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Tuning the Charge of Sliding Water Drops
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Langmuir

Cite this: Langmuir 2022, 38, 19, 6224–6230
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https://doi.org/10.1021/acs.langmuir.2c00941
Published May 2, 2022

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Abstract

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When a water drop slides over a hydrophobic surface, it usually acquires a positive charge and deposits the negative countercharge on the surface. Although the electrification of solid surfaces induced after contact with a liquid is intensively studied, the actual mechanisms of charge separation, so-termed slide electrification, are still unclear. Here, slide electrification is studied by measuring the charge of a series of water drops sliding down inclined glass plates. The glass was coated with hydrophobic (hydrocarbon/fluorocarbon) and amine-terminated silanes. On hydrophobic surfaces, drops charge positively while the surfaces charge negatively. Hydrophobic surfaces coated with a mono-amine (3-aminopropyltriethyoxysilane) lead to negatively charged drops and positively charged surfaces. When coated with a multiamine (N-(3-trimethoxysilylpropyl)diethylenetriamine), a gradual transition from positively to negatively charged drops is observed. We attribute this tunable drop charging to surface-directed ion transfer. Some of the protons accepted by the amine-functionalized surfaces (−NH2 with H+ acceptor) remain on the surface even after drop departure. These findings demonstrate the facile tunability of surface-controlled slide electrification.

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Introduction

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Charge formation on solid–gas interfaces after liquid contact (i.e., charge separation (1,2)) is a universal but incompletely understood phenomenon. (3) Charge separation is commonly attributed to ion transfer to the solid because of the presence of ions, such as H+ and OH– in water. (4−6) During the sliding contact of water drops on surfaces, charging (i.e., slide electrification) is induced by sequential ionization, dissociation, and adsorption of ions onto the solid surface. The charges are then left on the surface after departure of the drop. Thereafter, they are neutralized by the environment, (7) which may be caused by the flow of electrons, ion migration, and/or desorption of ions. (8,9) However, despite significant research in charge separation and slide electrification, much dispute still remains. (10,11) In particular, the origin of charge formation can still be attributed to ion (8,11−16) and/or electron (10,11,16,17) transfer.
The aim of this work is to improve the understanding of ion-transfer mechanisms by changing the surface chemistry of hydrophobic surfaces. Based on this knowledge, we intend to control the strength and polarity of slide electrification. To date, almost all slide electrification experiments have produced positively charged drops and negatively charged surfaces. (8,10−12,14,18−20) The rare exception of producing negatively charged drops was achieved only through the use of low pH (pH < 3) by Sosa et al., (12) thus below the surface’s pH of zero charge, that is, the pH at which the net charge of the surface is zero.
To our knowledge, the use of hydrophilic or water-interactive functionalities (i.e., amines) was never achieved under continuous drop slide electrification. Hydrophobic surfaces are often used because drops do not easily slide on hydrophilic surfaces. Sliding drops become unstable, and satellite droplets are formed. (21) The formation and presence of remnant satellite droplets affects the charging–discharging dynamics and the continuation of drop slide electrification (i.e., the surface eventually appears uncharged as bulk water actively discharges surfaces). To exploit the use of hydrophilic moieties (i.e., amines), a two-step functionalization process is used. In the first step, a hydrophobic (hydro/fluorocarbon) primary layer is functionalized. In the second step, the sequential functionalization of secondary amine layers confers charge modification characteristics. These amine-integrated hydrophobic surfaces preserve low contact angle hysteresis and inhibit the formation of satellite droplets during slide electrification. The integrated amines experience a hydrolytic reaction during wetting, consuming H+ ions, R-NH2 + H+ → R-NH3+, thus producing negatively charged water drops.

Experimental Section

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Chemical Vapor Deposition Synthesis of Surfaces

Cleaning and Activation of Amorphous Glass Substrates

Amorphous glass substrates (1 mm thick) (Thermoscientific) were washed in streams of ethanol and acetone before blow-drying by dry nitrogen. Care was taken to avoid the use of visually evident defective glass (stains, spots, cracks, etc.). The glass substrates were then O2-plasma-treated for 10 min at 100% power (Diener Electronic Plasma Surface Technology: Femto BLS, Ebhausen, Germany).

