Jeong, Jae-Ho; Choe, Seong-Kyu; Kim, Jin-Man; Yamamoto, Masahiro; Choy, Hyon E.; Park, Jin Bong; Jo, Eun-Kyeong
    Nature Communications (2018), 9 (1), 1-17CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
    Gamma-aminobutyric acid (GABA) is the principal inhibitory neurotransmitter in the brain; however, the roles of GABA in antimicrobial host defenses are largely unknown. Here we demonstrate that GABAergic activation enhances antimicrobial responses against intracellular bacterial infection. Intracellular bacterial infection decreases GABA levels in vitro in macrophages and in vivo in sera. Treatment of macrophages with GABA or GABAergic drugs promotes autophagy activation, enhances phagosomal maturation and antimicrobial responses against mycobacterial infection. In macrophages, the GABAergic defense is mediated via macrophage type A GABA receptor (GABAAR), intracellular calcium release, and the GABA type A receptor-assocd. protein-like 1 (GABARAPL1; an Atg8 homolog). Finally, GABAergic inhibition increases bacterial loads in mice and zebrafish in vivo, suggesting that the GABAergic defense plays an essential function in metazoan host defenses. Our study identified a previously unappreciated role for GABAergic signaling in linking antibacterial autophagy to enhance host innate defense against intracellular bacterial infection.
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    Wang, Z.; Cang, Y.; Kremer, F.; Thomas, E. L.; Fytas, G. Determination of the Complete Elasticity of Nephila Pilipes Spider Silk. Biomacromolecules 2020, 21, 1179– 1185,  DOI: 10.1021/acs.biomac.9b01607
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    Determination of the Complete Elasticity of Nephila pilipes Spider Silk
    Wang, Zuyuan; Cang, Yu; Kremer, Friedrich; Thomas, Edwin L.; Fytas, George
    Biomacromolecules (2020), 21 (3), 1179-1185CODEN: BOMAF6; ISSN:1525-7797. (American Chemical Society)
    Spider silks are remarkable materials designed by nature to have extraordinary elasticity. Their elasticity, however, remains poorly understood, as typical stress-strain expts. only allow access to the axial Young's modulus. In this work, micro-Brillouin light spectroscopy (micro-BLS), a noncontact, nondestructive technique, is utilized to probe the direction-dependent phonon propagation in the Nephila pilipes spider silk and hence solve its full elasticity. To the best of our knowledge, this is the first demonstration on the detn. of the anisotropic Young's moduli, shear moduli, and Poisson's ratios of a single spider fiber. The axial and lateral Young's moduli are found to be 20.9 ± 0.8 and 9.2 ± 0.3 GPa, resp., and the anisotropy of the Young's moduli further increases upon stretching. In contrast, the shear moduli and Poisson's ratios exhibit very weak anisotropy and are robust to stretching.
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    Park, J.-G.; Kim, S.-H.; Magkiriadou, S.; Choi, T. M.; Kim, Y.-S.; Manoharan, V. N. Full-Spectrum Photonic Pigments with Non-Iridescent Structural Colors through Colloidal Assembly. Angew. Chem., Int. Ed. 2014, 53, 2899– 2903,  DOI: 10.1002/anie.201309306
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    Full-Spectrum Photonic Pigments with Non-iridescent Structural Colors through Colloidal Assembly
    Park, Jin-Gyu; Kim, Shin-Hyun; Magkiriadou, Sofia; Choi, Tae Min; Kim, Young-Seok; Manoharan, Vinothan N.
    Angewandte Chemie, International Edition (2014), 53 (11), 2899-2903CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Structurally colored materials could potentially replace dyes and pigments in many applications, but it is challenging to fabricate structural colors that mimic the appearance of absorbing pigments. The authors demonstrate the microfluidic fabrication of photonic pigments consisting of microcapsules contg. dense amorphous packings of core-shell colloidal particles. These microcapsules show noniridescent structural colors that are independent of viewing angle, a crit. requirement for applications such as displays or coatings. The design of the microcapsules facilitates the suppression of incoherent and multiple scattering, enabling the fabrication of photonic pigments with colors spanning the visible spectrum. The authors' findings should provide new insights into the design and synthesis of materials with structural colors.
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    Lee, J. N.; Park, C.; Whitesides, G. M. Solvent Compatibility of Poly(Dimethylsiloxane)-Based Microfluidic Devices. Anal. Chem. 2003, 75, 6544– 6554,  DOI: 10.1021/ac0346712
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    Solvent Compatibility of Poly(dimethylsiloxane)-Based Microfluidic Devices
    Lee, Jessamine Ng; Park, Cheolmin; Whitesides, George M.
    Analytical Chemistry (2003), 75 (23), 6544-6554CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
    This paper describes the compatibility of poly(dimethylsiloxane) (PDMS) with org. solvents; this compatibility is important in considering the potential of PDMS-based microfluidic devices in a no. of applications, including that of microreactors for org. reactions. The three aspects of compatibility, the swelling of PDMS in a solvent, the partitioning of solutes between a solvent and PDMS, and the dissoln. of PDMS oligomers in a solvent are considered. Of these three parameters that det. the compatibility of PDMS with a solvent, the swelling of PDMS had the greatest effect. Exptl. measurements of swelling were correlated with the soly. parameter, δ (cal1/2 cm-3/2), which is based on the cohesive energy densities, c (cal/cm3), of the materials. Solvents that swelled PDMS the least included water, nitromethane, DMSO, ethylene glycol, perfluorotributylamine, perfluorodecalin, acetonitrile, and propylene carbonate; solvents that swelled PDMS the most were diisopropylamine, triethylamine, pentane, and xylenes. Highly swelling solvents were useful for extg. contaminants from bulk PDMS and for changing the surface properties of PDMS. The feasibility of performing org. reactions in PDMS was demonstrated by performing a Diels-Alder reaction in a microchannel.
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    Pietsch, W. Size Enlargement by Agglomeration. Handbook of Powder Science and Technology; Springer US: Boston, MA, 1997; pp 202– 377.
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    Bika, D. G.; Gentzler, M.; Michaels, J. N. Mechanical Properties of Agglomerates. Powder Technol. 2001, 117, 98– 112,  DOI: 10.1016/S0032-5910(01)00318-7
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    Mechanical properties of agglomerates
    Bika, D. G.; Gentzler, M.; Michaels, J. N.
    Powder Technology (2001), 117 (1-2), 98-112CODEN: POTEBX; ISSN:0032-5910. (Elsevier Science S.A.)
    The mech. properties of dry and wet agglomerates are reviewed in the context of continuum solid and fluid mechanics and fracture mechanics. The focus is on practical measurements of tensile strength, yield strength, hardness and fracture toughness, and how they define the attrition behavior of agglomerates. Well-established mech. testing methods can be applied to agglomerates, but certain limitations apply due to the nature of agglomerates being inherently non-equil. (glassy), anisotropic, and compressible. The mech. response of agglomerates may vary from brittle, elastic-plastic (for most dry agglomerates) to elastoviscoplastic and fully plastic (for wet agglomerates) depending on prepn. method, environment, structure and loading conditions. This transition from solid to liq.-like behavior can be followed by applying solid/fracture mechanics and rheol.-based testing, resp. It is clear that most available practical measures of agglomerate mech. behavior are not intrinsic, i.e. independent of test specimen geometry and the manner in which stress is applied. Therefore, selection and execution of measurements must be guided by loading conditions and agglomerate size and structure from the process of concern. Micromech. modeling addresses some of the dependence of mech. properties on the structure of agglomerates [e.g., porosity] and the properties of their primary constituents, but it cannot describe quant. bulk deformation and fracture of agglomerates. For this reason, agglomerate formulations are still tailored to achieve desired performance by empirical correlation of primary particle and agglomerate structure to mech. properties.
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    Pérez, R.; Gumbsch, P. Directional Anisotropy in the Cleavage Fracture of Silicon. Phys. Rev. Lett. 2000, 84, 5347– 5350,  DOI: 10.1103/PhysRevLett.84.5347
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    Directional Anisotropy in the Cleavage Fracture of Silicon
    Perez, Ruben; Gumbsch, Peter
    Physical Review Letters (2000), 84 (23), 5347-5350CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    Total-energy pseudopotential calcns. were used to study the cleavage anisotropy in Si. Cracks propagate easily on {111} and {110} planes provided crack propagation proceeds in the 〈110〉 direction. In contrast, if the crack is driven in a 〈001〉 direction on a {110} plane the bond breaking process is discontinuous and assocd. with pronounced relaxations of the surrounding atoms, which results in a large lattice trapping. The different lattice trapping for different crack propagation directions can explain the exptl. obsd. cleavage anisotropy in Si single crystals.
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    Hueckel, T.; Hocky, G. M.; Palacci, J.; Sacanna, S. Ionic Solids from Common Colloids. Nature 2020, 580, 487– 490,  DOI: 10.1038/s41586-020-2205-0
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    Ionic solids from common colloids
    Hueckel, Theodore; Hocky, Glen M.; Palacci, Jeremie; Sacanna, Stefano
    Nature (London, United Kingdom) (2020), 580 (7804), 487-490CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
    From rock salt to nanoparticle superlattices, complex structure can emerge from simple building blocks that attract each other through Coulombic forces1-4. On the micrometre scale, however, colloids in water defy the intuitively simple idea of forming crystals from oppositely charged partners, instead forming non-equil. structures such as clusters and gels5-7. Although various systems have been engineered to grow binary crystals8-11, native surface charge in aq. conditions has not been used to assemble cryst. materials. Here we form ionic colloidal crystals in water through an approach that we refer to as polymer-attenuated Coulombic self-assembly. The key to crystn. is the use of a neutral polymer to keep particles sepd. by well defined distances, allowing us to tune the attractive overlap of elec. double layers, directing particles to disperse, crystallize or become permanently fixed on demand. The nucleation and growth of macroscopic single crystals is demonstrated by using the Debye screening length to fine-tune assembly. Using a variety of colloidal particles and com. polymers, ionic colloidal crystals isostructural to caesium chloride, sodium chloride, aluminum diboride and K4C60 are selected according to particle size ratios. Once fixed by simply dilg. out soln. salts, crystals are pulled out of the water for further manipulation, demonstrating an accurate translation from soln.-phase assembly to dried solid structures. In contrast to other assembly approaches, in which particles must be carefully engineered to encode binding information12-18, polymer-attenuated Coulombic self-assembly enables conventional colloids to be used as model colloidal ions, primed for crystn.
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    Li, J. T.; Caldwell, K. D.; Rapoport, N. Surface Properties of Pluronic-Coated Polymeric Colloids. Langmuir 1994, 10, 4475– 4482,  DOI: 10.1021/la00024a016
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    Surface Properties of Pluronic-Coated Polymeric Colloids
    Li, Jenq-Thun; Caldwell, Karin D.; Rapoport, Natalya
    Langmuir (1994), 10 (12), 4475-82CODEN: LANGD5; ISSN:0743-7463.
    With the help of field-flow fractionation, photon correlation spectroscopy, and ESR techniques, as well as more conventional labeling approaches, we have accomplished the anal. characterization of the adsorption complexes formed between, on the one hand, a series of selected triblock Pluronic polymeric surfactants with comparable lengths of their central poly(propylene oxide) (PPO) block but with varying poly(ethylene oxide) (PEO) block lengths and, on the other, a series of differently sized polystyrene colloids. For a given surfactant, it was found that both surface concns. and adlayer thicknesses are strongly related to the particle size, such that smaller particles take up fewer polymer mols. per unit area than the larger ones. The reduced crowding around each PEO chain results in thinner adlayers and higher chain mobilities. In adsorption complexes involving 69-nm polystyrene particles, the adlayer thickness is close to the diam. calcd. for the free PEO chain in aq. soln. In addn., for particles of a given size, it is the size of the surfactant's hydrophobic center block (PPO), rather than its flanking tails (PEO), that dets. the surface concn. Thus, triblocks of similar PPO size showed comparable surface concn., while the longer PEO chains were assocd. with thicker adlayers as well as greater chain dynamics.
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    Pitto-Barry, A.; Barry, N. P. E. Pluronic® Block-Copolymers in Medicine: From Chemical and Biological Versatility to Rationalisation and Clinical Advances. Polym. Chem. 2014, 5, 3291– 3297,  DOI: 10.1039/C4PY00039K
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    Pluronic block-copolymers in medicine: from chemical and biological versatility to rationalization and clinical advances
    Pitto-Barry, Anais; Barry, Nicolas P. E.
    Polymer Chemistry (2014), 5 (10), 3291-3297CODEN: PCOHC2; ISSN:1759-9962. (Royal Society of Chemistry)
    A review. This mini-review highlights the latest advances in the chem. and biol. of Pluronic triblock copolymers. We focus on their applications in medicine, as drug delivery carriers, biol. response modifiers, and pharmaceutical ingredients. Examples of drug delivery systems and formulations currently in clin. use, clin. trials or preclin. development are highlighted. We also discuss the role that Pluronic copolymers may play in the innovative design of new nanomedicines in the near future.
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    Graczykowski, B.; Vogel, N.; Bley, K.; Butt, H. J.; Fytas, G. Multiband Hypersound Filtering in Two-Dimensional Colloidal Crystals: Adhesion, Resonances, and Periodicity. Nano Lett. 2020, 20, 1883– 1889,  DOI: 10.1021/acs.nanolett.9b05101
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    93
    Multiband Hypersound Filtering in Two-Dimensional Colloidal Crystals: Adhesion, Resonances, and Periodicity
    Graczykowski, Bartlomiej; Vogel, Nicolas; Bley, Karina; Butt, Hans-Jurgen; Fytas, George
    Nano Letters (2020), 20 (3), 1883-1889CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    The hypersonic phonon propagation in large-area two-dimensional colloidal crystals is probed by spontaneous micro Brillouin light scattering. The dispersion relation of thermally populated Lamb waves reveals multiband filtering due to three distinct types of acoustic band gaps. We find Bragg gaps accompanied by two types of hybridization gaps in both sub- and superwavelength regimes resulting from contact-based resonances and nanoparticle eigenmodes, resp. The operating GHz frequencies can be tuned by particle size and depend on the adhesion at the contact interfaces. The exptl. dispersion relations are well represented by a finite element method simulation, physicochem. enabling identification of obsd. modes. The presented approach also allows for contactless study of the contact stiffness of submicrometer particles, which reveals size effect deviating from macroscopic predictions.
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    Babacic, V.; Varghese, J.; Coy, E.; Kang, E.; Pochylski, M.; Gapinski, J.; Fytas, G.; Graczykowski, B. Mechanical Reinforcement of Polymer Colloidal Crystals by Supercritical Fluids. J. Colloid Interface Sci. 2020, 579, 786– 793,  DOI: 10.1016/j.jcis.2020.06.104
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    94
    Mechanical reinforcement of polymer colloidal crystals by supercritical fluids
    Babacic, Visnja; Varghese, Jeena; Coy, Emerson; Kang, Eunsoo; Pochylski, Mikolaj; Gapinski, Jacek; Fytas, George; Graczykowski, Bartlomiej
    Journal of Colloid and Interface Science (2020), 579 (), 786-793CODEN: JCISA5; ISSN:0021-9797. (Elsevier B.V.)
    Colloidal crystals realized by self-assembled polymer nanoparticles have prominent attraction as a platform for various applications from assembling photonic and phononic crystals, acoustic metamaterials to coating applications. However, the fragility of these systems limits their application horizon. In this work the uniform mech. reinforcement and tunability of 3D polystyrene colloidal crystals by means of cold soldering are reported. This structural strengthening is achieved by high pressure gas (N2 or Ar) plasticization at temps. well below the glass transition. Brillouin light scattering is employed to monitor in-situ the mech. vibrations of the crystal and thereby det. preferential pressure, temp. and time ranges for soldering, i.e. formation of phys. bonding among the nanoparticles while maintaining the shape and translational order. This low-cost method is potentially useful for fabrication and tuning of durable devices including applications in photonics, phononics, acoustic metamaterials, optomechanics, surface coatings and nanolithog.
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This article is cited by 5 publications.

