Selective Vapor Condensation for the Synthesis and Assembly of Spherical Colloids with a Precise Rough Patch

Patchy particles occupy an increasingly important space in soft matter research due to their ability to assemble into intricate phases and states. Being able to fine-tune the interactions among these particles is essential to understanding the principles governing the self-assembly processes. However, current fabrication techniques often yield patches that deviate chemically and physically from the native particles, impeding the identification of the driving forces behind self-assembly. To overcome this challenge, we propose a new approach to synthesizing spherical colloids with a well-defined rough patch on their surface. By treating polystyrene microspheres with vapors of a good solvent, here an acetone–water mixture, we achieve selective polymer corrugation on the particle surface resulting in a chemically similar yet rough surface patch. The key step is the selective condensation of the acetone–water vapors on the apex of the polystyrene microparticles immobilized on a substrate, which leads to rough patch formation. We leverage the ability to tune the vapor–liquid equilibrium of the volatile acetone–water mixture to precisely control the polymer corrugation on the particle surface. We demonstrate the dependence of patch formation on particle and substrate wettability, with the condensation occurring on the particle apex only when it is more wettable than the substrate, which is consistent with Volmer’s classical nucleation theory. By combining experiments and molecular dynamics simulations, we identify the role of the rough patch in the depletion interaction-driven self-assembly of the microspheres, which is crucial for designing programmable supracolloidal structures.

scheme with a timestep of 0.01 (mσ 2 /ε) 1/2 .The simulations were run for 2 × 10 7 steps.The snapshots of the simulations were created by the VMD software.
The final structure formed by the microparticles was analyzed using types of the contact points between neighboring particles.A contact between a specific pair was defined when the distance between the center of mass of two particles is less than 3σ.Three types of contact: S-S, S-R and R-R are classified by identifying the point types with a minimum distance  89: between two contacted nanoparticles. 89: is calculated as below: 89: = min { 9,< } where  9,< is the Euclidean distance between beads i and j from each of the particles in the contact pair.The index of i and j are recorded as I and J when their distance is minimum.If both beads I and J are type R, the contact is R-R.If both beads I and J are type S, the contact is S-S.Otherwise, it's a S-R contact.

Figure S1 .
Figure S1.Size distribution for the synthesized PS particles used for experiments.The frequency distribution is obtained analyzing the optical microscope images.The bars represent the measured data, and the line represents the fit to the data using log-normal distribution.The particles show a mean radius of ~1.1 µm.

Figure S2 .
Figure S2.3-D model and photographs of dual chamber system used in our experiments.(A) Scheme depicting the mechanism allowing the inner Chamber 1 to open while the outer Chamber 2 remains sealed.This allows for the acetone-water mixture to remain in equilibrium while controlling particle exposure.(B-C) Exterior of the outer Chamber 2. (D-E) Interior of the outer chamber with a closed (D) and open inner Chamber 1 (E).

Figure S3 .
Figure S3.Vapor-liquid equilibrium (VLE) relation for acetone-water mixtures.This relationship was simulated through Aspen+ utilizing the National Institute of Standards and Technology (NIST) databanks available.Graphs were generated both through the NRTL (Non-random two-liquid) (black, solid line) and UNIFAC (UNIQUAC functional-group activity coefficients) (blue, dotted line) property methods.For experimentation, liquid composition of each component was directly measured, and vapor composition was estimated through this relationship.

Figure S4 .
Figure S4.(a) ATR-FTIR of the smooth and rough PS particles.The spectrum shows no significant chemical change in the particles upon introduction of the rough patch.(b) Size exclusion chromatogram (SEC) showing the identical molar mass distribution of the polymer forming the smooth and patchy particles.

Figure S5 .Figure S6 .
Figure S5.Increase in the radius i.e. swelling of the non-crosslinked PS particles upon changing the fraction of the acetone present in the continuous medium.The change in size was determined using optical microscopy.The circles are the mean of the measured values for a set of at least 1500 particles, and bars represent the standard error.The particles show shape deformation beyond ɸL>0.4,and thus were not used for estimating particle swelling.Note that this swelling was measured by dispersing the PS particles in the liquid acetone-water mixture.

Figure S7 .
Figure S7.AFM images and height profiles of particles at various acetone molar fractions (φL = 0.0, 0.4, 0.6, 0.8).(A) Illustrates the AFM 3D reconstruction of polystyrene particle surfaces subjected to varying acetone-water mixtures.(B) Depicts the corresponding height profiles at each molar fraction.The x-axis indicates the distance covered by the cantilever across the surface, while the y-axis represents the height traversed by the cantilever.

Figure S8 .
Figure S8.Roughness of patches at various acetone molar fractions (φL = 0.0, 0.4, 0.6, 0.8).The symbols depict the maximum roughness of the patches based on over ten measurements.The error bars indicate the standard error.Each group underwent exposure to the corresponding acetone vapor for a duration of 20 minutes.

Figure S9 .
Figure S9.SEM image showing the deformation in the shape of the particles when the substrate was more wettable than the particles, i.e.  !>  " .

Figure S10 .
Figure S10.SEM images of PS particles after being exposed to vapors of acetone-water mixture with ɸL = 0.36 for t = 60 minutes.(A) Top view of particle surface.Rough patch covers entire visible surface.(B) Top view of PS monolayer.Effects of exposure have begun to show signs of shape distortion.(C) Side view of PS monolayer (camera tilted 74°).Aspect ratio begins change by this timepoint.Scale bars represent 2 µm.

Figure S11 .
Figure S11.Synthesis of interlinked particle assemblies.(A) Schematic depicting the method for particle interlinking.(B) Particle clusters linked together through isotropic assembly, followed by acetone vapor exposure.(C) Particle clusters linked through anisotropic assembly followed by exposure to acetone vapor.(D, E) Three-particle cluster (D) and chain (E) interlinked and imaged in aqueous solution.Consecutive images display different time points of the same interlinked particle, showing its movement as a single unit.Scale bar represents 1 µm.

Figure S11 .
Figure S11.Cluster size distribution obtained for the experiments and simulations for patch sizes f = 0.2 and 0.3.Here the cluster size is defined as the number of particles within a cluster.

Figure S13 .
Figure S13.Data processing performed using Gwyddion SPM analysis software.Panels (a-c) display the progression from raw AFM scan images to processed representations.(a) Presents the initial raw image captured by the AFM.(b) Illustrates the image post second degree polynomial background removal, and (c) showcases the image following a 5-pixel Gaussian blur application.Corresponding 3D reconstructions at each processing step are presented in panels (d-f).