Primary Functionalization Layers

Cleaned and activated glass substrates were placed into a desiccator (25 cm diameter, V = 9.2 L) at a distance of ca. 11 cm from the center, where 1 mL of the functionalizing agent was deposited, ca. 1 cm lower than the glass substrates. Silanes were deposited into the desiccator before evacuation. The hydrocarbon-based trichloro(propyl)silane, TCPS, was evacuated to ca. 200 mbar while the fluorocarbon-based trichloro(1H,1H,2H,2H-perfluorooctyl)silane, PFOTS, was evacuated to ca. 50 mbar. A reaction time of 30 min was allowed for primary layers. A separate desiccator was used for each chemical (TCPS and PFOTS) to avoid cross-contamination.

Secondary Functionalization Layers

If the primary layers are to be integrated with a secondary layer (see below), only 5 min (TCPS) and 10 (PFOTS) min of reaction time was allowed, respectively. Then, secondary layers of (3-aminopropyl)triethoxysilane, APTES or N-(3-trimethoxysilylpropyl)diethylenetriamine, NTDET, were deposited as 1 mL of liquid into their own respective desiccator, again at ca. 11 cm from the center. The pressure was then evacuated to ca. 50 mbar and kept for 6 h.

Completion of Functionalization and Equilibration Time

After functionalization, the glass substrates were kept at 50 mbar without any silane present to vent unreacted silanes. Afterward, the coated glass substrates were left to equilibrate with the ambient air environment (T = 20 °C, humidity = 30–70%) for 5 days before testing. Because of the tri-functionality of silanes used, (29−31) sequential surface hydrolysis and condensation reactions will result in a cross-linked network with both material configurations layered.

Charge Analysis

The charge measurement setup is kept in an electrically grounded Faraday cage, as described before. (7) Peripheral electrical devices, that is, the pump and the current amplifier were kept outside of the Faraday cage. Test surfaces were mounted on a tilted stage at an angle of 50°. A flat-tipped syringe needle, 2 mm inner diameter, was mounted 5 mm above the top part of the sample. A peristaltic pump (Gilson Minipuls 3, Wisconson, USA) was used to produce V = 45 μL deionized water drops (Sartorius Arium Pro VF, 18.2 MΩ cm resistivity, Germany) at an interval of between 2 and 16 s. The drops fell approximately 0.5 cm, at minimum natural detachment height, before landing on the coated glass surfaces. As drops slide down, they contact a series of two electrodes. The first electrode (silver wire, upon landing) grounds the drop. The second electrode (gold-tipped electrode, at 4 cm) measures the drop current via a low noise current amplifier (response time: 5 ms, FEMTO DLPCA-200, Berlin, Germany). Upon leaving the second electrode, drops roll over the extended end of the coated glass surface, and into a collection dish. Before every new experiment, an ionizing air stream (Simco-Ion, Pennsylvania, USA) was directed over the surface for 1 min in order to neutralize the surface. Data were collected and recorded using a National Instruments data acquisition card (NI USB-6366 X-Series) and LabVIEW software. Drops (typically 500 drops) run successively over the coated glass surface at defined time intervals (typically 2 s). For every drop, a current spike was recorded using the second electrode. Current signals were integrated for every drop to quantify the drop charge. Repeats are typically performed over 3–5 days of cross-batch samples and multiple repeats on each day (up to 3 readings), culminating in a total of 10 data points for each surface variant analyzed.

Wetting Analysis

Roll-off angle, and advancing and receding contact angles were measured using 45 μL water drops using an OCA 35 contact angle goniometer (Dataphysics, Germany, zoom factor 1.0). Drops were tilted (1°/s) until they rolled off. The contact angle hysteresis and roll-off angles are reported respectively for the surface variants over three measurements.