  1. Yu Cang, Rebecca Sainidou, Pascal Rembert, Giulia Magnabosco, Tim Still, Nicolas Vogel, Bartlomiej Graczykowski, George Fytas. Origin of the Acoustic Bandgaps in Hypersonic Colloidal Phononics: The Role of the Elastic Impedance. The Journal of Physical Chemistry B 2022, 126 (34) , 6575-6584. https://doi.org/10.1021/acs.jpcb.2c03923
  2. Umair Sultan, Alexander Götz, Carola Schlumberger, Dominik Drobek, Gudrun Bleyer, Teresa Walter, Erik Löwer, Urs Alexander Peuker, Matthias Thommes, Erdmann Spiecker, Benjamin Apeleo Zubiri, Alexandra Inayat, Nicolas Vogel. From Meso to Macro: Controlling Hierarchical Porosity in Supraparticle Powders. Small 2023, 134 , 2300241. https://doi.org/10.1002/smll.202300241
  3. Jeena Varghese, Reza Mohammadi, Mikolaj Pochylski, Visnja Babacic, Jacek Gapinski, Nicolas Vogel, Hans-Juergen Butt, George Fytas, Bartlomiej Graczykowski. Size-dependent nanoscale soldering of polystyrene colloidal crystals by supercritical fluids. Journal of Colloid and Interface Science 2023, 633 , 314-322. https://doi.org/10.1016/j.jcis.2022.11.090
  4. Sarah Wenderoth, Gudrun Bleyer, Jakob Endres, Johannes Prieschl, Nicolas Vogel, Susanne Wintzheimer, Karl Mandel. Spray‐Dried Photonic Balls with a Disordered/Ordered Hybrid Structure for Shear‐Stress Indication. Small 2022, 18 (48) , 2203068. https://doi.org/10.1002/smll.202203068
  5. Sarah Wenderoth, Andreas Eigen, Susanne Wintzheimer, Johannes Prieschl, Andreas Hirsch, Marcus Halik, Karl Mandel. Supraparticles with a Mechanically Triggerable Color‐Change‐Effect to Equip Coatings with the Ability to Report Damage. Small 2022, 18 (15) , 2107513. https://doi.org/10.1002/smll.202107513
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  • Abstract

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    Figure 1

    Figure 1. Emulsion-templated fabrication of supraparticles. (a) Schematic illustration of the supraparticle self-assembly in an emulsion droplet and the role of surfactants. (b) Nonionic perfluoropolyether-block- (polyethylene glycol-block-polypropylene glycol-block-polyethylene glycol)-block perfluoropolyether surfactant (PPP) used in supraparticle fabrication. (c) Anionic Krytox surfactant used in supraparticle fabrication. (d) Monodispersed water in oil droplets (gray) encapsulating polystyrene colloidal particles produced by the microfluidic device. (e, f) Colloidal supraparticles with spherical shape and ordered surface structure, made with PPP and Krytox surfactant, respectively. (g) Colloidal supraparticle under a nanoindenter tip.

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    Figure 2

    Figure 2. Supraparticles with nonionic PPP surfactant exhibit ductile mechanical properties and high stability from favorable surfactant-particle interactions. (a) Supraparticles formed by polystyrene primary particles and the nonionic PPP surfactant show a reproducible compression behavior with ductile fracture. Multiple, defined plateaus indicate multiple crack formation events without breakage of the supraparticle. (b–d) SEM investigation of the supraparticle surface and morphology. (b) Supraparticle after compression shows several large cracks at the periphery but remains as a single piece. (c) Close up of a deformed supraparticle. Individual primary particles are heavily deformed and solid material bridging neighboring particles is seen. (d) Rig-like residues at the primary particle surface indicate the formation of solid deposits at the contact points between primary particles.

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    Figure 3

    Figure 3. Supraparticles with Krytox surfactant exhibit brittle mechanical properties and low stability from unfavorable surfactant-particle interactions. (a) Supraparticles formed by primary polystyrene particles and the anionic Krytox surfactant show brittle fracture with reduced fracture stress and strain. (b) Supraparticles fracture in a brittle way without visible deformation of the individual primary particles. (c) Surfactant traces reside on primary particle surface as tiny spherical dots.

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    Figure 4

    Figure 4. Hypothesis on the influence of surfactant in supraparticle mechanical stability. (a) Nonionic PPP surfactant has a PPO-PEO-PPO block that adsorbs to polystyrene surface due to hydrophobic interactions, the polymer chains in the PFPE blocks from different surfactant molecules are entangled. The PFPE polymer chains are viscous at room temperature and promote adhesion between neighboring particles. (b) Anionic Krytox surfactant has a carboxyl group that deprotonates in ambient humidity to carry a negative charge, which causes an electrostatic repulsion with the negatively charged particle surface. The PFPE block also dewets the particle surface. The surfactant molecule resides at the surface without providing extra adhesive forces. The sketch is not drawn to scale.

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    Figure 5

    Figure 5. Brillouin light spectroscopy (BLS) spectra (circles) of a colloidal crystal of polystyrene primary particles in (a) normal I(f) and (b) normalized I(f)·f2 presentations. The spectra show the as prepared, dry colloidal crystal film (gray), as well as the film infiltrated with HFE oil containing 0.1% nonionic PPP surfactant (red), pure HFE oil (green), and HFE oil containing 0.1% anionic Krytox surfactant (purple). Dashed lines show for Gaussian line shape fits. Inset to (a): The single peak BLS spectrum recorded in a backscattering geometry shows the same film infiltrated only with PPP. (c) SEM images of the colloidal crystal film as fabricated. Scale bars in (c) are 5 μm (upper) and 200 nm (lower panel).

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    Figure 6

    Figure 6. BLS spectra of supraparticles fabricated in two different surfactants. The red and purple curves correspond to crystalline and disordered supraparticles using Krytox surfactant. The interaction-induced split of the (1,2) mode is represented by two Gaussian peaks (dashed lines). The green curve corresponds to the supraparticle using the nonionic PPP surfactant. Notably, no particle vibrations are resolved, and the BLS spectrum shows signatures characteristic of two propagating acoustic phonons.

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  • References

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      Self-assembly of anisotropic nanoparticles into functional superstructures
      Deng, Kerong; Luo, Zhishan; Tan, Li; Quan, Zewei
      Chemical Society Reviews (2020), 49 (16), 6002-6038CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      A review. Self-assembly of colloidal nanoparticles (NPs) into superstructures offers a flexible and promising pathway to manipulate the nanometer-sized particles and thus make full use of their unique properties. This bottom-up strategy builds a bridge between the NP regime and a new class of transformative materials across multiple length scales for technol. applications. In this field, anisotropic NPs with size- and shape-dependent phys. properties as self-assembly building blocks have long fascinated scientists. Self-assembly of anisotropic NPs not only opens up exciting opportunities to engineer a variety of intriguing and complex superlattice architectures, but also provides access to discover emergent collective properties that stem from their ordered arrangement. Thus, this has stimulated enormous research interests in both fundamental science and technol. applications. This present review comprehensively summarizes the latest advances in this area, and highlights their rich packing behaviors from the viewpoint of NP shape. We provide the basics of the exptl. techniques to produce NP superstructures and structural characterization tools, and detail the delicate assembled structures. Then the current understanding of the assembly dynamics is discussed with the assistance of in situ studies, followed by emergent collective properties from these NP assemblies. Finally, we end this article with the remaining challenges and outlook, hoping to encourage further research in this field.
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    13. 13
      Liu, W.; Midya, J.; Kappl, M.; Butt, H.-J.; Nikoubashman, A. Segregation in Drying Binary Colloidal Droplets. ACS Nano 2019, 13, 4972– 4979,  DOI: 10.1021/acsnano.9b00459
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      13
      Segregation in Drying Binary Colloidal Droplets
      Liu, Wendong; Midya, Jiarul; Kappl, Michael; Butt, Hans-Juergen; Nikoubashman, Arash
      ACS Nano (2019), 13 (5), 4972-4979CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      When a colloidal suspension droplet evaps. from a solid surface, it leaves a characteristic deposit in the contact region. These deposits are common and important for many applications in printing, coating, or washing. By the use of superamphiphobic surfaces as a substrate, the contact area can be reduced so that evapn. is almost radially sym. While drying, the droplets maintain a nearly perfect spherical shape. Here, we exploit this phenomenon to fabricate supraparticles from bidisperse colloidal aq. suspensions. The supraparticles have a core-shell morphol. The outer region is predominantly occupied by small colloids, forming a close-packed cryst. structure. Toward the center, the no. of large colloids increases and they are packed amorphously. The extent of this stratification decreases with decreasing the evapn. rate. Complementary simulations indicate that evapn. leads to a local increase in d., which, in turn, exerts stronger inward forces on the larger colloids. A comparison between expts. and simulations suggest that hydrodynamic interactions between the suspended colloids reduce the extent of stratification. Our findings are relevant for the fabrication of supraparticles for applications in the fields of chromatog., catalysis, drug delivery, photonics, and a better understanding of spray-drying.
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    14. 14
      Zellmer, S.; Garnweitner, G.; Breinlinger, T.; Kraft, T.; Schilde, C. Hierarchical Structure Formation of Nanoparticulate Spray-Dried Composite Aggregates. ACS Nano 2015, 9, 10749– 10757,  DOI: 10.1021/acsnano.5b05220
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      14
      Hierarchical Structure Formation of Nanoparticulate Spray-Dried Composite Aggregates
      Zellmer, Sabrina; Garnweitner, Georg; Breinlinger, Thomas; Kraft, Torsten; Schilde, Carsten
      ACS Nano (2015), 9 (11), 10749-10757CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      The design of hierarchically structured nano- and microparticles of different sizes, porosities, surface areas, compns., and internal structures from nanoparticle building blocks is important for new or enhanced application properties of high-quality products in a variety of industries. Spray-drying processes are well-suited for the design of hierarchical structures of multicomponent products. This structure design using various nanoparticles as building blocks is one of the most important challenges for the future to create products with optimized or completely new properties. Furthermore, the transfer of designed nanomaterials to large-scale products with favorable handling and processing can be achieved. The resultant aggregate structure depends on the utilized nanoparticle building blocks as well as on a large no. of process and formulation parameters. In this study, structure formation and segregation phenomena during the spray drying process were investigated to enable the synthesis of tailor-made nanostructures with defined properties. Moreover, a theor. model of this segregation and structure formation in nanosuspensions is presented using a discrete element method simulation.
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    15. 15
      Zhao, Y.; Shang, L.; Cheng, Y.; Gu, Z. Spherical Colloidal Photonic Crystals. Acc. Chem. Res. 2014, 47, 3632– 3642,  DOI: 10.1021/ar500317s
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      15
      Spherical Colloidal Photonic Crystals
      Zhao, Yuanjin; Shang, Luoran; Cheng, Yao; Gu, Zhongze
      Accounts of Chemical Research (2014), 47 (12), 3632-3642CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
      A review. The authors outline various types of spherical colloidal PhCs, such as close-packed, nonclose-packed, inverse opal, biphasic or multiphasic Janus structured, and core-shell structured geometries. Based on their unique optical properties, applications of the spherical colloidal PhCs for displays, sensors, barcodes, and cell culture microcarriers are presented. Future developments of the spherical colloidal PhC materials are also envisioned.
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    16. 16
      Goerlitzer, E. S. A.; Klupp Taylor, R. N.; Vogel, N. Bioinspired Photonic Pigments from Colloidal Self-Assembly. Adv. Mater. 2018, 30, 1706654,  DOI: 10.1002/adma.201706654
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      There is no corresponding record for this reference.
    17. 17
      Xiao, M.; Hu, Z.; Wang, Z.; Li, Y.; Tormo, A. D.; Le Thomas, N.; Wang, B.; Gianneschi, N. C.; Shawkey, M. D.; Dhinojwala, A. Bioinspired Bright Noniridescent Photonic Melanin Supraballs. Sci. Adv. 2017, 3, e1701151,  DOI: 10.1126/sciadv.1701151
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      17
      Bioinspired bright noniridescent photonic melanin supraballs
      Xiao, Ming; Hu, Ziying; Wang, Zhao; Li, Yiwen; Tormo, Alejandro Diaz; Le Thomas, Nicolas; Wang, Boxiang; Gianneschi, Nathan C.; Shawkey, Matthew D.; Dhinojwala, Ali
      Science Advances (2017), 3 (9), e1701151/1-e1701151/7CODEN: SACDAF; ISSN:2375-2548. (American Association for the Advancement of Science)
      Structural colors enable the creation of a spectrum of nonfading colors without pigments, potentially replacing toxic metal oxides and conjugated org. pigments. However, significant challenges remain to achieve the contrast needed for a complete gamut of colors and a scalable process for industrial application. We demonstrate a feasible soln. for producing structural colors inspired by bird feathers. We have designed core-shell nanoparticles using high-refractive index (RI) (∼1.74)melanin cores and low-RI (∼1.45) silica shells. The design of these nanoparticles was guided by finite-difference time-domain simulations. These nanoparticles were self-assembled using a one-pot reverse emulsion process, which resulted in bright and noniridescent supraballs. With the combination of only two ingredients, synthetic melanin and silica, we can generate a full spectrum of colors. These supraballs could be directly added to paints, plastics, and coatings and also used as UV-resistant inks or cosmetics.
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    18. 18
      Song, D. P.; Zhao, T. H.; Guidetti, G.; Vignolini, S.; Parker, R. M. Hierarchical Photonic Pigments via the Confined Self-Assembly of Bottlebrush Block Copolymers. ACS Nano 2019, 13, 1764– 1771,  DOI: 10.1021/acsnano.8b07845
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      18
      Hierarchical Photonic Pigments via the Confined Self-Assembly of Bottlebrush Block Copolymers
      Song, Dong-Po; Zhao, Tianheng H.; Guidetti, Giulia; Vignolini, Silvia; Parker, Richard M.
      ACS Nano (2019), 13 (2), 1764-1771CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Hierarchical, structurally colored materials offer a wide variety of visual effects that cannot be achieved with std. pigments or dyes. However, their fabrication requires simultaneous control over multiple length-scales. Here we introduce a robust strategy for the fabrication of hierarchical photonic pigments via the confined self-assembly of bottlebrush block copolymers within emulsified microdroplets. The bottlebrush block copolymer self-assembles into highly ordered concentric lamellae, giving rise to a near perfect photonic multilayer in the solid state, with reflectivity up to 100%. The reflected color can be readily tuned across the whole visible spectrum by either altering the mol. wt. or by blending the bottlebrush block copolymers. Furthermore, the developed photonic pigments are responsive, with a selective and reversible color change obsd. upon swelling in different solvents. Our system is particularly suited for the scalable prodn. of photonic pigments, arising from their rapid self-assembly mechanism and size-independent color.
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    19. 19
      Choi, T. M.; Lee, G. H.; Kim, Y. S.; Park, J. G.; Hwang, H.; Kim, S. H. Photonic Microcapsules Containing Single-Crystal Colloidal Arrays with Optical Anisotropy. Adv. Mater. 2019, 31, 1900693,  DOI: 10.1002/adma.201900693
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    20. 20
      Kim, S.-H.; Park, J.-G.; Choi, T. M.; Manoharan, V. N.; Weitz, D. A. Osmotic-Pressure-Controlled Concentration of Colloidal Particles in Thin-Shelled Capsules. Nat. Commun. 2014, 5, 3068,  DOI: 10.1038/ncomms4068
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      Osmotic-pressure-controlled concentration of colloidal particles in thin-shelled capsules
      Kim Shin-Hyun; Park Jin-Gyu; Manoharan Vinothan N; Weitz David A; Choi Tae Min
      Nature communications (2014), 5 (), 3068 ISSN:.
      Colloidal crystals are promising structures for photonic applications requiring dynamic control over optical properties. However, for ease of processing and reconfigurability, the crystals should be encapsulated to form 'ink' capsules rather than confined in a thin film. Here we demonstrate a class of encapsulated colloidal photonic structures whose optical properties can be controlled through osmotic pressure. The ordering and separation of the particles within the microfluidically created capsules can be tuned by changing the colloidal concentration through osmotic pressure-induced control of the size of the individual capsules, modulating photonic stop band. The rubber capsules exhibit a reversible change in the diffracted colour, depending on osmotic pressure, a property we call osmochromaticity. The high encapsulation efficiency and capsule uniformity of this microfluidic approach, combined with the highly reconfigurable shapes and the broad control over photonic properties, make this class of structures particularly suitable for photonic applications such as electronic inks and reflective displays.
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    21. 21
      Vogel, N.; Utech, S.; England, G. T.; Shirman, T.; Phillips, K. R.; Koay, N.; Burgess, I. B.; Kolle, M.; Weitz, D. A.; Aizenberg, J. Color from Hierarchy: Diverse Optical Properties of Micron-Sized Spherical Colloidal Assemblies. Proc. Natl. Acad. Sci. U. S. A. 2015, 112, 10845– 10850,  DOI: 10.1073/pnas.1506272112
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      21
      Color from hierarchy: Diverse optical properties of micron-sized spherical colloidal assemblies
      Vogel, Nicolas; Utech, Stefanie; England, Grant T.; Shirman, Tanya; Phillips, Katherine R.; Koay, Natalie; Burgess, Ian B.; Kolle, Mathias; Weitz, David A.; Aizenberg, Joanna
      Proceedings of the National Academy of Sciences of the United States of America (2015), 112 (35), 10845-10850CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
      Materials are characterized by structural order over multiple length scales have evolved for max. performance and multifunctionality, and are often produced by self-assembly processes. A striking example of this design principle is structural coloration, where interference, diffraction, and absorption effects result in vivid colors. Mimicking this emergence of complex effects from simple building blocks is a key challenge for man-made materials. Here, a simple confined self-assembly process leads to a complex hierarchical geometry that displays a variety of optical effects. Colloidal crystn. in an emulsion droplet creates micron-sized superstructures, termed photonic balls. The curvature imposed by the emulsion droplet leads to frustrated crystn. The authors observe spherical colloidal crystals with ordered, cryst. layers and a disordered core. This geometry produces multiple optical effects. The ordered layers give rise to structural color from Bragg diffraction with limited angular dependence and unusual transmission due to the curved nature of the individual crystals. The disordered core contributes nonresonant scattering that induces a macroscopically whitish appearance, which the authors mitigate by incorporating absorbing Au nanoparticles that suppress scattering and macroscopically purify the color. With increasing size of the constituent colloidal particles, grating diffraction effects dominate, which result from order along the crystal's curved surface and induce a vivid polychromatic appearance. The control of multiple optical effects induced by the hierarchical morphol. in photonic balls paves the way to use them as building blocks for complex optical assemblies-potentially as more efficient mimics of structural color as it occurs.
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    22. 22
      Park, J. G.; Kim, S. H.; Magkiriadou, S.; Choi, T. M.; Kim, Y. S.; Manoharan, V. N. Full-Spectrum Photonic Pigments with Non-Iridescent Structural Colors through Colloidal Assembly. Angew. Chem., Int. Ed. 2014, 53, 2899– 2903,  DOI: 10.1002/anie.201309306
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      22
      Full-Spectrum Photonic Pigments with Non-iridescent Structural Colors through Colloidal Assembly
      Park, Jin-Gyu; Kim, Shin-Hyun; Magkiriadou, Sofia; Choi, Tae Min; Kim, Young-Seok; Manoharan, Vinothan N.
      Angewandte Chemie, International Edition (2014), 53 (11), 2899-2903CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Structurally colored materials could potentially replace dyes and pigments in many applications, but it is challenging to fabricate structural colors that mimic the appearance of absorbing pigments. The authors demonstrate the microfluidic fabrication of photonic pigments consisting of microcapsules contg. dense amorphous packings of core-shell colloidal particles. These microcapsules show noniridescent structural colors that are independent of viewing angle, a crit. requirement for applications such as displays or coatings. The design of the microcapsules facilitates the suppression of incoherent and multiple scattering, enabling the fabrication of photonic pigments with colors spanning the visible spectrum. The authors' findings should provide new insights into the design and synthesis of materials with structural colors.
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    23. 23
      Klein, S. M.; Manoharan, V. N.; Pine, D. J.; Lange, F. F. Synthesis of Spherical Polymer and Titania Photonic Crystallites. Langmuir 2005, 21, 6669– 6674,  DOI: 10.1021/la0469957
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      23
      Synthesis of Spherical Polymer and Titania Photonic Crystallites
      Klein, Sascha M.; Manoharan, Vinothan N.; Pine, David J.; Lange, Fred F.
      Langmuir (2005), 21 (15), 6669-6674CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      The fabrication of small structured spherical particles that are essentially small photonic crystals is described. The particles are 1-50 μm in diam. and are porous with nearly close-packed monodisperse pores whose size is comparable to the wavelength of light. The solid matrix of the particles is titania, which provides a large refractive index contrast between the particle matrix and pores. The particles are made by encapsulating polymer colloidal particles in emulsion droplets of hexanes in which a titanium alkoxide precursor is dissolved. Subsequent osmotic removal of the hexanes from the droplets and condensation of the alkoxide precursor leads to spherical aggregates of polymer spheres with titania filling the spaces between the polymer spheres. The polymer particles are then burned out leaving behind the desired porous titania particles. The size and structure of the pores and high refractive index of the titania matrix are expected to produce particles that are very efficient scatterers of light, making them useful as pigments.
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    24. 24
      Wang, J.; Sultan, U.; Goerlitzer, E. S. A.; Mbah, C. F.; Engel, M.; Vogel, N. Structural Color of Colloidal Clusters as a Tool to Investigate Structure and Dynamics. Adv. Funct. Mater. 2020, 30, 1907730,  DOI: 10.1002/adfm.201907730
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      24
      Structural Color of Colloidal Clusters as a Tool to Investigate Structure and Dynamics
      Wang, Junwei; Sultan, Umair; Goerlitzer, Eric S. A.; Mbah, Chrameh Fru; Engel, Michael; Vogel, Nicolas
      Advanced Functional Materials (2020), 30 (26), 1907730CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Colloidal assemblies have applications as photonic crystals and templates for functional porous materials. While there has been significant progress in controlling colloidal assemblies into defined structures, their 3D order remains difficult to characterize. Simple, low-cost techniques are sought that characterize colloidal structures and assist optimization of process parameters. Here, structural color is presented to image the structure and dynamics of colloidal clusters prepd. by a confined self-assembly process in emulsion droplets. It is shown that characteristic anisotropic structural color motifs such as circles, stripes, triangles, or bowties arise from the defined interior grain geometry of such colloidal clusters. The optical detection of these motifs reliably distinguishes icosahedral, decahedral, and face-centered cubic colloidal clusters and thus enables a simple yet precise characterization of their internal structure. In addn., the rotational motion and dynamics of such micrometer-scale clusters suspended in a liq. can be followed in real time via their anisotropic coloration. Finally, monitoring the evolution of structural color provides real-time information about the crystn. pathway within the confining emulsion droplet. Together, this work demonstrates that structural color is a simple and versatile tool to characterize the structure and dynamic properties of colloidal clusters.
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    25. 25
      Clough, J. M.; Guimard, E.; Rivet, C.; Sprakel, J.; Kodger, T. E. Photonic Paints: Structural Pigments Combined with Water-Based Polymeric Film-Formers for Structurally Colored Coatings. Adv. Opt. Mater. 2019, 7, 1900218,  DOI: 10.1002/adom.201900218
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      There is no corresponding record for this reference.
    26. 26
      Zheng, F.; Cheng, Y.; Wang, J.; Lu, J.; Zhang, B.; Zhao, Y.; Gu, Z. Aptamer-Functionalized Barcode Particles for the Capture and Detection of Multiple Types of Circulating Tumor Cells. Adv. Mater. 2014, 26, 7333– 7338,  DOI: 10.1002/adma.201403530
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      26
      Aptamer-Functionalized Barcode Particles for the Capture and Detection of Multiple Types of Circulating Tumor Cells
      Zheng, Fuyin; Cheng, Yao; Wang, Jie; Lu, Jie; Zhang, Bin; Zhao, Yuanjin; Gu, Zhongze
      Advanced Materials (Weinheim, Germany) (2014), 26 (43), 7333-7338CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      We developed a novel barcode-particle technol. that can simultaneously capture, detect and release multiple types of CTCs from a complex sample.
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      Liu, W.; Kappl, M.; Butt, H. J. Tuning the Porosity of Supraparticles. ACS Nano 2019, 13, 13949– 13956,  DOI: 10.1021/acsnano.9b05673
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      27
      Tuning the Porosity of Supraparticles
      Liu, Wendong; Kappl, Michael; Butt, Hans-Juergen