Drop Mobility Analysis

To measure the dynamics of sliding drops, 33 μL water drops were deposited at intervals of 2 s on tilted surfaces. The syringe needle was grounded and kept at 5 mm above the surfaces. A peristaltic pump (Minipuls 3, Gilson) delivers the drops. Drops were imaged at a frame rate of 1000 FPS (FASTCAM MINI UX100, Photron with a Titan TL telecentric lens, 0.268×, 1″ C-Mount, Edmund Optics), from the front and the side over a slide length of 4 cm. Analysis of the video images allows extraction of drop velocity and advancing/receding contact angles. All parameters vary with time and position. To extract advancing/receding contact angles, the open drop shape analysis package from MATLAB (DSAfM) is adopted. Dynamic contact angles were determined by applying a polynomial fit to every contour image.

Results and Discussion

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Surface Functionalization and Characterization

Plain soda lime glass was functionalized (see the Experimental Section) either by a primary layer of trichloro(propyl)silane (TCPS) or trichloro(1H,1H,2H,2H-perfluorooctyl)silane (PFOTS) (Figure 1). Thereafter, a secondary layer of a mono-amine: (3-aminopropyl)triethoxysilane (APTES) or a multi-amine: N-(3-trimethoxysilylpropyl)diethylenetriamine (NTDET) was added. In contrast to the simpler mono-amine, NTDET has an alternating molecular configuration as follows: a −C3H6–, a secondary amine −NH–, followed by −C2H4–, a second secondary amine −NH–, followed by −C2H4–, and a primary terminal amine −NH2.

Figure 1

Figure 1. Concept: bi-layered surface functionalization. To inhibit wetting, (a) plain soda lime glass was functionalized with a primary layer of (b) hydrocarbon (trichloro(propyl)silane, TCPS) or (c) fluorocarbon (trichloro(1H,1H,2H,2H-perfluorooctyl)silane, PFOTS). Thereafter, secondary layers of (d,e) (3-aminopropyl)triethoxysilane, APTES, or (f) N-(3-trimethoxysilylpropyl)diethylenetriamine, NTDET, were added. The relevant functional molecular groups are hydrocarbon: green, fluorocarbon: purple, amine: blue, and unreacted hydroxyls: orange, respectively. The (+) and (−) symbols indicate the nature of polarity during slide electrification.

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The contact angles (advancing and receding) observed on TCPS-functionalized (hydrocarbon) surfaces are slightly lower than those on the PFOTS-functionalized (fluorocarbon) surfaces (Table 1). When integrated with the secondary layer of APTES or NTDET, the contact angles did not decrease but even show a slight increase, despite the fact that the pure APTES or NTDET layers are not hydrophobic. Atomic force microscopy (AFM) measurements (Figure S1 and Table 1, last column) show that the hydrocarbon (TCPS)- and fluorocarbon (PFOTS)-functionalized primary layers are composed of a homogeneous layer and bumps that are up to 10–20 and 30–45 nm high (Figure S1), respectively. Secondary amine (APTES/NTDET) layers are much smoother (<5 nm textures). During repeated sliding drop contact (i.e., 500 drops), these macroscopic nanobumps remain. A complementary time-of-flight secondary ion mass spectrometry (TOF-SIMS) analysis (Figure S2 and Supplementary Discussion) shows that the amine-integrated surfaces have mixed chemical compositions (both amine- and hydro/fluorocarbon) despite minimal wettability variations from purely hydrophobic surfaces.
Table 1. Dynamic Wetting Analysis on Both Primary (Hydro/Fluorocarbon)- and Secondary (Amine)-Functionalized Surfacesa
Surface VariantAdvancing CAReceding CARoll-Off CAHRoll-Off AngleRMS Roughness (nm)
TCPS92 ± 173 ± 119 ± 117 ± 13.0 ± 0.6
PFOTS107 ± 189 ± 217 ± 221 ± 26.9 ± 1.0
APTES67 ± 333 ± 134 ± 425 ± 30.9 ± 0.3
NTDET67 ± 535 ± 233 ± 631 ± 70.5 ± 0.1
TCPS-APTES94 ± 177 ± 39 ± 116 ± 15.7 ± 0.6
PFOTS-APTES111 ± 1102 ± 218 ± 118 ± 17.9 ± 1.7
PFOTS-NTDET113 ± 491 ± 422 ± 430 ± 65.7 ± 1.6
a

Wetting analysis (roll-off angle and contact angle hysteresis) was performed using 45 μL water drops that were tilted at 1°/s until rolling off.