      ACS Nano (2019), 13 (12), 13949-13956CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Supraparticles consisting of nano- or microparticles have potential applications as, for example, photonic crystals, drug carriers, or heterogeneous catalysts. To avoid the use of solvent or processing liq., one can make supraparticles by evapg. droplets of aq. suspensions from super-liq.-repellent surfaces. Herein, a method to adjust the porosity of supraparticles is described; a high porosity is desired, for example, in catalysis. To prep. highly porous TiO2 supraparticles, polymer nanoparticles are co-dispersed in the suspension. Supraparticles are formed through evapn. of aq. suspension droplets on superamphiphobic surfaces followed by calcination of the sacrificial polymer particles. The increase of porosity of up to 92% resulted in enhanced photocatalytic activity while maintaining sufficient mech. stability.
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    28. 28
      Hou, K.; Han, J.; Tang, Z. Formation of Supraparticles and Their Application in Catalysis. ACS Mater. Lett. 2020, 2, 95– 106,  DOI: 10.1021/acsmaterialslett.9b00446
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      28
      Formation of Supraparticles and Their Application in Catalysis
      Hou, Ke; Han, Jianyu; Tang, Zhiyong
      ACS Materials Letters (2020), 2 (1), 95-106CODEN: AMLCEF; ISSN:2639-4979. (American Chemical Society)
      A review. Monodisperse supraparticles has been an emerging hot research topic in the fields of chem., materials science and nanotechnol. in the past several years. Great attention has been paid to construct monodisperse supraparticles with different sizes, morphologies, and compns. Their distinctive hierarchical structures and collective properties from various building blocks offer huge benefits towards catalyzing the reactions. In this review, the authors firstly outline the recent progress on the formation of supraparticles. Afterwards, their applications in photocatalysis, electrocatalysis and thermal catalysis are summarized. At last, the prospect in this research area is discussed.
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    29. 29
      Egly, S.; Fröhlich, C.; Vogel, S.; Gruenewald, A.; Wang, J.; Detsch, R.; Boccaccini, A. R.; Vogel, N. Bottom-Up Assembly of Silica and Bioactive Glass Supraparticles with Tunable Hierarchical Porosity. Langmuir 2018, 34, 2063– 2072,  DOI: 10.1021/acs.langmuir.7b03904
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      29
      Bottom-Up Assembly of Silica and Bioactive Glass Supraparticles with Tunable Hierarchical Porosity
      Egly, Steffen; Froehlich, Christina; Vogel, Stefanie; Gruenewald, Alina; Wang, Junwei; Detsch, Rainer; Boccaccini, Aldo R.; Vogel, Nicolas
      Langmuir (2018), 34 (5), 2063-2072CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      The authors study the formation of spherical supraparticles with controlled and tunable porosity on the nanometer and micrometer scales using the self-organization of a binary mixt. of small (nanometer scale) oxidic particles with large (micrometer scale) polystyrene particles in the confinement of an emulsion droplet. The external confinement dets. the final, spherical structure of the hybrid assembly, while the small particles form the matrix material. The large particles act as templating porogens to create micropores after combustion at elevated temps. The authors control the pore sizes on the micrometer scale by varying the size of the coassembled polystyrene microspheres and produce supraparticles from both SiO2- and Ca-contg. CaO/SiO2 particles. Although porous supraparticles are obtained in both cases, the presence of Ca ions substantially complicated the fabrication process since the increased ionic strength of the dispersion compromises the colloidal stability during the assembly process. The authors minimized these stability issues via the addn. of a steric stabilizing agent and by mixing bioactive and SiO2 colloidal particles. The authors studied the interaction of the porous particles with bone marrow stromal cells and found an increase in cell attachment with increasing pore size of the self-assembled supraparticles.
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    30. 30
      Canziani, H.; Chiera, S.; Schuffenhauer, T.; Kopp, S.; Metzger, F.; Bück, A.; Schmidt, M.; Vogel, N. Bottom-Up Design of Composite Supraparticles for Powder-Based Additive Manufacturing. Small 2020, 16, 2002076,  DOI: 10.1002/smll.202002076
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      30
      Bottom-Up Design of Composite Supraparticles for Powder-Based Additive Manufacturing
      Canziani, Herbert; Chiera, Salvatore; Schuffenhauer, Thomas; Kopp, Sebastian-Paul; Metzger, Florian; Bueck, Andreas; Schmidt, Michael; Vogel, Nicolas
      Small (2020), 16 (30), 2002076CODEN: SMALBC; ISSN:1613-6810. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Additive manufg. promises high flexibility and customized product design. Powder bed fusion processes use a laser to melt a polymer powder at predefined locations and iterate the scheme to build 3D objects. The design of flowable powders is a crit. parameter for a successful fabrication process that currently limits the choice of available materials. Here, a bottom-up process is introduced to fabricate tailored polymer- and composite supraparticles for powder-based additive manufg. processes by controlled aggregation of colloidal primary particles. These supraparticles exhibit a near-spherical shape and tailored compn., morphol., and surface roughness. These parameters can be precisely controlled by the mixing and size ratio of the primary particles. Polystyrene/silica composite particles are chosen as a model system to establish structure-property relations connecting shape, morphol., and surface roughness to the adhesion within the powder, which is accessed by tensile strength measurements. The adhesive properties are then connected to powder flowability and it is shown that the resulting powders allow the formation of dense powder films with uniform coverage. Finally, successful powder bed fusion is demonstrated by producing macroscopic single layer specimens with uniform distribution of nanoscale silica additives.
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    31. 31
      Nam, S. K.; Kim, J. B.; Han, S. H.; Kim, S.-H. Photonic Janus Balls with Controlled Magnetic Moment and Density Asymmetry. ACS Nano 2020, 14, 15714– 15722,  DOI: 10.1021/acsnano.0c06672
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      31
      Photonic Janus Balls with Controlled Magnetic Moment and Density Asymmetry
      Nam, Seong Kyeong; Kim, Jong Bin; Han, Sang Hoon; Kim, Shin-Hyun
      ACS Nano (2020), 14 (11), 15714-15722CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Colloidal crystals show structural colors through wavelength-selective diffraction at photonic stopbands. Here, we design photonic Janus balls with a controlled magnetic moment for programmable structural color switching. The Janus balls are produced from microfluidically produced paired drops of two distinct photocurable resins. The lighter resin contains magnetic nanoparticles and carbon black, whereas heavier one contains silica particles at a high vol. fraction. The paired drops spontaneously align vertically due to the d. asymmetry. The magnetic moment is assigned in the vertically aligned drops by aligning magnetic nanoparticles with an external field and capturing them through photopolymn. Silica particles in the heavier compartment spontaneously form cryst. arrays due to interparticle repulsion, developing structural colors. The resulting photonic Janus balls vertically align without an external field, like a roly-poly toy, so that carbon-black-laden compartments face upward. With an external magnetic field, the Janus balls align their magnetic moment to the field and display structural colors. Importantly, the direction of the magnetic moment is set by the direction of the external field during photopolymn., which enables the simultaneous manipulation of orientations of distinct photonic Janus balls in a programmed manner. These photonic Janus balls are potentially useful as active color inks for anti-counterfeiting tags.
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    32. 32
      Schneider, M.; Tschöpe, A.; Hanselmann, D.; Ballweg, T.; Gellermann, C.; Franzreb, M.; Mandel, K. Adsorber Particles with Magnetically-Supported Improved Electrochemical Conversion Behavior for Waste Water Treatment Processes. Part. Part. Syst. Charact. 2020, 37, 1900487,  DOI: 10.1002/ppsc.201900487
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      Adsorber Particles with Magnetically-Supported Improved Electrochemical Conversion Behavior for Waste Water Treatment Processes
      Schneider, Michael; Tschoepe, Andre; Hanselmann, Doris; Ballweg, Thomas; Gellermann, Carsten; Franzreb, Matthias; Mandel, Karl
      Particle & Particle Systems Characterization (2020), 37 (2), 1900487CODEN: PPCHEZ; ISSN:1521-4117. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Micron-sized supraparticles, consisting of a plurality of discrete nano- and microscale functional units, are assembled and fused by means of a droplet extrusion process. By combining nano magnetite, activated carbon, and conductive carbon with a polymeric binder matrix, particles are obtained which unite good magnetic properties, elec. cond., and adsorber activity through the high accessible surface area of the incorporated activated carbon of about 570 m2 g-1, thereby enabling a new approach toward sustainable water treatment processes. Due to the interplay of the components, it is possible to adsorb target substances, dissolved in the water which is demonstrated by the adsorption of the model dye methylene blue. A very fast adsorption kinetic and an adsorption capacity of about 400 mg g-1 is detd. By using the developed composite particles, it is also possible to electrochem. alter substances flowing through a magnetically-stabilized fluidized-bed reactor by electrochem. charging/discharging, significantly supported by the magnetic field enabling alternatingly optimum mobility/adsorption phases with contact/charging intervals. The electrochem. conversion can be increased up to 151% depending on the applied flow-rate and elec. voltage. By applying an external magnetic field, a further increase of electrochem. conversion of up to 70% can be obsd.
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    33. 33
      Miller, F.; Wintzheimer, S.; Prieschl, J.; Strauss, V.; Mandel, K. A Supraparticle-Based Five-Level-Identification Tag That Switches Information Upon Readout. Adv. Opt. Mater. 2021, 9, 2001972,  DOI: 10.1002/adom.202001972
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      A Supraparticle-Based Five-Level-Identification Tag That Switches Information Upon Readout
      Miller, Franziska; Wintzheimer, Susanne; Prieschl, Johannes; Strauss, Volker; Mandel, Karl
      Advanced Optical Materials (2021), 9 (4), 2001972CODEN: AOMDAX; ISSN:2195-1071. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Product identification tags are of great importance in a globalized world with increasingly complex trading routes and networks. Beyond currently used coding strategies, such as QR codes, higher data d., flexible application as well as miniaturization and readout indication are longed for in the next generation of security tags. In this work, micron-sized supraparticles (SPs) with encoded information (ID) are produced that not only exhibit multiple initially covert identification levels but are also irreversibly marked as "read" upon readout. To achieve this, lanthanide doped CaF2 nanoparticles are assembled in various quantity-weighted ratios via spray-drying in presence of a broad-spectrum stealth fluorophore (StFl), yielding covert spectrally encoded ID-SPs. Using these as pigments, QR codes, initially dominated by the green fluorescence of the StFl, could be generated. Upon thermal energy input, these particle-based tags irreversibly switch to an activated state revealing not only multiple luminescent colors but also spectral IDs. This strategy provides the next generation of material-based security tags with a high data d. and security level that switch information upon readout and can be, therefore, used as seal of quality.
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    34. 34
      Müssig, S.; Granath, T.; Schembri, T.; Fidler, F.; Haddad, D.; Hiller, K.-H.; Wintzheimer, S.; Mandel, K. Anisotropic Magnetic Supraparticles with a Magnetic Particle Spectroscopy Fingerprint as Indicators for Cold-Chain Breach. ACS Appl. Nano Mater. 2019, 2, 4698– 4702,  DOI: 10.1021/acsanm.9b00977
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      34
      Anisotropic Magnetic Supraparticles with a Magnetic Particle Spectroscopy Fingerprint as Indicators for Cold-Chain Breach
      Muessig, Stephan; Granath, Tim; Schembri, Tim; Fidler, Florian; Haddad, Daniel; Hiller, Karl-Heinz; Wintzheimer, Susanne; Mandel, Karl
      ACS Applied Nano Materials (2019), 2 (8), 4698-4702CODEN: AANMF6; ISSN:2574-0970. (American Chemical Society)
      Magnetic particle spectroscopy (MPS) is used in this work to obtain a magnetic fingerprint signal from anisotropic supraparticles, i.e., microrods assembled from superparamagnetic iron oxide nanoparticles. Exceeding its intended purpose of nanoparticle characterization for biomedical magnetic particle imaging, it is shown that MPS is capable of resolving structural differences between the anisotropic alignment of individual nanoparticles and its isotropic counterpart. Addnl., orientation-dependent MPS signal variations of anisotropic supraparticles are identifiable. This finding enables the detection of cold-chain breaches (for instance, during delivery of a product that needs to be cooled all of the time) by recording the initial and final MPS signals of microrod samples integrated into the container of a frozen product.
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    35. 35
      Montanarella, F.; Urbonas, D.; Chadwick, L.; Moerman, P. G.; Baesjou, P. J.; Mahrt, R. F.; Van Blaaderen, A.; Stöferle, T.; Vanmaekelbergh, D. Lasing Supraparticles Self-Assembled from Nanocrystals. ACS Nano 2018, 12, 12788– 12794,  DOI: 10.1021/acsnano.8b07896
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      35
      Lasing Supraparticles Self-Assembled from Nanocrystals
      Montanarella, Federico; Urbonas, Darius; Chadwick, Luke; Moerman, Pepijn G.; Baesjou, Patrick J.; Mahrt, Rainer F.; van Blaaderen, Alfons; Stoeferle, Thilo; Vanmaekelbergh, Daniel
      ACS Nano (2018), 12 (12), 12788-12794CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      One of the most attractive com. applications of semiconductor nanocrystals (NCs) is their use in lasers. Thanks to their high quantum yield, tunable optical properties, photostability, and wet-chem. processability, NCs have arisen as promising gain materials. Most of these applications, however, rely on incorporation of NCs in lasing cavities sep. produced using sophisticated fabrication methods and often difficult to manipulate. Here, we present whispering gallery mode lasing in supraparticles (SPs) of self-assembled NCs. The SPs composed of NCs act as both lasing medium and cavity. Moreover, the synthesis of the SPs, based on an in-flow microfluidic device, allows precise control of the dimensions of the SPs, i.e. the size of the cavity, in the micrometer range with polydispersity as low as several percent. The SPs presented here show whispering gallery mode resonances with quality factors up to 320. Whispering gallery mode lasing is evidenced by a clear threshold behavior, coherent emission, and emission lifetime shortening due to the stimulation process.
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    36. 36
      Mazzanti, A.; Yang, Z.; Silva, M. G.; Yang, N.; Rizza, G.; Coulon, P.-E.; Manzoni, C.; de Paula, A. M.; Cerullo, G.; Della Valle, G. Light–Heat Conversion Dynamics in Highly Diversified Water-Dispersed Hydrophobic Nanocrystal Assemblies. Proc. Natl. Acad. Sci. U. S. A. 2019, 116, 8161– 8166,  DOI: 10.1073/pnas.1817850116
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      36
      Light-heat conversion dynamics in highly diversified water-dispersed hydrophobic nanocrystal assemblies
      Mazzanti, Andrea; Yang, Zhijie; Silva, Mychel G.; Yang, Nailiang; Rizza, Giancarlo; Coulon, Pierre-EugA ne; Manzoni, Cristian; de Paula, Ana Maria; Cerullo, Giulio; Della Valle, Giuseppe; Pileni, Marie-Paule
      Proceedings of the National Academy of Sciences of the United States of America (2019), 116 (17), 8161-8166CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
      The authors study, with a combination of ultrafast optical spectroscopy and semiclassical modeling, the photothermal properties of various water-sol. nanocrystal assemblies. Broadband pump-probe expts. with ~100 fs time resoln. in the visible and near IR reveal a complex scenario for their transient optical response that is dictated by their hybrid compn. at the nanoscale, comprising metallic (Au) or semiconducting (Fe3O4) nanostructures and a matrix of org. ligands. The authors track the whole chain of energy flow that starts from light absorption by the individual nanocrystals and subsequent excitation of out-of-equil. carriers followed by the electron-phonon equilibration, occurring in a few picoseconds, and then by the heat release to the matrix on the 100-ps timescale. Two-dimensional finite-element method electromagnetic simulations of the composite nanostructure and multi-temp. modeling of the energy flow dynamics enable the authors to identify the key mechanism presiding over the light-heat conversion in these kinds of nanomaterials. Hybrid (org.-inorg.) nanocrystal assemblies can operate as efficient nanoheaters by exploiting the high absorption from the individual nanocrystals, enabled by the diln. of the inorg. phase that is followed by a relatively fast heating of the embedding org. matrix, occurring on the 100-ps timescale.
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    37. 37
      Nicolas-Boluda, A.; Yang, Z.; Guilbert, T.; Fouassier, L.; Carn, F.; Gazeau, F.; Pileni, M. P. Self-Assemblies of Fe 3 O 4 Nanocrystals: Toward Nanoscale Precision of Photothermal Effects in the Tumor Microenvironment. Adv. Funct. Mater. 2021, 31, 2006824,  DOI: 10.1002/adfm.202006824
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      Self-Assemblies of Fe3O4 Nanocrystals: Toward Nanoscale Precision of Photothermal Effects in the Tumor Microenvironment
      Nicolas-Boluda, Alba; Yang, Zhijie; Guilbert, Thomas; Fouassier, Laura; Carn, Florent; Gazeau, Florence; Pileni, Marie Paule
      Advanced Functional Materials (2021), 31 (4), 2006824CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Fe3O4 nanocrystals are self-assembled into two different conformations: colloidosome and supraball that confer them with distinct properties detg. their photo-induced heating capacities. These self-assemblies are assessed for photothermal therapy, an adjuvant strategy for tumor therapy. The tumor microenvironment is a heterogeneous ecosystem including immune cells and the extracellular matrix. The interactions between photothermal therapy agents and the different components of the tumor microenvironment det. the outcome of this therapy. In this study, the fate of both colloidosomes and supraballs within the tumor microenvironment in comparison to their Fe3O4 nanocrystal building blocks is revealed. This study highlights how these two hybrid self-assemblies target different compartments of the tumor microenvironment and trigger local photothermal damages that are inaccessible for isolated nanocrystals and not predicted by global temp. measurements.
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    38. 38
      Donaldson, J. G.; Schall, P.; Rossi, L. Magnetic Coupling in Colloidal Clusters for Hierarchical Self-Assembly. ACS Nano 2021, 15, 4989– 4999,  DOI: 10.1021/acsnano.0c09952
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      Magnetic Coupling in Colloidal Clusters for Hierarchical Self-Assembly
      Donaldson, Joe G.; Schall, Peter; Rossi, Laura
      ACS Nano (2021), 15 (3), 4989-4999CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Manipulating the way in which colloidal particles self-organize is a central challenge in the design of functional soft materials. Meeting this challenge requires the use of building blocks that interact with one another in a highly specific manner. Their fabrication, however, is limited by the complexity of the available synthesis procedures. Here, we demonstrate that, starting from exptl. available magnetic colloids, we can create a variety of complex building blocks suitable for hierarchical self-organization through a simple scalable process. Using computer simulations, we compress spherical and cubic magnetic colloids in spherical confinement, and investigate their suitability to form small clusters with reproducible structural and magnetic properties. We find that, while the structure of these clusters is highly reproducible, their magnetic character depends on the particle shape. Only spherical particles have the rotational degrees of freedom to produce consistent magnetic configurations, whereas cubic particles frustrate the minimization of the cluster energy, resulting in various magnetic configurations. To highlight their potential for self-assembly, we demonstrate that already clusters of three magnetic particles form highly nontrivial Archimedean lattices, namely, staggered kagome, bounce, and honeycomb, when focusing on different aspects of the same monolayer structure. The work presented here offers a conceptually different way to design materials by utilizing preassembled magnetic building blocks that can readily self-organize into complex structures.