Slide Electrification

Slide electrification experiments (7) were performed (Figure 2a) using deionized water (≈pH 5). Drops (45 μL) were deposited at a height of 0.5 cm on surfaces tilted at an angle of 50°. Drops were discharged with the first electrode (ground electrode) directly after landing. Drops then slide for 4 cm. The sliding drop collects charges from the contacting surface as it moves. This charge is accumulated until the drop reaches a second electrode (charge electrode) where it discharges the drop as a pulse (width of 2.5 ms, Figure 2b). The pulse travels through a current amplifier which collects and amplifies the charges for measurements. Drop charge is obtained by integrating the current signal (Figure 2b). The fixed drop size (V = 45 μL), drop interval (Δt = 2 s), and slide length (x = 4 cm) ensure that measurements across surfaces are consistent. Repeats were performed over 10 separate runs (3–5 sampling batches) and averaged (Figure 2). Further details are included in the Experimental Section.

Figure 2

Figure 2. Tunable slide electrification. Slide electrification is achieved by sequentially dropping-and-sliding liquid water drops on tilted, functionalized glass substrates. (a) The drop current is measured using an electrode, which is then amplified for analysis. (b) Depending on the surface, the charges collected can be positive (hydro/fluorocarbon, as shown: PFOTS-functionalized) or negative (amines, as shown: PFOTS-APTES-functionalized). (c) The drop charge saturation for all variants is achieved within 500 drops (10 repeat runs, presented as average ± standard deviation). Both pure hydrophobic variants (PFOTS, purple, and TCPS, green) show positive initial and saturation charges. Secondary functionalization of the mono-amine (APTES) with both hydrophobic variants (PFOTS-APTES, blue, and TCPS-APTES, cyan) results in negative drop charging, showing both negative initial and saturation charges.

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Positive Charging

The first drop sliding down the surface carries the highest charge. For subsequent drops, the charge decreases and reaches a steady state after ca. ten drops. At the steady state, incoming drops charge the surface as quickly as the surface discharges. On hydrocarbon (TCPS)- or fluorocarbon (PFOTS)-functionalized surfaces, drops charge positively (Figure 2c, green: TCPS, purple: PFOTS). (7,8,10−12,14,18−20) Hydrocarbon (TCPS) surfaces charge lower, at an initial charge of ca. 0.5 nC, which saturates to ca. 0.05 nC ± 0.03 nC. The fluorocarbon (PFOTS) variant charges higher, at an initial charge of ca. 1.5 nC, which saturates to ca. 0.71 nC ± 0.08 nC. Current curves for PFOTS are also included in Figure 2b, left panel. This positive drop electrification effect is widely attributed to the autolysis of water on hydrophobic surfaces and the adsorption of the hydroxide ion, OH–. We assume that a part of these adsorbed OH– ions do not recombine at the receding side of the drop but remain on the surface. A net surplus of protons, H+, is thus found in the departing drop. (14,22) The phenomenon of charging highly positive drops on PFOTS is well-known, (11,12,14) with more recent studies (18,19) corroborating the higher charging capacity of fluorocarbon vs hydrocarbons or siloxanes.

Negative Charging

After the secondary-functionalization by a mono-amine (APTES), the hydrophobic surfaces (TCPS and PFOTS) charge contacting drops negatively (Figure 2c, cyan: TCPS-APTES, blue: PFOTS-APTES). Drops charge negatively (from the onset) and at similar magnitudes for both hydro- and fluoro-carbon-based surfaces (TCPS-APTES and PFOTS-APTES). The initial charge was ca. −0.6 nC with a charge saturation of ca. −0.18 nC ± 0.04 nC (TCPS-APTES) and −0.15 ± 0.03 nC (PFOTS-APTES). The magnitudes of the average initial and saturation drop charges were almost identical, regardless of the primary sublayer (TCPS or PFOTS). Current curves are also included for PFOTS-APTES in Figure 2b, right panel. We note that pure APTES surfaces cannot be used under these test conditions as drops fragment during sliding.
The polarity flip induced by the functionalization of the mono-amine (APTES) is attributed to the active hydrolytic protonation during contact with water. This reaction consumes H+ ions from the sliding drop, R-NH2 + H+ → R-NH3+, thus resulting in negative drop electrification. To our knowledge, this is the first instance where continuous water drops of normal pH are shown to charge negatively during slide electrification.

Adaptive (Positive-to-Negative) Charging

On the NTDET functionalized PFOTS surfaces (PFOTS-NTDET), we observe a rapid decrease in charge magnitude which eventually leads to a flip in the drop charge polarity (to negative values) after 3−4 drops (Figure 3a): the initial drop charge is positive between ca. 0.2 and 0.4 nC, turning negative at ca. −0.15 nC ± 0.03 nC. Saturation of equilibrium charge is complete after ca. 50 drops, after a minor overshoot at ca. 10 drops (Figure 3b).

Figure 3

Figure 3. Adaptive charging phenomenon (PFOTS-NTDET). Sequential drop deposition (45 μL, 2 s interval) shows an (a) initial positive drop charge but adapts and saturates at a negative drop charge equilibrium. (b) The positive charge persists for ca. 3 drops before flipping toward a negative charge. The drop charge saturation is achieved, albeit first showing a minor overshoot, within 50 drops (10 repeat runs, presented as average ± standard deviation). (c) The positive-to-negative drop charging adaptation in PFOTS-NTDET is reversible (dark blue), with a longer drop interval (top x-axis, in seconds) enabling reversal back to a positive charging behavior. (d) Drop sliding velocity was analyzed using a conductive silicon wafer substrate that removes the influence of charging on drop mobility. Without the influence of electrostatics, and over the course of 1000 sequential drops (33 μL), drop mobility adapts (i.e., changes), indicative strongly of surface chemistry adaptation. (c, d) PFOTS (purple) is included as a control for comparative purposes.

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This polarity flip of PFOTS-NTDET surfaces may be attributed to surface adaptation. (23−25) Adaptive surfaces can show a reversible alteration of their physical properties under exposure to external stimuli (e.g., wetting). To demonstrate the reversibility of the charge polarity flip, the drop interval (Δt) was increased every 200 drops from the initial 2 s up to 16 s (Figure 3c, PFOTS-NTDET). Under these conditions, the initial positive-to-negative transition (Δt = 2 s) reverses at a drop interval of Δt = 16 s. At the end of the negative-to-positive reversal, the surface could be made to recreate negative drop charging by reshortening drop intervals (Δt = 2 s). Notably, the initial positive surface charge can also recover by letting the surface dry for several minutes. In contrast, control PFOTS surfaces do not exhibit this characteristic reversible flip in charge polarity (Figure 3c, PFOTS).
To confirm wetting-induced adaptation for PFOTS-NTDET surfaces, drop mobility can be analyzed. However, the influence of charging on drop mobility must first be removed. To this end, a silicon substrate was employed (Figures 3d and S3). The high permittivity of silicon substrates reduces electrostatic forces, thus eliminating the influence of charging. (26) The surface preparation was identical to that on glass. One thousand drops (33 μL) were then deposited (Δt = 2 s) on the functionalized silicon substrates at a tilt angle of 50° (i.e., charge measurement conditions). On PFOTS-NTDET surfaces, we observe large changes in drop mobility. Drops significantly slow down (ca., 2–3 times) over the course of 1000 sliding drop events (Figure 3d). This change in drop-on-surface mobility without the influence of electrostatic forces strongly indicates the presence of surface adaptation.
Such adaptation is not seen in control experiments with PFOTS (Figure 3d, PFOTS) or PFOTS-APTES-coated silicon wafers (Figure S3). Changes in surface chemistry can influence drop mobility by affecting wettability (hydrophilicity), thus surface adhesion, and subsequently the drop profiles and contact line behaviors. Here, the detected drop profiles (i.e., drop length, Figure S4) are notably altered in PFOTS-NTDET vs PFOTS during continuous drop contact. There appears to be minimal changes in contact angles (advancing or receding, Figure S4), but this may be due to the detection limit of the optical method employed. Any changes in receding contact angles at a sub-10 μm scale may be undetectable. Based on these collective observations, surface adaptation is likely, which influences the resultant dynamic drop profile, hence adhesion and mobility.