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    39. 39
      Yang, Y.; Wang, B.; Shen, X.; Yao, L.; Wang, L.; Chen, X.; Xie, S.; Li, T.; Hu, J.; Yang, D. Scalable Assembly of Crystalline Binary Nanocrystal Superparticles and Their Enhanced Magnetic and Electrochemical Properties. J. Am. Chem. Soc. 2018, 140, 15038– 15047,  DOI: 10.1021/jacs.8b09779
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      Scalable Assembly of Crystalline Binary Nanocrystal Superparticles and Their Enhanced Magnetic and Electrochemical Properties
      Yang, Yuchi; Wang, Biwei; Shen, Xiudi; Yao, Luyin; Wang, Lei; Chen, Xiao; Xie, Songhai; Li, Tongtao; Hu, Jianhua; Yang, Dong; Dong, Angang
      Journal of the American Chemical Society (2018), 140 (44), 15038-15047CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Self-assembled binary nanocrystal superlattices (BNSLs) represent an important class of solid-state materials with potentially designed properties. In pursuit of widening the range of applications for binary superlattice materials, it is desirable to develop scalable assembly methods that enable high-quality BNSLs with tailored compns., structures, and morphologies. We report the gram-scale assembly of cryst. binary nanocrystal superparticles with high phase purity through an emulsion-based process. The structure of the resulting BNSL colloids can be tuned in a wide range (AB13, AlB2, MgZn2, NaCl, and CaCu5) by varying the size and(or) no. ratios of the 2 nanocrystal components. Access to large-scale, phase-pure BNSL colloids offers vast opportunities for investigating their physiochem. properties, as exemplified by AB13-type CoFe2O4-Fe3O4 binary superparticles. CoFe2O4-Fe3O4 binary superparticles not only display enhanced magnetic coupling but also exhibit superior lithium-storage properties. The nonclosed-packed NC packing arrangements of AB13-type binary superparticles are found to play a key role in facilitating lithiation/delithiation kinetics and maintaining structural integrity during repeated cycling. Our work establishes the scalable assembly of high-quality BNSL colloids, which is beneficial for accelerating the exploration of multicomponent nanocrystal superlattices toward various applications.
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    40. 40
      Wang, D.; Dasgupta, T.; van der Wee, E. B.; Zanaga, D.; Altantzis, T.; Wu, Y.; Coli, G. M.; Murray, C. B.; Bals, S.; Dijkstra, M. Binary Icosahedral Clusters of Hard Spheres in Spherical Confinement. Nat. Phys. 2021, 17, 128– 134,  DOI: 10.1038/s41567-020-1003-9
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      Binary icosahedral clusters of hard spheres in spherical confinement
      Wang, Da; Dasgupta, Tonnishtha; van der Wee, Ernest B.; Zanaga, Daniele; Altantzis, Thomas; Wu, Yaoting; Coli, Gabriele M.; Murray, Christopher B.; Bals, Sara; Dijkstra, Marjolein; van Blaaderen, Alfons
      Nature Physics (2021), 17 (1), 128-134CODEN: NPAHAX; ISSN:1745-2473. (Nature Research)
      Abstr.: The influence of geometry on the local and global packing of particles is important to many fundamental and applied research themes, such as the structure and stability of liqs., crystals and glasses. Here we show by expts. and simulations that a binary mixt. of hard-sphere-like nanoparticles crystg. into a MgZn2 Laves phase in bulk spontaneously forms icosahedral clusters in slowly drying droplets. Using advanced electron tomog., we are able to obtain the real-space coordinates of all the spheres in the icosahedral clusters of up to about 10,000 particles. The local structure of 70-80% of the particles became similar to that of the MgCu2 Laves phase. These observations are important for photonic applications. In addn., we obsd. in simulations that the icosahedral clusters nucleated away from the spherical boundary, which is distinctly different from that of the single species clusters. Our findings open the way for particle-level studies of nucleation and growth of icosahedral clusters, and of binary crystn.
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    41. 41
      Wang, J.; Mbah, C. F.; Przybilla, T.; Englisch, S.; Spiecker, E.; Engel, M.; Vogel, N. Free Energy Landscape of Colloidal Clusters in Spherical Confinement. ACS Nano 2019, 13, 9005– 9015,  DOI: 10.1021/acsnano.9b03039
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      Free Energy Landscape of Colloidal Clusters in Spherical Confinement
      Wang, Junwei; Mbah, Chrameh Fru; Przybilla, Thomas; Englisch, Silvan; Spiecker, Erdmann; Engel, Michael; Vogel, Nicolas
      ACS Nano (2019), 13 (8), 9005-9015CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      The structure of finite self-assembling systems depends sensitively on the no. of constituent building blocks. Recently, it was demonstrated that hard sphere-like colloidal particles show a magic no. effect when confined in emulsion droplets. Geometric construction rules permit a few dozen magic nos. that correspond to a discrete series of completely filled concentric icosahedral shells. Here, we investigate the free energy landscape of these colloidal clusters as a function of the no. of their constituent building blocks for system sizes up to several thousand particles. We find that min. in the free energy landscape, arising from the presence of filled, concentric shells, are significantly broadened, compared to their at. analogs. Colloidal clusters in spherical confinement can flexibly accommodate excess particles by ordering icosahedrally in the cluster center while changing the structure near the cluster surface. In between these magic no. regions, the building blocks cannot arrange into filled shells. Instead, we observe that defects accumulate in a single wedge and therefore only affect a few tetrahedral grains of the cluster. We predict the existence of this wedge by simulation and confirm its presence in expt. using electron tomog. The introduction of the wedge minimizes the free energy penalty by confining defects to small regions within the cluster. In addn., the remaining ordered tetrahedral grains can relax internal strain by breaking icosahedral symmetry. Our findings demonstrate how multiple defect mechanisms collude to form the complex free energy landscape of colloidal clusters.
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    42. 42
      Wang, X. D.; Chen, B.; Wang, H. F.; Wang, Z. S. Adhesion between Submicrometer Polystyrene Spheres. Powder Technol. 2011, 214, 447– 450,  DOI: 10.1016/j.powtec.2011.08.047
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      Adhesion between submicrometer polystyrene spheres
      Wang, Xiao-Dong; Chen, Bo; Wang, Hai-Feng; Wang, Zhan-Shan
      Powder Technology (2011), 214 (3), 447-450CODEN: POTEBX; ISSN:0032-5910. (Elsevier B.V.)
      The sphere-substrate contact method was usually used to study adhesion theory because it is rather difficult to make two micrometer or submicrometer spheres contact precisely. Here, we used sphere-sphere contact method by a novel, simple process to investigate deformations of spheres. The polystyrene particles size ranges from 60 nm to 600 nm. We found that the polystyrene particles underwent plastic deformations due to van der Waals interaction. The contact radii were obsd. by the scanning electron microscope (SEM).
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    43. 43
      Paul, J.; Romeis, S.; Mačković, M.; Marthala, V. R. R.; Herre, P.; Przybilla, T.; Hartmann, M.; Spiecker, E.; Schmidt, J.; Peukert, W. In Situ Cracking of Silica Beads in the SEM and TEM - Effect of Particle Size on Structure-Property Correlations. Powder Technol. 2015, 270, 337– 347,  DOI: 10.1016/j.powtec.2014.10.026
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      In situ cracking of silica beads in the SEM and TEM - Effect of particle size on structure-property correlations
      Paul, J.; Romeis, S.; Mackovic, M.; Marthala, V. R. R.; Herre, P.; Przybilla, T.; Hartmann, M.; Spiecker, E.; Schmidt, J.; Peukert, W.
      Powder Technology (2015), 270 (Part_A), 337-347CODEN: POTEBX; ISSN:0032-5910. (Elsevier B.V.)
      St.ovrddot.ober Fink Bohn (SFB) silica is a well-known colloidal system: monodisperse spherical particles are readily obtained by this wet chem. synthesis route. Despite the frequent application of SFB silica, its mech. properties are not comprehensively understood. Within this account, size dependent mech. properties of single SFB particles are systematically investigated using in situ SEM and TEM techniques: particle properties are addressed in the size range of 1.5 μm down to 200 nm. Both, untreated particles and heat treated particles densified at 1000°C are investigated. Structural characterization by vibrational spectroscopy, by solid state NMR spectroscopy, by SEM and by focused ion beam machining allows correlating mech. properties and the internal particle structure. For untreated particles comprised of a weakly cross-linked silica network a high degree of plasticity assocd. with low Young's moduli and hardness values is found. At large strains only cracking without full fracture of the particles occurs, whereby the crack path is clearly linked to the synthesis conditions: for larger particles obtained from a multistep growth process cracks propagate along weak intraparticle interfaces and are deflected around an inner core structure. After the thermal treatment crosslinking of the silica network is significantly enhanced and Young's moduli comparable to vitreous silica accompanied by increased hardness values are found. Only the smallest particles exhibit cracking; fracture into two or more pieces is distinguished for particles above 500 nm. It is found by SEM imaging that the relative occurrence of cracking into several pieces is increasing with size - a behavior assocd. with the brittle to ductile transition of silica. By testing a large quantity of particles, statistical significance is ensured.
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      Paul, J.; Romeis, S.; Herre, P.; Peukert, W. Deformation Behavior of Micron-Sized Polycrystalline Gold Particles Studied by in Situ Compression Experiments and Frictional Finite Element Simulation. Powder Technol. 2015, 286, 706– 715,  DOI: 10.1016/j.powtec.2015.09.020
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      Deformation behavior of micron-sized polycrystalline gold particles studied by in situ compression experiments and frictional finite element simulation
      Paul, Jonas; Romeis, Stefan; Herre, Patrick; Peukert, Wolfgang
      Powder Technology (2015), 286 (), 706-715CODEN: POTEBX; ISSN:0032-5910. (Elsevier B.V.)
      We present a combined exptl. and finite element study on the deformation behavior of micron-sized polycryst. gold particles. This study enables detailed insights into the underlying deformation mechanisms of the particles. Scanning electron microscope supported in situ uniaxial compression expts. of the single spherical polycryst. gold particles were performed in the size range of 1 μm by using a custom built manipulation device. By testing a large no. of particles stress-strain data and information on the particle morphol. were obtained with statistical significance. The exptl. obsd. stress-strain behavior and the geometric shape of the stressed particles were found to be in excellent agreement with the elastic-perfectly plastic finite element model accounting for frictional effects at the contact interfaces. A significantly increased yield strength compared to bulk gold was found - grain size strengthening according to the Hall-Petch relation was identified as the main hardening mechanism. Hardness was found to vary with strain - an effect related to the altering geometric shape of the particles during compression. Comparison to a frictionless finite element model revealed the necessity of considering the effect of friction. These findings are not restricted to gold particles, but should be applicable to a wide range of elastic-perfectly plastic materials.
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    45. 45
      Herre, P.; Romeis, S.; Mačković, M.; Przybilla, T.; Paul, J.; Schwenger, J.; Torun, B.; Grundmeier, G.; Spiecker, E.; Peukert, W. Deformation Behavior of Nanocrystalline Titania Particles Accessed by Complementary in Situ Electron Microscopy Techniques. J. Am. Ceram. Soc. 2017, 100, 5709– 5722,  DOI: 10.1111/jace.15072
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      Deformation behavior of nanocrystalline titania particles accessed by complementary in situ electron microscopy techniques
      Herre, Patrick; Romeis, Stefan; Mackovic, Mirza; Przybilla, Thomas; Paul, Jonas; Schwenger, Jan; Torun, Boray; Grundmeier, Guido; Spiecker, Erdmann; Peukert, Wolfgang
      Journal of the American Ceramic Society (2017), 100 (12), 5709-5722CODEN: JACTAW; ISSN:0002-7820. (Wiley-Blackwell)
      The mech. behavior of nanostructured spherical submicrometer titania particles was studied by in situ uniaxial compression expts. in the scanning and transmission electron microscope (SEM and TEM). Mesoporous and amorphous titania particles were prepd. by a wet chem. sol-gel approach. To obtain nanocryst. (nc) single-phase anatase and rutile particles the amorphous particles were crystd. by high-temp. annealing. For each sample the deformation behavior of ≥50 particles was investigated by in situ compression expts. in the SEM. In all cases an elastic - predominantly plastic deformation behavior accompanied by crack initiation at exceptionally high engineering strain values of several percent were obsd. Crack propagation presumably along grain boundaries and a Weibull distributed fracture stress was shown for all nc particles. Complementary in situ TEM expts. and ex situ anal. of focused ion beam prepd. particle cross-sections were carried out to identify the underlying deformation mechanisms. Grain rotations and grain sliding are obsd. for nc anatase particles during in situ compression and are further identified to be linked to a densification of the mesoporous particle structure. Our dedicated prepn. and quant. in situ characterization methodol. provides an excellent basis for a better understanding of the mech. behavior of advanced ceramics.
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    46. 46
      Paul, J.; Romeis, S.; Tomas, J.; Peukert, W. A Review of Models for Single Particle Compression and Their Application to Silica Microspheres. Adv. Powder Technol. 2014, 25, 136– 153,  DOI: 10.1016/j.apt.2013.09.009
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      46
      A review of models for single particle compression and their application to silica microspheres
      Paul, Jonas; Romeis, Stefan; Tomas, Jurgen; Peukert, Wolfgang
      Advanced Powder Technology (2014), 25 (1), 136-153CODEN: APTEEE; ISSN:0921-8831. (Elsevier B.V.)
      A review. We report on the deformation behavior of single silica microspheres. For the first time a detailed discussion on the quant. evaluation of force-deformation data in the elastic and plastic regime is given. The microspheres are compressed between two flat plates of a custom built manipulation device supported by a scanning electron microscope. The device allows a high sample throughput enabling full statistical evaluation of force-deformation data of single microspheres. Existing theories describing the deformation behavior of single spheres are discussed and applied to the elastic and plastic deformation regime of silica microspheres. The results obtained from the theories are compared between each other and values reported in literature. The silica microspheres exhibit a significantly different deformation behavior than expected from bulk fused silica, i.e. a distinct plastic deformation behavior. Furthermore, a significant decrease in Young's modulus and hardness was obsd. caused by high porosity and reduced crosslinking of silicon atoms, resp., that is inherent to silica produced by Stoeber synthesis. However, the calcd. contact pressures exceed the compressive strength of bulk fused silica by at least a factor of two as an effect of a reduced amt. of material flaws in microstructures.
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    47. 47
      Cheong, Y. S.; Adams, M. J.; Routh, A. F.; Hounslow, M. J.; Salman, A. D. The Production of Binderless Granules and Their Mechanical Characteristics. Chem. Eng. Sci. 2005, 60, 4045– 4053,  DOI: 10.1016/j.ces.2005.02.033
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      The production of binderless granules and their mechanical characteristics
      Cheong, Y. S.; Adams, M. J.; Routh, A. F.; Hounslow, M. J.; Salman, A. D.
      Chemical Engineering Science (2005), 60 (14), 4045-4053CODEN: CESCAC; ISSN:0009-2509. (Elsevier Ltd.)
      A granulation procedure is described for prepg. model binderless granules from spherical polystyrene colloids. The deformation and breakage behavior of the granules was also studied. Impact and slow diametrical compression expts. were used to simulate the mech. response of the granules at high and low strain rates, resp. They were found to deform elasto-plastically before fracturing in a semi-brittle manner. Densification or rearrangement of particle packing in the deformed region was concluded to be the main mechanism for energy dissipation under both impact and diametrical compression. In addn., the surface chem. of the constituent particles within the granules was one of the factors that govern the strength.
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      Reynolds, G. K.; Fu, J. S.; Cheong, Y. S.; Hounslow, M. J.; Salman, A. D. Breakage in Granulation: A Review. Chem. Eng. Sci. 2005, 60, 3969– 3992,  DOI: 10.1016/j.ces.2005.02.029
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      Breakage in granulation: A review
      Reynolds, G. K.; Fu, J. S.; Cheong, Y. S.; Hounslow, M. J.; Salman, A. D.
      Chemical Engineering Science (2005), 60 (14), 3969-3992CODEN: CESCAC; ISSN:0009-2509. (Elsevier Ltd.)
      A review of breakage from the process scale down to the single granule and sub-granule scale, discussing largely exptl. results complemented with some modeling results. The study of breakage in granulation is important from a process and from a product quality perspective. Breakage is considered an important rate process in granulation, and plays roles in granule homogeneity and strength. Understanding this rate process has important implications in the design and control of the granulation process. From a product perspective, the study of breakage has important implications for the subsequent processing, transport, handling and final use of granular products. Breakage behavior of granules can be a strong signature of the consistency of properties between nominally identical granular products.
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      Antonyuk, S.; Tomas, J.; Heinrich, S.; Mörl, L. Breakage Behaviour of Spherical Granulates by Compression. Chem. Eng. Sci. 2005, 60, 4031– 4044,  DOI: 10.1016/j.ces.2005.02.038
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      Breakage behaviour of spherical granulates by compression
      Antonyuk, Sergiy; Tomas, Juergen; Heinrich, Stefan; Moerl, Lothar
      Chemical Engineering Science (2005), 60 (14), 4031-4044CODEN: CESCAC; ISSN:0009-2509. (Elsevier Ltd.)
      This paper describes the deformation and breakage behavior of granulates in single particle compression test. Three industrial spherical granulates-γ-Al2O3, the synthetic zeolite Koestrolith, and sodium benzoate (C6H5COONa) were used as model materials to study the mech. behavior from elastic to plastic range. The elastic compression behavior of granulates is described by means of force-displacement curves, by application of Hertz-Huber contact theory and continuum mechanics. An elastic-plastic contact model was proposed to describe the deformation behavior of elastic-plastic granules. The effects of granulate size and stressing velocity on the breakage force and contact stiffness during elastic and elastic-plastic displacement are examd. It is shown that the zeolite granulates with elastic-plastic behavior have viscous properties as well. Breakage mechanisms of granulates during elastic, elastic-plastic, and plastic deformation are also explained. The breakage probability is approximated by Weibull distribution function. The behavior of the granulate during compression under the repeated loading-unloading conditions was investigated.
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      Schilde, C.; Westphal, B.; Kwade, A. Effect of the Primary Particle Morphology on the Micromechanical Properties of Nanostructured Alumina Agglomerates. J. Nanopart. Res. 2012, 14, 745,  DOI: 10.1007/s11051-012-0745-4
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      Effect of the primary particle morphology on the micromechanical properties of nanostructured alumina agglomerates
      Schilde, Carsten; Westphal, Bastian; Kwade, Arno
      Journal of Nanoparticle Research (2012), 14 (3), 745/1-745/11CODEN: JNARFA; ISSN:1388-0764. (Springer)
      Depending on the application of nanoparticles, certain characteristics of the product quality such as size, morphol., abrasion resistance, sp. surface, dispersibility and tendency to agglomeration are important. These characteristics are a function of the physicochem. properties, i.e. the micromech. properties of the nanostructured material. The micromech. properties of these nanostructured agglomerates such as the max. indentation force, the plastic and elastic deformation energy and the strength give information on the product properties, e.g. the efficiency of a dispersion process of the agglomerates, and can be measured by nanoindentation. In this study a Berkovich indenter tip was used for the characterization of model aggregates out of sol-gel produced silica and pptd. alumina agglomerates with different primary particle morphologies (dimension of 15-40 nm). In general, the effect of the primary particle morphol. and the presence or absence of solid bonds can be characterized by the measurement of the micromech. properties via nanoindentation. The micromech. behavior of aggregates contg. solid bonds is strongly affected by the elastic-plastic deformation behavior of the solid bonds and the breakage of solid bonds. Moreover, varying the primary particle morphol. for similar particle material and approx. isotropic agglomerate behavior the particle-particle interactions within the agglomerates can be described by the elemental breaking stress according to the formula of Rumpf.
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    51. 51
      Sekido, T.; Wooh, S.; Fuchs, R.; Kappl, M.; Nakamura, Y.; Butt, H. J.; Fujii, S. Controlling the Structure of Supraballs by PH-Responsive Particle Assembly. Langmuir 2017, 33, 1995– 2002,  DOI: 10.1021/acs.langmuir.6b04648
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      Controlling the Structure of Supraballs by pH-Responsive Particle Assembly
      Sekido, Takafumi; Wooh, Sanghyuk; Fuchs, Regina; Kappl, Michael; Nakamura, Yoshinobu; Butt, Hans-Jurgen; Fujii, Syuji
      Langmuir (2017), 33 (8), 1995-2002CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      Supraballs of various sizes and compns. can be fabricated via drying of drops of aq. colloidal dispersions on super-liq.-repellent surfaces with no chem. waste and energy consumption. A "supraball" is a particle composed of colloids. Many properties, such as mech. strength and porosity, are detd. by the ordering of a colloidal assembly. To tune such properties, a colloidal assembly needs to be controlled when supraballs are formed during drying. Here, we introduce a method to control a colloidal assembly of supraballs by adjusting the dispersity of the colloids. Supraballs are fabricated on superamphiphobic surfaces from colloidal aq. dispersions of polystyrene microparticles carrying pH-responsive poly[2-(diethylamino)ethyl methacrylate]. Drying of dispersion drops at pH 3 on superamphiphobic surfaces leads to the formation of spherical supraballs with densely packed colloids. The pH 10 supraballs are more oblate and consist of more disordered colloids than the pH 3 supraballs, caused by particle aggregates with random sizes and shapes in the pH 10 dispersion. Thus, the shape, crystallinity, porosity, and mech. properties could be controlled by pH, which allows broader uses of supraballs.
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      Zellmer, S.; Lindenau, M.; Michel, S.; Garnweitner, G.; Schilde, C. Influence of Surface Modification on Structure Formation and Micromechanical Properties of Spray-Dried Silica Aggregates. J. Colloid Interface Sci. 2016, 464, 183– 190,  DOI: 10.1016/j.jcis.2015.11.028
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      Influence of surface modification on structure formation and micromechanical properties of spray-dried silica aggregates
      Zellmer, Sabrina; Lindenau, Maylin; Michel, Stephanie; Garnweitner, Georg; Schilde, Carsten
      Journal of Colloid and Interface Science (2016), 464 (), 183-190CODEN: JCISA5; ISSN:0021-9797. (Elsevier B.V.)
      Spray drying processes were utilized for the prodn. of hierarchical materials with defined structures. The structure formation during the spray drying process and the micromech. properties of the obtained aggregates depend on the particle-particle interactions, the primary particle size and morphol. as well as the process parameters of the spray drying process. Hence, the effect of different primary particle systems prepd. as stable dispersions with various surface modifications were investigated on the colloidal structure formation and the micromech. properties of silica particles as model aggregates and compared to theor. considerations. The obtained results show that the structure formation of aggregates during the spray drying process for stable suspensions is almost independent on the functional groups present at the particle surface. Further, the mech. properties of these aggregates differ considerably with the content of the bound ligand. This allows the defined adjustment of the aggregate properties, such as the strength and surface properties, as well as the formation of defined hierarchical aggregate structures.
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      Russell, A.; Müller, P.; Tomas, J. Quasi-Static Diametrical Compression of Characteristic Elastic-Plastic Granules: Energetic Aspects at Contact. Chem. Eng. Sci. 2014, 114, 70– 84,  DOI: 10.1016/j.ces.2014.04.016
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      Quasi-static diametrical compression of characteristic elastic-plastic granules: Energetic aspects at contact
      Russell, Alexander; Mueller, Peter; Tomas, Juergen
      Chemical Engineering Science (2014), 114 (), 70-84CODEN: CESCAC; ISSN:0009-2509. (Elsevier Ltd.)
      Comminution processes have traditionally been considered as empirical endeavors, with great difficulties in analyzing, predicting and controlling the obsd. behavior. Process industries have faced serious problems, ranging from poor product quality control up to managing product quantity losses due to unexpected attrition and breakage events. After a decade of ardent research, there has been a notable advancement in our understanding of granular behavior under mech. stresses. However, far less is known about the energetic aspects occurring at such conditions. In this article, we present crit. information from an energetic perspective concerning the deformation and breakage behavior of characteristic elastic-plastic granules under quasi-static compressive forces, studied using single granule diametrical compression tests at a const. strain controlled loading velocity. The force-displacement behavior has been approximated using the Hertz model (Hertz, 1882) for elastic loading, the Tomas model (Tomas, 2007a) for elastic-plastic loading and the Stronge-Antonyuk correlations (Stronge, 2000; Antonyuk, 2006) incorporated Hertz model (Hertz, 1882) for elastic unloading with addnl. displacements due to apparent viscous effects. The rate independent specific energy characteristics have been studied and a corresponding energetic coeff. of restitution has been derived. A comparison of the energetic characteristics at primary breakage of fresh and pre-loaded granules has been presented using breakage probability functions. The phenomenon of strain hardening during localized repetitive compressive loadings has also been analyzed. Furthermore, the influences of granule size, moisture content and stressing intensity have been considered in each of the investigated entities.
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      Thornton, C.; Ciomocos, M. T.; Adams, M. J. Numerical Simulations of Diametrical Compression Tests on Agglomerates. Powder Technol. 2004, 140, 258– 267,  DOI: 10.1016/j.powtec.2004.01.022
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      Numerical simulations of diametrical compression tests on agglomerates
      Thornton, C.; Ciomocos, M. T.; Adams, M. J.
      Powder Technology (2004), 140 (3), 258-267CODEN: POTEBX; ISSN:0032-5910. (Elsevier Science B.V.)
      The paper reports discrete element simulations of the diametrical compression test applied to two spherical agglomerates: one a dense agglomerate and the other a loosely packed agglomerate. The results obtained for the dense agglomerate show that the agglomerate fractures along a slightly inclined, approx. diametrical plane. Outwardly, the agglomerate shows all the characteristics of brittle fracture but half of the final no. of broken bonds was progressively broken during loading. In the simulation on the loose agglomerate, significant flattening occurred at the platens and the agglomerate failed by crushing.
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    55. 55
      Kozhar, S.; Dosta, M.; Antonyuk, S.; Heinrich, S.; Bröckel, U. DEM Simulations of Amorphous Irregular Shaped Micrometer-Sized Titania Agglomerates at Compression. Adv. Powder Technol. 2015, 26, 767– 777,  DOI: 10.1016/j.apt.2015.05.005
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      DEM simulations of amorphous irregular shaped micrometer-sized titania agglomerates at compression
      Kozhar, S.; Dosta, M.; Antonyuk, S.; Heinrich, S.; Broeckel, U.
      Advanced Powder Technology (2015), 26 (3), 767-777CODEN: APTEEE; ISSN:0921-8831. (Elsevier B.V.)
      Bulk solids are exposed to time-dependent mech. stressing due to particle-particle and particle-app. contact interactions during various manufg. processes and transportation steps. These interactions can be described by discrete element method (DEM) based on the contact models of particles. Usually in DEM simulations the particles are assumed to be spheres and as a consequence shape effects are neglected. However, most bulk solids processed in industry consist of irregular shaped particles. Therefore, in order to improve accuracy of numerical simulation the real shapes of particles must be considered in DEM. In this work, amorphous irregular shaped micrometer-sized titanium dioxide agglomerates were investigated. The force-displacement curves at compression were obtained with the help of a self-designed exptl. setup. Based on the exptl. data, several material parameters were detd. and implemented in viscoelastic and elastic-plastic contact models. To consider the shape effect in the estn. of contact parameters the DEM simulation of studied agglomerates was performed by multi-sphere approach and bonded-particle model. The shape and position of the agglomerates on the loading pin were obtained by X-ray computer tomog. and used in DEM simulations. From the obtained results it was pointed out that the bonded-particle model based on Maxwell viscoelastic model gives the best agreement with exptl. data from compression tests with titania agglomerates.
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      Plunkett, A.; Eldridge, C.; Schneider, G. A.; Domènech, B. Controlling the Large-Scale Fabrication of Supraparticles. J. Phys. Chem. B 2020, 124, 11263– 11272,  DOI: 10.1021/acs.jpcb.0c07306
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      Controlling the Large-Scale Fabrication of Supraparticles
      Plunkett, Alexander; Eldridge, Catriona; Schneider, Gerold A.; Domenech, Berta
      Journal of Physical Chemistry B (2020), 124 (49), 11263-11272CODEN: JPCBFK; ISSN:1520-5207. (American Chemical Society)
      Controlling the nanoscale interactions of colloidal building blocks is a key step for the transition from single nanoparticles to tailor-made, architected morphologies and their further integration into functional materials. Solvent evapn.-induced self-assembly within emulsion droplets emerges as a fast, versatile, and low-cost approach to obtain spherical, complex structures, such as supraparticles. Nevertheless, some process-structure relationships able to describe the effects of emulsion conditions on the synthesis outcomes still remain to be understood. Here, we explore the effect of different physicochem. parameters of emulsion-templated self-assembly (ETSA) on supraparticles' formation. Supraparticle size, size dispersity, microporosity, and sample homogeneity are rationalized based on the used surfactant formulation, stabilization mechanism, and viscosity of the emulsion. We further demonstrate the significance of the parameters found by optimizing a transferable, large-scale (gram-size) ETSA setup for the controlled synthesis of spherical supraparticles in a range of defined sizes (from 0.1-10μm). Ultimately, our results provide new key synthetic parameters able to control the process, promoting the development of supraparticle-based, functional nanomaterials for a wide range of applications.
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      Shang, L.; Cheng, Y.; Zhao, Y. Emerging Droplet Microfluidics. Chem. Rev. 2017, 117, 7964– 8040,  DOI: 10.1021/acs.chemrev.6b00848
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      Emerging Droplet Microfluidics
      Shang, Luoran; Cheng, Yao; Zhao, Yuanjin
      Chemical Reviews (Washington, DC, United States) (2017), 117 (12), 7964-8040CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review. Droplet microfluidics generates and manipulates discrete droplets through immiscible multiphase flows inside microchannels. Due to its remarkable advantages, droplet microfluidics bears significant value in an extremely wide range of area. In this review, a comprehensive and in-depth insight is provided into droplet microfluidics, covering fundamental research from microfluidic chip fabrication and droplet generation to the applications of droplets in bio(chem.) anal. and materials generation. The purpose of the review is to convey the fundamentals of droplet microfluidics, a crit. anal. on its current status and challenges, and opinions on its future development. The review will promote communications among biol., chem., physics, and materials science.
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      Park, S.; Hwang, H.; Kim, M.; Moon, J. H.; Kim, S.-H. Colloidal Assembly in Droplets: Structures and Optical Properties. Nanoscale 2020, 12, 18576– 18594,  DOI: 10.1039/D0NR04608F
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      Colloidal assembly in droplets: structures and optical properties
      Park, Sanghyuk; Hwang, Hyerim; Kim, Minjung; Moon, Jun Hyuk; Kim, Shin-Hyun
      Nanoscale (2020), 12 (36), 18576-18594CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      A review. Colloidal assembly in emulsion drops provides fundamental tools for studying optimum particle arrangement under spherical confinement and practical means for producing photonic microparticles. Recent progress has revealed that energetically favored cluster configurations are different from conventional supraballs, which could enhance optical performance. This paper reviews state-of-the-art emulsion-templated colloidal clusters, and particularly focuses on recently reported novel structures such as icosahedral, decahedral, and single-cryst. face-centered cubic (fcc) clusters. We classify the clusters according to the no. of component particles as small (N < O(102)), medium (O(102) ≤ N ≤ O(104)), and large (N ≥ O(105)). For each size of clusters, we discuss the detailed structures, mechanisms of cluster formation, and optical properties and potential applications. Finally, we outline current challenges and questions that require further investigation.
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      Kim, S. H.; Lee, S. Y.; Yi, G. R.; Pine, D. J.; Yang, S. M. Microwave-Assisted Self-Organization of Colloidal Particles in Confining Aqueous Droplets. J. Am. Chem. Soc. 2006, 128, 10897– 10904,  DOI: 10.1021/ja063528y
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      Microwave-Assisted Self-Organization of Colloidal Particles in Confining Aqueous Droplets
      Kim, Shin-Hyun; Lee, Su Yeon; Yi, Gi-Ra; Pine, David J.; Yang, Seung-Man
      Journal of the American Chemical Society (2006), 128 (33), 10897-10904CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Monodisperse aq. emulsion droplets encapsulating colloidal particles were produced in the oil phase, and controlled microwave irradn. of the aq. drop phase created spherical colloidal crystals by so-called evapn.-induced self-organization of the colloidal particles. Unlike usual colloidal crystals, colloidal crystals in spherical symmetry (or photonic balls) possessed photonic band gaps for the normal incident light independent of the position all over the spherical surface. While the consolidation of colloidal particles in emulsion droplets in an oven took several hours, the present microwave-assisted evapn. could reduce the time for complete evapn. to a few tens of minutes. Under the microwave irradn., the aq. phase in emulsions was superheated selectively and the evapn. rate of water could be controlled easily by adjusting the microwave intensity. The result showed that the packing quality of colloidal crystals obtained by the microwave-assisted self-organization was good enough to show photonic band gap characteristics. The reflectance of our photonic balls responded precisely to any change in phys. properties including the size of colloidal particles, refractive index mismatch, and angle of the incident beam. In particular, for polymeric particles, the photonic band gap could be tuned by the intensity of microwave irradn., and the reflection color was red-shifted with stronger microwave irradn. Finally, for better photonic band gap properties, inverted photonic balls were prepd. by using the spherical colloidal crystals as sacrificial templates.
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      Pileni, M.-P. Mechanical Properties of Supracrystals. EPL (Europhysics Letters). 2017, 119, 37002,  DOI: 10.1209/0295-5075/119/37002
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      Junwei, W.; Jan, S.; Andreas, S.; Patrick, F.; Patrick, H.; Stefan, R.; Wolfgang, P.; Benoit, M.; Nicolas, V. Mechanics of Colloidal Supraparticles Under Compression. Sci. Adv. 2021,  DOI: 10.1126/sciadv.abj0954
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      Kim, Y. R.; Lee, T. W.; Park, S.; Jang, J.; Ahn, C. W.; Choi, J. J.; Hahn, B. D.; Choi, J. H.; Yoon, W. H.; Bae, S. H. Supraparticle Engineering for Highly Dense Microspheres: Yttria-Stabilized Zirconia with Adjustable Micromechanical Properties. ACS Nano 2021, 15, 10264– 10274,  DOI: 10.1021/acsnano.1c02408
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      Supraparticle Engineering for Highly Dense Microspheres: Yttria-Stabilized Zirconia with Adjustable Micromechanical Properties
      Kim Young-Rok; Lee Tae Won; Park Seonhwa; Jang Jongmoon; Ahn Cheol-Woo; Choi Jong-Jin; Hahn Byung-Dong; Choi Joon-Hwan; Yoon Woon-Ha; Min Yuho; Kim Young-Rok; Bae Sung-Hwan
      ACS nano (2021), 15 (6), 10264-10274 ISSN:.
      Various supraparticles have been extensively studied owing to their excellent catalytic properties that are attributed to their inherent porous structure; however, their mechanical properties have not garnered attention owing to their less dense structure. We demonstrate a rational approach for fabricating assembled supraparticles and, subsequently, highly dense microspheres. In addition, 3 mol % yttria-stabilized zirconia (3YSZ) and alumina particles were selected as building blocks and assembled into higher-order architectures using a droplet-based template method (spray drying) for validation with proof-of-concept. Moreover, structural features such as density, size, sphericity, and morphology of supraparticles were controlled by adjusting the competing kinetics occurring between the assembly of building blocks and evaporation of the solvent in the droplets. The preparatory aqueous suspension and process parameters were optimized as well. A detailed understanding of the formation mechanism facilitated the yield of tailor-made supraparticles and, thereafter, highly dense microspheres (approximate relative density = 99%) with excellent sphericity (>98%) via heat treatment. The microspheres displayed highest hardness (26.77 GPa) and superior elastic modulus (210.19 GPa) compared with the mechanical properties of the 3YSZ samples reported to date. Ultimately, the proposed supraparticle engineering provided insight for controlling the structural features and resultant micromechanical properties, which widely extends the applicability of supraparticle-based functional materials for practical purposes that require materials with high density and excellent mechanical properties.
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    63. 63
      Giuntini, D.; Zhao, S.; Krekeler, T.; Li, M.; Blankenburg, M.; Bor, B.; Schaan, G.; Domenech, B.; Muller, M.; Scheider, I.; Ritter, M.; Schneider, G. A. Defects and Plasticity in Ultrastrong Supercrystalline Nanocomposites. Sci. Adv. 2021, 7, 1– 11,  DOI: 10.1126/sciadv.abb6063
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      Dreyer, A.; Feld, A.; Kornowski, A.; Yilmaz, E. D.; Noei, H.; Meyer, A.; Krekeler, T.; Jiao, C.; Stierle, A.; Abetz, V. Organically Linked Iron Oxide Nanoparticle Supercrystals with Exceptional Isotropic Mechanical Properties. Nat. Mater. 2016, 15, 522– 528,  DOI: 10.1038/nmat4553
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      Organically linked iron oxide nanoparticle supercrystals with exceptional isotropic mechanical properties
      Dreyer, Axel; Feld, Artur; Kornowski, Andreas; Yilmaz, Ezgi D.; Noei, Heshmat; Meyer, Andreas; Krekeler, Tobias; Jiao, Chengge; Stierle, Andreas; Abetz, Volker; Weller, Horst; Schneider, Gerold A.
      Nature Materials (2016), 15 (5), 522-528CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
      The authors show self-assembly of nearly spherical iron oxide nanoparticles in supercrystals linked together by a thermally induced crosslinking reaction of oleic acid mols. leads to a nanocomposite with exceptional bending modulus of 114 GPa, hardness of up to 4 GPa and strength of up to 630 MPa. By using a nanomech. model, the authors detd. that these exceptional mech. properties are dominated by the covalent backbone of the linked org. mols. Oleic acid has been broadly used as nanoparticle ligand, our crosslinking approach should be applicable to a large variety of nanoparticle systems.
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      Ayouch, A.; Dieudonné, X.; Vaudel, G.; Piombini, H.; Vallé, K.; Gusev, V.; Belleville, P.; Ruello, P. Elasticity of an Assembly of Disordered Nanoparticles Interacting via either Van der Waals-Bonded or Covalent-Bonded Coating Layers. ACS Nano 2012, 6, 10614– 10621,  DOI: 10.1021/nn303631d