Possible Mechanisms in Polarity-Flipping: Adaptation and Charging Kinetics

With the multiamine NTDET, the presence of more amine functionalities should promote further protonation events, secondary R-NH-R + H+ → R-NH2+-R and primary R-NH2 + H+ → R-NH3+. However, the observed charging of the multiamine did not lead to a larger magnitude in negatively charged drops during slide electrification. Instead, surfaces experience the as-observed flip in charge polarity. This effect can be tentatively explained using a combination of structural adaptation and/or charging kinetics.
One possible explanation of the flip in polarity is to assume that in its pristine state, the amine-functional groups of NTDET are covered by hydrophobic (CxHy or PFOTS’s CxFz) moieties at the solid–air interface. These hydrophobic groups capture negative charges, q1–, that is, hydroxyl (OH–) ions, thus leading to temporally positive charges (1–3 drops) during initial wetting. During continuous drop sliding/wetting (>1 drop), the hydrophilic NH1/2 groups begin to capture positive charges or protons (H+), q2+, in a protonation reaction. From a structural adaptation perspective, hydrophilic NH1/2 groups can even reorientate to enrich at the water interface, replacing the originally surface-dominant hydrophobic CxHy or CxFz groups. (27,28) These structural changes further enhance protonation (increasing q2+).
With the monoamine, PFOTS-APTES, positive drop/negative surface charging is never observed (i.e., net q2+ > net q1–). This is likely due to the shorter hydrocarbon chain in APTES (C3N1) vs NTDET (C7N3). In NTDET, the flexibility of the longer chains may allow a certain degree of molecular-level shielding, with hydrocarbons covering amines (at the solid–air interface) in the pristine dry state, thus allowing for an initial onset of positive drop charging (albeit temporally unstable). In APTES, the shorter chains are more rigid and may not permit shielding, with exposed amines immediately inducing negative drop charging. This hypothesis can tentatively explain our experimental observations: the longer hydrocarbon chain of the multiamine NTDET may have provided a secondary (wetting-sensitive) negative charge capture characteristic, q3–, in contrast to that in which APTES does not confer.
As a consequence, the PFOTS-NTDET surface charges via qnet = k1q1– + k2q2+ + k3(t) · q3– instead of qnet = k1q1– + k2q2+ as exhibited by PFOTS-APTES. k1 – 2 are constants with minimal adaptivity, while k3 is wetting time-dependent, completely decaying (to a negligible level) within a few seconds of wetting. Drops are charged at −qnet. Therefore, if the PFOTS-NTDET surface is wetted with an increased drop interval (i.e., Δt = 16 s), the reacted amines can deprotonate and/or hydro/fluorocarbon groups can reorientate. k3(t) returns to its finite pristine value. As a result, drop charging returns to positive, that is, polarity reversal. The combination of structural adaptation and/or charging kinetics tentatively provides a possible mechanism behind the as-observed net negative or positive drop charging behavior at low or high drop intervals (Δt), respectively.

Conclusions

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Water drops sliding over hydrophobic layers doped with amine groups acquire a negative drop charge. This was achieved by coating glass surfaces using sequential chemical vapor deposition of fluoro- or hydrocarbon silanes followed by amine-functionalized silanes. This coating method preserved hydrophobicity while conferring the amine-terminated functionality. The amine groups charge positively in water, R-NH2 + H+ → R-NH3+. We believe that part of this charge remains on the surface even after the drop has departed. When using a short-chained mono-amine, APTES, drops charge immediately and saturate negatively. With a longer-chained multiamine, NTDET, the polarity of the saturation drop charge depends on the time interval between drops. These surface-directed polarity switching designs depict guidelines toward the future of tunable drop slide electrification.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.langmuir.2c00941.