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      65
      Elasticity of an Assembly of Disordered Nanoparticles Interacting via Either van der Waals-Bonded or Covalent-Bonded Coating Layers
      Ayouch, Adil; Dieudonne, Xavier; Vaudel, Gwenaelle; Piombini, Herve; Valle, Karine; Gusev, Vitalyi; Belleville, Philippe; Ruello, Pascal
      ACS Nano (2012), 6 (12), 10614-10621CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Tailoring phys. and chem. properties at the nanoscale by assembling nanoparticles currently paves the way for new functional materials. Obtaining the desired macroscopic properties is usually detd. by a perfect control of the contact between nanoparticles. Therefore, the physics and chem. of nanocontacts are one of the central issues for the design of the nanocomposites. Since the birth of at. force microscopy, crucial advances were achieved in the quant. evaluation of van der Waals and Casimir forces in nanostructures and of adhesion between the nanoparticles. The authors present here a study, by a noncontact method, of the elasticity of an assembly of nanoparticles interacting via either van der Waals-bonded or covalent-bonded coating layers. The authors demonstrate indeed that the ultrafast opto-acoustic technique, based on the generation and detection of hypersound by femtosecond laser pulses, is very sensitive to probe the properties of the nanocontacts. In particular, the authors observe and evaluate how much the subnanometric mols. present at nanocontacts influence the coherent acoustic phonon propagation along the network of the interconnected SiO2 nanoparticles. Finally, this ultrafast opto-acoustic technique provides quant. ests. of the rigidity/stiffness of the nanocontacts.
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      Cang, Y.; Liu, B.; Das, S.; Xu, X.; Xie, J.; Deng, X.; Fytas, G. Surface Contacts Strongly Influence the Elasticity and Thermal Conductivity of Silica Nanoparticle Fibers. Phys. Chem. Chem. Phys. 2021, 23, 3707– 3715,  DOI: 10.1039/D0CP05377E
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      66
      Surface contacts strongly influence elasticity and thermal conductivity of silica nanoparticle fibers
      Cang, Yu; Liu, Bohai; Das, Sudatta; Xu, Xiangfan; Xie, Jingli; Deng, Xu; Fytas, George
      Physical Chemistry Chemical Physics (2021), 23 (6), 3707-3715CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      Granular materials are often encountered in science and engineering disciplines, in which controlling the particle contacts is one of the crit. issues for the design, engineering, and utilization of their desired properties. The achievable rapid fabrication of nanoparticles with tunable phys. and chem. properties facilitates tailoring the macroscopic properties of particle assemblies through contacts at the nanoscale. Models have been developed to predict the mech. properties of macroscopic granular materials; however, their predicted power in the case of nanoparticle assemblies is still uncertain. Here, we investigate the influence of nanocontacts on the elasticity and thermal cond. of a granular fiber comprised of close-packed silica nanoparticles. A complete elastic moduli characterization was realized by non-contact and non-destructive Brillouin light spectroscopy, which also allowed resolving the stiffness of the constituent particles in situ. In the framework of effective medium models, the strong enhancement of the elastic moduli is attributed to the formation of adhesive nanocontacts with phys. and/or chem. bondings. The nanoparticle contacts are also responsible for the increase in the fiber thermal cond. that emphasizes the role of interface thermal resistance, which tends to be ignored in most porosity models. This insight into the fundamental understanding of structure-property relationships advances knowledge on the manipulation of granular systems at the nanoscale.
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    67. 67
      Holtze, C.; Rowat, A. C.; Agresti, J. J.; Hutchison, J. B.; Angile, F. E.; Schmitz, C. H. J.; Koster, S.; Duan, H.; Humphry, K. J.; Scanga, R. A. Biocompatible Surfactants for Water-in-Fluorocarbon Emulsions. Lab Chip 2008, 8, 1632– 1639,  DOI: 10.1039/b806706f
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      Biocompatible surfactants for water-in-fluorocarbon emulsions
      Holtze, C.; Rowat, A. C.; Agresti, J. J.; Hutchison, J. B.; Angile, F. E.; Schmitz, C. H. J.; Koster, S.; Duan, H.; Humphry, K. J.; Scanga, R. A.; Johnson, J. S.; Pisignano, D.; Weitz, D. A.
      Lab on a Chip (2008), 8 (10), 1632-1639CODEN: LCAHAM; ISSN:1473-0197. (Royal Society of Chemistry)
      Drops of water-in-fluorocarbon emulsions have great potential for compartmentalizing both in vitro and in vivo biol. systems; however, surfactants to stabilize such emulsions are scarce. Here we present a novel class of fluorosurfactants that we synthesize by coupling oligomeric perfluorinated polyethers (PFPE) with polyethyleneglycol (PEG). We demonstrate that these block copolymer surfactants stabilize water-in-fluorocarbon oil emulsions during all necessary steps of a drop-based expt. including drop formation, incubation, and reinjection into a second microfluidic device. Furthermore, we show that aq. drops stabilized with these surfactants can be used for in vitro translation (IVT), as well as encapsulation and incubation of single cells. The compatibility of this emulsion system with both biol. systems and polydimethylsiloxane (PDMS) microfluidic devices makes these surfactants ideal for a broad range of high-throughput, drop-based applications.
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      Etienne, G.; Kessler, M.; Amstad, E. Influence of Fluorinated Surfactant Composition on the Stability of Emulsion Drops. Macromol. Chem. Phys. 2017, 218, 1600365,  DOI: 10.1002/macp.201600365
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      Seemann, R.; Brinkmann, M.; Pfohl, T.; Herminghaus, S. Droplet Based Microfluidics. Rep. Prog. Phys. 2012, 75, 016601,  DOI: 10.1088/0034-4885/75/1/016601
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      Droplet based microfluidics
      Seemann, Ralf; Brinkmann, Martin; Pfohl, Thomas; Herminghaus, Stephan
      Reports on Progress in Physics (2012), 75 (1), 016601/1-016601/41CODEN: RPPHAG; ISSN:0034-4885. (Institute of Physics Publishing)
      A review. Droplet based microfluidics is a rapidly growing interdisciplinary field of research combining soft matter physics, biochem., and microsystems engineering. Its applications range from fast anal. systems or the synthesis of advanced materials to protein crystn., and biol. assays for living cells. Precise control of droplet vols. and reliable manipulation of individual droplets such as coalescence, mixing of their contents, and sorting in combination with fast anal. tools allow to perform chem. reactions inside the droplets under defined conditions. In this paper, reviews available drop generation and manipulation techniques. The main focus of the review is not to be comprehensive and explain all techniques in great detail but to identify and shed light on similarities and underlying phys. principles. Since geometry and wetting properties of the microfluidic channels are crucial factors for droplet generation, typical device fabrication methods in droplet based microfluidics are also briefly described. Examples of applications and reaction schemes which rely on the discussed manipulation techniques are also presented, such as the fabrication of special materials and biophys. expts.
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      Penciu, R. S.; Fytas, G.; Economou, E. N.; Steffen, W.; Yannopoulos, S. N. Acoustic Excitations in Suspensions of Soft Colloids. Phys. Rev. Lett. 2000, 85, 4622– 4625,  DOI: 10.1103/PhysRevLett.85.4622
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      Acoustic Excitations in Suspensions of Soft Colloids
      Penciu, R. S.; Fytas, G.; Economou, E. N.; Steffen, W.; Yannopoulos, S. N.
      Physical Review Letters (2000), 85 (21), 4622-4625CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      Vibrational modes in suspensions of soft colloids in a fluid can be detected exptl. by Brillouin light scattering. Besides the usual acoustic mode, being essentially the longitudinal phonon of the liq. matrix, an optic-like mode is obsd. in giant starlike micelles at low vol. fractions. Probably this optic-like mode is due mainly to the internal vibration of each hairy particle.
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      Kuok, M. H.; Lim, H. S.; Ng, S. C.; Liu, N. N.; Wang, Z. K. Brillouin Study of the Quantization of Acoustic Modes in Nanospheres. Phys. Rev. Lett. 2003, 90, 4,  DOI: 10.1103/PhysRevLett.90.255502
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      Courty, A.; Mermet, A.; Albouy, P. A.; Duval, E.; Pileni, M. P. Vibrational Coherence of Self-Organized Silver Nanocrystals in f.c.c. Supra-Crystals. Nat. Mater. 2005, 4, 395– 398,  DOI: 10.1038/nmat1366
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      Vibrational coherence of self-organized silver nanocrystals in f.c.c. supra-crystals
      Courty, A.; Mermet, A.; Albouy, P. A.; Duval, E.; Pileni, M. P.
      Nature Materials (2005), 4 (5), 395-398CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
      Fabrication of devices from inorg. nanocrystals normally requires that they are self-organized into ordered structures. It has now been demonstrated that nanocrystals are able to self-organize in a 'supra'-crystal with a face-centered cubic (f.c.c.) structure. The phys. properties of nanocrystals self-organized into compact arrays are quite different from those of both isolated nanocrystals and the bulk phase. The collective optical and magnetic properties of these nanocrystal assemblies are governed mainly by dipolar interactions. Here, we show that nanocrystals vibrate coherently when they are self-organized in f.c.c. supra-crystals. Hence, a phase relation exists between the vibrations of all of the nanocrystals in a supra-crystal. This vibrational coherence can be obsd. by a substantial change of the quadrupolar low-frequency Raman scattering peak. Although a change in electronic transport properties has previously been obsd. on self-organization of silver nanocrystals, vibrational coherence represents the first intrinsic property of f.c.c. supra-crystals.
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      Cheng, W.; Wang, J.; Jonas, U.; Fytas, G.; Stefanou, N. Observation and Tuning of Hypersonic Bandgaps in Colloidal Crystals. Nat. Mater. 2006, 5, 830– 836,  DOI: 10.1038/nmat1727
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      Observation and tuning of hypersonic bandgaps in colloidal crystals
      Cheng, Wei; Wang, Jianjun; Jonas, Ulrich; Fytas, George; Stefanou, Nikolaos
      Nature Materials (2006), 5 (10), 830-836CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
      Composite materials with periodic variations of d. and/or sound velocities, so-called phononic crystals, can exhibit bandgaps where propagation of acoustic waves is forbidden. Phononic crystals are the elastic analog of the well-established photonic crystals and show potential for manipulating the flow of elastic energy. So far, the exptl. realization of phononic crystals was restricted to macroscopic systems with sonic or ultrasonic bandgaps in the sub-MHz frequency range. Using high-resoln. Brillouin spectroscopy the authors report the 1st observation of a hypersonic bandgap in fcc. colloidal crystals formed by self-assembly of polystyrene nanoparticles with subsequent fluid infiltration. Depending on the particle size and the sound velocity in the infiltrated fluid, the frequency and the width of the gap can be tuned. Promising technol. applications of hypersonic crystals, ranging from tunable filters and heat management to acoustooptical devices, are anticipated.
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      Still, T.; Sainidou, R.; Retsch, M.; Jonas, U.; Spahn, P.; Hellmann, G. P.; Fytas, G. The “Music” of Core-Shell Spheres and Hollow Capsules: Influence of the Architecture on the Mechanical Properties at the Nanoscale. Nano Lett. 2008, 8, 3194– 3199,  DOI: 10.1021/nl801500n
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      The "Music" of Core-Shell Spheres and Hollow Capsules: Influence of the Architecture on the Mechanical Properties at the Nanoscale
      Still, T.; Sainidou, R.; Retsch, M.; Jonas, U.; Spahn, P.; Hellmann, G. P.; Fytas, G.
      Nano Letters (2008), 8 (10), 3194-3199CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      The elastic vibrational modes were measured in core-shell spheres (silica-poly(Me methacrylate), SiO2-PMMA) and corresponding spherical hollow capsules (PMMA) with different particle size and shell thickness using Brillouin light scattering, supported by numerical calcns. These localized modes allow access to the mech. moduli down to a few tens of nanometers. Reduced mech. strength of the porous silica core and for the core-shell spheres a striking increase of the moduli in both the SiO2 core and the PMMA shell were obsd. The peculiar behavior of the vibrational modes in the hollow capsules is attributed to antagonistic dependence on overall size and layer thickness in agreement with theor. predictions.
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      Still, T.; Mattarelli, M.; Kiefer, D.; Fytas, G.; Montagna, M. Eigenvibrations of Submicrometer Colloidal Spheres. J. Phys. Chem. Lett. 2010, 1, 2440– 2444,  DOI: 10.1021/jz100774b
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      Eigenvibrations of Submicrometer Colloidal Spheres
      Still, T.; Mattarelli, M.; Kiefer, D.; Fytas, G.; Montagna, M.
      Journal of Physical Chemistry Letters (2010), 1 (16), 2440-2444CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Brillouin light scattering (BLS) can record particle resonances, which allow access to thermomech. properties on a submicrometer scale. The full theor. representation of BLS eigenmode spectra of polymer and SiO2 colloids resolves a longstanding discussion showing that both even and odd l spheroidal modes are active. The BLS spectra of transparent SiO2 suspensions fully support the predicted dependence of the strength of the individual resonance modes on the probing wavevector. It is this dependence that selects which spheroidal modes will be revealed exptl.
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      Schneider, D.; Beltramo, P. J.; Mattarelli, M.; Pfleiderer, P.; Vermant, J.; Crespy, D.; Montagna, M.; Furst, E. M.; Fytas, G. Elongated Polystyrene Spheres as Resonant Building Blocks in Anisotropic Colloidal Crystals. Soft Matter 2013, 9, 9129– 9136,  DOI: 10.1039/c3sm50959a
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      Elongated polystyrene spheres as resonant building blocks in anisotropic colloidal crystals
      Schneider, Dirk; Beltramo, Peter J.; Mattarelli, Maurizio; Pfleiderer, Patrick; Vermant, Jan; Crespy, Daniel; Montagna, Maurizio; Furst, Eric M.; Fytas, George
      Soft Matter (2013), 9 (38), 9129-9136CODEN: SMOABF; ISSN:1744-683X. (Royal Society of Chemistry)
      Colloidal crystals have gained increasing importance due to their fascinating ability to mold the flow of light and sound (heat). The characteristics of these ordered assemblies of particles are strongly detd. by the resp. building blocks, which require complete understanding of their phys. properties. In this study the mech. properties of stretched polystyrene colloids (spheroids) are addressed. The non-invasive technique of Brillouin light scattering captures the vibrational spectra at hypersonic (GHz) frequencies. Resolved eigenmodes are considered fingerprints of the particles' shape, size and composing materials. A single particle model is used to simulate the exptl. data by calcn. of all active modes and subsequent evaluation of their contribution to the spectrum. Compared to spheres (high symmetry) more modes contribute to the spectra that limit the resoln. at very high frequencies, due to the lifted mode degeneracy. Knowing the nature of the principal modes of spheroids is a precondition to understand the phononic dispersion in the resp. colloidal crystals, in particular those responsible for anticrossing interactions with the effective medium acoustic phonon.
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      Tchebotareva, A. L.; Van Dijk, M. A.; Ruijgrok, P. V.; Fokkema, V.; Hesselberth, M. H. S.; Lippitz, M.; Orrit, M. Acoustic and Optical Modes of Single Dumbbells of Gold Nanoparticles. ChemPhysChem 2009, 10, 111– 114,  DOI: 10.1002/cphc.200800289
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      Acoustic and optical modes of single dumbbells of gold nanoparticles
      Tchebotareva, Anna L.; van Dijk, Meindert A.; Ruijgrok, Paul V.; Fokkema, Vincent; Hesselberth, Marcel H. S.; Lippitz, Markus; Orrit, Michel
      ChemPhysChem (2009), 10 (1), 111-114CODEN: CPCHFT; ISSN:1439-4235. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Gold vibrations: A new elastic (stretching) mode, appearing in individual dumbbells of gold nanospheres at 5-7 GHz (see figure), is a function of the contact area. This can be used to est. the contact area between the particles, which plays an important role in the local enhancement of electromagnetic fields in such nanoantenna structures.
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      Mattarelli, M.; Montagna, M.; Still, T.; Schneider, D.; Fytas, G. Vibration Spectroscopy of Weakly Interacting Mesoscopic Colloids. Soft Matter 2012, 8, 4235– 4243,  DOI: 10.1039/c2sm07034k
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      Vibration spectroscopy of weakly interacting mesoscopic colloids
      Mattarelli, Maurizio; Montagna, Maurizio; Still, Tim; Schneider, Dirk; Fytas, George
      Soft Matter (2012), 8 (15), 4235-4243CODEN: SMOABF; ISSN:1744-683X. (Royal Society of Chemistry)
      Brillouin spectroscopy has been used to study the vibrational dynamics of clusters of spherical polystyrene (PS) particles with different size. In a first approxn., the spectra can be described by a single particle model within the continuum approxn. of the Lamb theory. The model yields excellent results for particles with diam. d ⪆ 500 nm, but fails in accounting for the lineshapes of the obsd. lowest frequency signals in the vibration spectrum of smaller particles due to a broadening and shift of the lines of confined vibrations. The model also fails to predict an addnl. very low frequency broad band in the case of small particles (d .ltorsim. 400nm). This band is attributed to phonon propagation in multiple-particle clusters governed by the interactions among particles. These interactions also produce extended modes in close relation to the Lamb modes of the single sphere. A simple model for the particle interaction allows to represent the new spectral features and est. the strength of the interactions and the long wavelength longitudinal velocity in the colloidal clusters. Their thermal annealing near the glass transition of PS enhances the interactions which are manifested in the low frequency particle vibration spectrum and the phonon bands assocd. with the vibrations of the individual spheres. The emerging particle vibration spectroscopy becomes a sensitive tool of the colloids' thermo-mech. properties as well their interactions.
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      Girard, A.; Gehan, H.; Mermet, A.; Bonnet, C.; Lermé, J.; Berthelot, A.; Cottancin, E.; Crut, A.; Margueritat, J. Acoustic Mode Hybridization in a Single Dimer of Gold Nanoparticles. Nano Lett. 2018, 18, 3800– 3806,  DOI: 10.1021/acs.nanolett.8b01072
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      Acoustic Mode Hybridization in a Single Dimer of Gold Nanoparticles
      Girard, Adrien; Gehan, Helene; Mermet, Alain; Bonnet, Christophe; Lerme, Jean; Berthelot, Alice; Cottancin, Emmanuel; Crut, Aurelien; Margueritat, Jeremie
      Nano Letters (2018), 18 (6), 3800-3806CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      The acoustic vibrations of single monomers and dimers of Au nanoparticles were studied by measuring for the 1st time their ultralow-frequency micro-Raman scattering. This expt. provides access not only to the frequency of the detected vibrational modes but also to their damping rate, which is obscured by inhomogeneous effects in measurements on ensembles of nano-objects. This allows a detailed anal. of the mech. coupling occurring between 2 close nanoparticles (mediated by the polymer surrounding them) in the dimer case. Such coupling induces the hybridization of the vibrational modes of each nanoparticle, leading to the appearance in the Raman spectra of 2 ultralow-frequency modes corresponding to the out-of-phase longitudinal and transverse (with respect to the dimer axis) quasi-translations of the nanoparticles. Addnl., it is also shown to shift the frequency of the quadrupolar modes of the nanoparticles. Exptl. results are interpreted using finite-element simulations, which enable the unambiguous identification of the detected modes and despite the simplifications made lead to a reasonable reprodn. of their measured frequencies and quality factors. The demonstrated feasibility of low-frequency Raman scattering expts. on single nano-objects opens up new possibilities to improve the understanding of nanoscale vibrations with this technique being complementary with single nano-object time-resolved spectroscopy as it gives access to different vibrational modes.