  • (1) Descriptions of supplementary experiments, (2) supplementary discussion on TOF-SIMS analysis of surface chemical compositions, (3) AFM analysis of surfaces, and (4) supplementary drop mobility experiments (PDF)

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Author Information

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  • Corresponding Authors
  • Authors
    • Pravash Bista - Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany
    • Xiaomei Li - Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany
    • Lothar Veith - Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany
    • Azadeh Sharifi-Aghili - Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany
    • Stefan A. L. Weber - Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany;  Orcidhttps://orcid.org/0000-0003-3052-326X
  • Author Contributions

    W.S.Y.W., P.B., and H.-J.B. designed the experiments, analyzed the data, and prepared the manuscript. W.S.Y.W. fabricated the surfaces and carried out the experiments and charging characterization. S.A.L.W. analyzed the surface characterization results. X.L. assisted with the drop mobility experiments. A.S. assisted with the AFM measurements. L.V. assisted with the TOF-SIMS measurements. W.S.Y.W. and H.-J. B. planned and wrote the manuscript. All authors reviewed and approved the manuscript.

  • Funding

    Open access funded by Max Planck Society.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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This work was supported by the European Union’s Horizon 2020 research and innovation program LubISS No. 722497 (W.S.Y.W. and H.-J.B.) and the ERC Advanced Grant No. 883631 “DynaMo” (W.S.Y.W. and H.-J.B.). We also thank Rüdiger Berger, Amy Stetten, Benjamin Leibauer, Fahime Darvish, Benedikt Straub, Franjo Weber, Doris Vollmer, Lukas Hauer, Abhinav Naga, and Katharina Hegner for stimulating discussions.