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      Noual, A.; Kang, E.; Maji, T.; Gkikas, M.; Djafari-Rouhani, B.; Fytas, G. Optomechanic Coupling in Ag Polymer Nanocomposite Films. J. Phys. Chem. C 2021, 125, 14854– 14864,  DOI: 10.1021/acs.jpcc.1c04549
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      Optomechanic Coupling in Ag Polymer Nanocomposite Films
      Noual, Adnane; Kang, Eunsoo; Maji, Tanmoy; Gkikas, Manos; Djafari-Rouhani, Bahram; Fytas, George
      Journal of Physical Chemistry C (2021), 125 (27), 14854-14864CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Particle vibrational spectroscopy has emerged as a new tool for the measurement of elasticity, glass transition, and interactions at a nanoscale. For colloid-based materials, the weakly localized particle resonances in a fluid or solid medium renders their detection difficult. The strong amplification of the inelastic light scattering near surface plasmon resonance of metallic nanoparticles (NPs) allowed not only the detection of single NP eigenvibrations but also the interparticle interaction effects on the acoustic vibrations of NPs mediated by strong optomech. coupling. The rattling and quadrupolar modes of Ag/polymer and polymer-grafted Ag NPs with different diams. in their assemblies are probed by Brillouin light spectroscopy (BLS). The authors present thorough theor. 3D calcns. for anisotropic Ag elasticity to quantify the frequency and intensity of the rattling mode and hence its BLS activity for different interparticle sepns. and matrix rigidity. Theor., a liq.-like environment, e.g., poly(isobutylene) (PIB) does not support rattling vibration of Ag dimers but unexpectedly hardening of the extremely confined graft melt renders both activation of the former and a frequency blue shift of the fundamental quadrupolar mode in the grafted nanoparticle Ag@PIB film.
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      A review with 197 refs. is given. Microfabrication, the generation of small structures, is essential to much of modern science and technol.; it supports information technol. and permeates society through its role in microelectronics and optoelectronics. The patterning required in microfabrication is usually carried out with photolithog. Although it is difficult to find another technol. with more dominant influence, photolithog. nonetheless has disadvantages. The sizes of the features it can produce are limited by optical diffraction, and the high-energy radiation needed for small features requires complex facilities and technologies. Photolithog. is expensive; it cannot be easily applied to nonplanar surfaces; it tolerates little variation in the materials that can be used; and it provides almost no control over the chem. of patterned surfaces, esp. when complex org. functional groups of the sorts needed in chem., biochem., and biol. are involved. We wished to develop alternative, non-photolithog. microfabrication methods that would complement photolithog. These techniques would ideally circumvent the diffraction limits of photolithog., provide access to three-dimensional structures, tolerate a wide range of materials and surface chemistries, and be inexpensive, exptl. convenient, and accessible to mol. scientists. We have developed a set of such methods that we call "soft lithog.", since all members share the common feature that they use a patterned elastomer as the mask, stamp, or mold. We describe here soft lithog., and survey its ability to provide routes to high-quality patterns and structures with lateral dimensions of about 30 nm to 500 μm in systems presenting problems in topol., materials, or mol.-level definition that cannot (or at least not easily) be solved by photolithog.
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      Kim, H.; Cang, Y.; Kang, E.; Graczykowski, B.; Secchi, M.; Montagna, M.; Priestley, R. D.; Furst, E. M.; Fytas, G. Direct Observation of Polymer Surface Mobility via Nanoparticle Vibrations. Nat. Commun. 2018, 9, 1– 11,  DOI: 10.1038/s41467-018-04854-w
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      Nature Communications (2018), 9 (1), 1-17CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
      Gamma-aminobutyric acid (GABA) is the principal inhibitory neurotransmitter in the brain; however, the roles of GABA in antimicrobial host defenses are largely unknown. Here we demonstrate that GABAergic activation enhances antimicrobial responses against intracellular bacterial infection. Intracellular bacterial infection decreases GABA levels in vitro in macrophages and in vivo in sera. Treatment of macrophages with GABA or GABAergic drugs promotes autophagy activation, enhances phagosomal maturation and antimicrobial responses against mycobacterial infection. In macrophages, the GABAergic defense is mediated via macrophage type A GABA receptor (GABAAR), intracellular calcium release, and the GABA type A receptor-assocd. protein-like 1 (GABARAPL1; an Atg8 homolog). Finally, GABAergic inhibition increases bacterial loads in mice and zebrafish in vivo, suggesting that the GABAergic defense plays an essential function in metazoan host defenses. Our study identified a previously unappreciated role for GABAergic signaling in linking antibacterial autophagy to enhance host innate defense against intracellular bacterial infection.
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      Wang, Z.; Cang, Y.; Kremer, F.; Thomas, E. L.; Fytas, G. Determination of the Complete Elasticity of Nephila Pilipes Spider Silk. Biomacromolecules 2020, 21, 1179– 1185,  DOI: 10.1021/acs.biomac.9b01607
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      Spider silks are remarkable materials designed by nature to have extraordinary elasticity. Their elasticity, however, remains poorly understood, as typical stress-strain expts. only allow access to the axial Young's modulus. In this work, micro-Brillouin light spectroscopy (micro-BLS), a noncontact, nondestructive technique, is utilized to probe the direction-dependent phonon propagation in the Nephila pilipes spider silk and hence solve its full elasticity. To the best of our knowledge, this is the first demonstration on the detn. of the anisotropic Young's moduli, shear moduli, and Poisson's ratios of a single spider fiber. The axial and lateral Young's moduli are found to be 20.9 ± 0.8 and 9.2 ± 0.3 GPa, resp., and the anisotropy of the Young's moduli further increases upon stretching. In contrast, the shear moduli and Poisson's ratios exhibit very weak anisotropy and are robust to stretching.
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      Park, J.-G.; Kim, S.-H.; Magkiriadou, S.; Choi, T. M.; Kim, Y.-S.; Manoharan, V. N. Full-Spectrum Photonic Pigments with Non-Iridescent Structural Colors through Colloidal Assembly. Angew. Chem., Int. Ed. 2014, 53, 2899– 2903,  DOI: 10.1002/anie.201309306
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      Full-Spectrum Photonic Pigments with Non-iridescent Structural Colors through Colloidal Assembly
      Park, Jin-Gyu; Kim, Shin-Hyun; Magkiriadou, Sofia; Choi, Tae Min; Kim, Young-Seok; Manoharan, Vinothan N.
      Angewandte Chemie, International Edition (2014), 53 (11), 2899-2903CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Structurally colored materials could potentially replace dyes and pigments in many applications, but it is challenging to fabricate structural colors that mimic the appearance of absorbing pigments. The authors demonstrate the microfluidic fabrication of photonic pigments consisting of microcapsules contg. dense amorphous packings of core-shell colloidal particles. These microcapsules show noniridescent structural colors that are independent of viewing angle, a crit. requirement for applications such as displays or coatings. The design of the microcapsules facilitates the suppression of incoherent and multiple scattering, enabling the fabrication of photonic pigments with colors spanning the visible spectrum. The authors' findings should provide new insights into the design and synthesis of materials with structural colors.
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      This paper describes the compatibility of poly(dimethylsiloxane) (PDMS) with org. solvents; this compatibility is important in considering the potential of PDMS-based microfluidic devices in a no. of applications, including that of microreactors for org. reactions. The three aspects of compatibility, the swelling of PDMS in a solvent, the partitioning of solutes between a solvent and PDMS, and the dissoln. of PDMS oligomers in a solvent are considered. Of these three parameters that det. the compatibility of PDMS with a solvent, the swelling of PDMS had the greatest effect. Exptl. measurements of swelling were correlated with the soly. parameter, δ (cal1/2 cm-3/2), which is based on the cohesive energy densities, c (cal/cm3), of the materials. Solvents that swelled PDMS the least included water, nitromethane, DMSO, ethylene glycol, perfluorotributylamine, perfluorodecalin, acetonitrile, and propylene carbonate; solvents that swelled PDMS the most were diisopropylamine, triethylamine, pentane, and xylenes. Highly swelling solvents were useful for extg. contaminants from bulk PDMS and for changing the surface properties of PDMS. The feasibility of performing org. reactions in PDMS was demonstrated by performing a Diels-Alder reaction in a microchannel.
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      The mech. properties of dry and wet agglomerates are reviewed in the context of continuum solid and fluid mechanics and fracture mechanics. The focus is on practical measurements of tensile strength, yield strength, hardness and fracture toughness, and how they define the attrition behavior of agglomerates. Well-established mech. testing methods can be applied to agglomerates, but certain limitations apply due to the nature of agglomerates being inherently non-equil. (glassy), anisotropic, and compressible. The mech. response of agglomerates may vary from brittle, elastic-plastic (for most dry agglomerates) to elastoviscoplastic and fully plastic (for wet agglomerates) depending on prepn. method, environment, structure and loading conditions. This transition from solid to liq.-like behavior can be followed by applying solid/fracture mechanics and rheol.-based testing, resp. It is clear that most available practical measures of agglomerate mech. behavior are not intrinsic, i.e. independent of test specimen geometry and the manner in which stress is applied. Therefore, selection and execution of measurements must be guided by loading conditions and agglomerate size and structure from the process of concern. Micromech. modeling addresses some of the dependence of mech. properties on the structure of agglomerates [e.g., porosity] and the properties of their primary constituents, but it cannot describe quant. bulk deformation and fracture of agglomerates. For this reason, agglomerate formulations are still tailored to achieve desired performance by empirical correlation of primary particle and agglomerate structure to mech. properties.
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      Pérez, R.; Gumbsch, P. Directional Anisotropy in the Cleavage Fracture of Silicon. Phys. Rev. Lett. 2000, 84, 5347– 5350,  DOI: 10.1103/PhysRevLett.84.5347
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      Total-energy pseudopotential calcns. were used to study the cleavage anisotropy in Si. Cracks propagate easily on {111} and {110} planes provided crack propagation proceeds in the 〈110〉 direction. In contrast, if the crack is driven in a 〈001〉 direction on a {110} plane the bond breaking process is discontinuous and assocd. with pronounced relaxations of the surrounding atoms, which results in a large lattice trapping. The different lattice trapping for different crack propagation directions can explain the exptl. obsd. cleavage anisotropy in Si single crystals.
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      Hueckel, T.; Hocky, G. M.; Palacci, J.; Sacanna, S. Ionic Solids from Common Colloids. Nature 2020, 580, 487– 490,  DOI: 10.1038/s41586-020-2205-0
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      From rock salt to nanoparticle superlattices, complex structure can emerge from simple building blocks that attract each other through Coulombic forces1-4. On the micrometre scale, however, colloids in water defy the intuitively simple idea of forming crystals from oppositely charged partners, instead forming non-equil. structures such as clusters and gels5-7. Although various systems have been engineered to grow binary crystals8-11, native surface charge in aq. conditions has not been used to assemble cryst. materials. Here we form ionic colloidal crystals in water through an approach that we refer to as polymer-attenuated Coulombic self-assembly. The key to crystn. is the use of a neutral polymer to keep particles sepd. by well defined distances, allowing us to tune the attractive overlap of elec. double layers, directing particles to disperse, crystallize or become permanently fixed on demand. The nucleation and growth of macroscopic single crystals is demonstrated by using the Debye screening length to fine-tune assembly. Using a variety of colloidal particles and com. polymers, ionic colloidal crystals isostructural to caesium chloride, sodium chloride, aluminum diboride and K4C60 are selected according to particle size ratios. Once fixed by simply dilg. out soln. salts, crystals are pulled out of the water for further manipulation, demonstrating an accurate translation from soln.-phase assembly to dried solid structures. In contrast to other assembly approaches, in which particles must be carefully engineered to encode binding information12-18, polymer-attenuated Coulombic self-assembly enables conventional colloids to be used as model colloidal ions, primed for crystn.
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      With the help of field-flow fractionation, photon correlation spectroscopy, and ESR techniques, as well as more conventional labeling approaches, we have accomplished the anal. characterization of the adsorption complexes formed between, on the one hand, a series of selected triblock Pluronic polymeric surfactants with comparable lengths of their central poly(propylene oxide) (PPO) block but with varying poly(ethylene oxide) (PEO) block lengths and, on the other, a series of differently sized polystyrene colloids. For a given surfactant, it was found that both surface concns. and adlayer thicknesses are strongly related to the particle size, such that smaller particles take up fewer polymer mols. per unit area than the larger ones. The reduced crowding around each PEO chain results in thinner adlayers and higher chain mobilities. In adsorption complexes involving 69-nm polystyrene particles, the adlayer thickness is close to the diam. calcd. for the free PEO chain in aq. soln. In addn., for particles of a given size, it is the size of the surfactant's hydrophobic center block (PPO), rather than its flanking tails (PEO), that dets. the surface concn. Thus, triblocks of similar PPO size showed comparable surface concn., while the longer PEO chains were assocd. with thicker adlayers as well as greater chain dynamics.
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      A review. This mini-review highlights the latest advances in the chem. and biol. of Pluronic triblock copolymers. We focus on their applications in medicine, as drug delivery carriers, biol. response modifiers, and pharmaceutical ingredients. Examples of drug delivery systems and formulations currently in clin. use, clin. trials or preclin. development are highlighted. We also discuss the role that Pluronic copolymers may play in the innovative design of new nanomedicines in the near future.
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      Graczykowski, B.; Vogel, N.; Bley, K.; Butt, H. J.; Fytas, G. Multiband Hypersound Filtering in Two-Dimensional Colloidal Crystals: Adhesion, Resonances, and Periodicity. Nano Lett. 2020, 20, 1883– 1889,  DOI: 10.1021/acs.nanolett.9b05101
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      Multiband Hypersound Filtering in Two-Dimensional Colloidal Crystals: Adhesion, Resonances, and Periodicity
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      Nano Letters (2020), 20 (3), 1883-1889CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      The hypersonic phonon propagation in large-area two-dimensional colloidal crystals is probed by spontaneous micro Brillouin light scattering. The dispersion relation of thermally populated Lamb waves reveals multiband filtering due to three distinct types of acoustic band gaps. We find Bragg gaps accompanied by two types of hybridization gaps in both sub- and superwavelength regimes resulting from contact-based resonances and nanoparticle eigenmodes, resp. The operating GHz frequencies can be tuned by particle size and depend on the adhesion at the contact interfaces. The exptl. dispersion relations are well represented by a finite element method simulation, physicochem. enabling identification of obsd. modes. The presented approach also allows for contactless study of the contact stiffness of submicrometer particles, which reveals size effect deviating from macroscopic predictions.
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      Babacic, V.; Varghese, J.; Coy, E.; Kang, E.; Pochylski, M.; Gapinski, J.; Fytas, G.; Graczykowski, B. Mechanical Reinforcement of Polymer Colloidal Crystals by Supercritical Fluids. J. Colloid Interface Sci. 2020, 579, 786– 793,  DOI: 10.1016/j.jcis.2020.06.104
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      Mechanical reinforcement of polymer colloidal crystals by supercritical fluids
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      Journal of Colloid and Interface Science (2020), 579 (), 786-793CODEN: JCISA5; ISSN:0021-9797. (Elsevier B.V.)
      Colloidal crystals realized by self-assembled polymer nanoparticles have prominent attraction as a platform for various applications from assembling photonic and phononic crystals, acoustic metamaterials to coating applications. However, the fragility of these systems limits their application horizon. In this work the uniform mech. reinforcement and tunability of 3D polystyrene colloidal crystals by means of cold soldering are reported. This structural strengthening is achieved by high pressure gas (N2 or Ar) plasticization at temps. well below the glass transition. Brillouin light scattering is employed to monitor in-situ the mech. vibrations of the crystal and thereby det. preferential pressure, temp. and time ranges for soldering, i.e. formation of phys. bonding among the nanoparticles while maintaining the shape and translational order. This low-cost method is potentially useful for fabrication and tuning of durable devices including applications in photonics, phononics, acoustic metamaterials, optomechanics, surface coatings and nanolithog.
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      Kang, E.; Kim, H.; Gray, L. A. G.; Christie, D.; Jonas, U.; Graczykowski, B.; Furst, E. M.; Priestley, R. D.; Fytas, G. Ultrathin Shell Layers Dramatically Influence Polymer Nanoparticle Surface Mobility. Macromolecules 2018, 51, 8522– 8529,  DOI: 10.1021/acs.macromol.8b01804
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      Macromolecules (Washington, DC, United States) (2018), 51 (21), 8522-8529CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)
      Advances in nanoparticle synthesis, self-assembly, and surface coating or patterning have enabled a diverse array of applications ranging from photonic and phononic crystal fabrication to drug delivery vehicles. One of the key obstacles restricting its potential is structural and thermal stability. The presence of a glass transition can facilitate deformation within nanoparticles, thus resulting in a significant alteration in structure and performance. Recently, we detected a glassy-state transition within individual polystyrene nanoparticles and related its origin to the presence of a surface layer with enhanced dynamics compared to the bulk. The presence of this mobile layer could have a dramatic impact on the thermal stability of polymer nanoparticles. Here, we demonstrate how the addn. of a shell layer, as thin as a single polymer chain, atop the nanoparticles could completely eliminate any evidence of enhanced mobility at the surface of polystyrene nanoparticles. The ultrathin polymer shell layers were placed atop the nanoparticles via two approaches: (i) covalent bonding or (ii) electrostatic interactions. The temp. dependence of the particle vibrational spectrum, as recorded by Brillouin light scattering, was used to probe the surface mobility of nanoparticles with and without a shell layer. Beyond suppression of the surface mobility, the presence of the ultrathin polymer shell layers impacted the nanoparticle glass transition temp. and shear modulus, albeit to a lesser extent. The implication of this work is that the core-shell architecture allows for tailoring of the nanoparticle elasticity, surface softening, and glass transition temp.
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      Randazzo, K.; Bartkiewicz, M.; Graczykowski, B.; Cangialosi, D.; Fytas, G.; Zuo, B.; Priestley, R. Direct Visualization and Characterization of Interfacially-Adsorbed Polymer atop Nanoparticles and within Nanocomposites. Macromolecules 2021,This paper is accepted for publication.
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Please note: If you switch to a different device, you may be asked to login again with only your ACS ID.

Please note: If you switch to a different device, you may be asked to login again with only your ACS ID.

MENDELEY PAIRING EXPIRED
Your Mendeley pairing has expired. Please reconnect