References

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    Nanogenerators (NGs) are a field that uses Maxwell's displacement current as the driving force for effectively converting mech. energy into elec. power/signal, which have broad applications in energy science, environmental protection, wearable electronics, self-powered sensors, medical science, robotics and artificial intelligence. NGs are usually based on three effects: piezoelectricity, triboelectricity (contact electrification), and pyroelectricity. In this paper, a formal theory for NGs is presented starting from Maxwell's equations. Besides the general expression for displacement vector D = εE used for deriving classical electromagnetic dynamics, we added an addnl. term Ps in D, which represents the polarization created by the electrostatic surface charges owing to piezoelectricity and/or triboelectricity as a result of mech. triggering in NG. In contrast to P that is resulted from the elec. field induced medium polarization and vanishes if E = 0, Ps remains even when there is no external elec. field. We reformulated the Maxwell equations that include both the medium polarizations due to elec. field (P) and non-elec. field (such as strain) (Ps) induced polarization terms, from which, the output power, electromagnetic behavior and current transport equation for a NG are systematically derived. A general soln. is presented for the modified Maxwell equations, and anal. solns. about the output potential are provided for a few cases. The displacement current arising from ε.vdelta.E/.vdelta.t is responsible for the electromagnetic waves, while the newly added term .vdelta.Ps/.vdelta.t is the application of Maxwell's equations in energy and sensors. This work sets the first principle theory for quantifying the performance and electromagnetic behavior of a nanogenerator in general.
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  4. 4
    Kudin, K. N.; Car, R. Why Are Water–Hydrophobic Interfaces Charged?. J. Am. Chem. Soc. 2008, 130, 3915– 3919,  DOI: 10.1021/ja077205t
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    4
    Why Are Water-Hydrophobic Interfaces Charged?
    Kudin, Konstantin N.; Car, Roberto
    Journal of the American Chemical Society (2008), 130 (12), 3915-3919CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    We report ab initio mol. dynamics simulations of hydroxide and hydronium ions near a hydrophobic interface, indicating that both ions behave like amphiphilic surfactants that stick to a hydrophobic hydrocarbon surface with their hydrophobic side. We show that this behavior originates from the asymmetry of the mol. charge distribution which makes one end of the ions strongly hydrophobic while the other end is even more hydrophilic than the regular water (H2O) mols. The effect is more pronounced for the hydroxide than for the hydronium. Our results are consistent with several exptl. observations and explain why hydrophobic surfaces in contact with water acquire a net neg. charge, a phenomenon that has important implications for biol. and polymer science.
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  5. 5
    Zimmermann, R.; Rein, N.; Werner, C. Water ion adsorption dominates charging at nonpolar polymer surfaces in multivalent electrolytes. Phys. Chem. Chem. Phys. 2009, 11, 4360– 4364,  DOI: 10.1039/b900755e
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    5
    Water ion adsorption dominates charging at nonpolar polymer surfaces in multivalent electrolytes
    Zimmermann Ralf; Rein Nelly; Werner Carsten
    Physical chemistry chemical physics : PCCP (2009), 11 (21), 4360-4 ISSN:1463-9076.
    We systematically applied microslit electrokinetic experiments to explore the role of various electrolytes (KCl, CaCl(2), K(2)SO(4) and LaCl(3)) in the pH dependent charging of nonpolar poly(tetrafluoroethylene-co-2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole) (Teflon(R) AF) thin films. The results impressively reveal the dominating role of hydroxide and hydronium ions for the charging of water/hydrophobic wall interfaces even in the presence multivalent electrolyte ions. In any system the charging primarily reflected the preferential adsorption of hydroxide over hydronium ions while the electrolyte ions were concluded to determine the magnitude of the surface charge according to their hydration characteristics.
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  6. 6
    McCarty, L. S.; Whitesides, G. M. Electrostatic Charging Due to Separation of Ions at Interfaces: Contact Electrification of Ionic Electrets. Angew. Chem., Int. Ed. 2008, 47, 2188– 2207,  DOI: 10.1002/anie.200701812
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    6
    Electrostatic charging due to separation of ions at interfaces: contact electrification of ionic electrets
    McCarty, Logan S.; Whitesides, George M.
    Angewandte Chemie, International Edition (2008), 47 (12), 2188-2207CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. This Review discusses ionic electrets: their prepn., their mechanisms of formation, tools for their characterization, and their applications. An electret is a material that has a permanent, macroscopic elec. field at its surface; this field can arise from a net orientation of polar groups in the material, or from a net, macroscopic electrostatic charge on the material. An ionic electret is a material that has a net electrostatic charge due to a difference in the no. of cationic and anionic charges in the material. Any material that has ions at its surface, or accessible in its interior, has the potential to become an ionic electret. When such a material is brought into contact with some other material, ions can transfer between them. If the anions and cations have different propensities to transfer, the unequal transfer of these ions can result in a net transfer of charge between the two materials. This Review focuses on the exptl. evidence and theor. models for the formation of ionic electrets through this ion-transfer mechanism, and proposes-as a still-unproved hypothesis- that this ion-transfer mechanism may also explain the ubiquitous contact electrification ("static electricity") of materials, such as org. polymers, that do not explicitly have ions at their surface.
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  7. 7
    Stetten, A. Z.; Golovko, D. S.; Weber, S. A. L.; Butt, H.-J. Slide electrification: charging of surfaces by moving water drops. Soft Matter 2019, 15, 8667– 8679,  DOI: 10.1039/C9SM01348B
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    7
    Slide electrification: charging of surfaces by moving water drops
    Stetten, Amy Z.; Golovko, Dmytro S.; Weber, Stefan A. L.; Butt, Hans-Juergen
    Soft Matter (2019), 15 (43), 8667-8679CODEN: SMOABF; ISSN:1744-6848. (Royal Society of Chemistry)
    We investigate the charge sepn. caused by the motion of a water drop across a hydrophobic, insulating solid surface. Although the phenomenon of liq. charging has been consistently reported, these reports are primarily observational, results are difficult to reproduce, and no quant. theory has been developed. In this work, we address both the exptl. and theor. sides of this problem. We reproducibly measure the charge gained by water drops sliding down a substrate, and we outline an anal. theory to describe this charging process. As an exptl. system, we choose water drops moving down an inclined plane of glass hydrophobized with perfluoro octadecyltrichlorosilane (PFOTS). On this surface, sliding drops gain a pos. charge. We observe charge satn. in three variables: increasing drop no., increasing interval between drops, and increasing drop-sliding length. These charge saturations indicate a limited "storage capacity" of the system, as well as a gradual discharging of the surface. To explain these results, we theorize that some fraction of the charge in the Debye layer is transferred to the surface rather than being neutralized as the drop passes. This fraction, or "transfer coeff.", is dependent on the elec. potentials of surface and drop. All of our exptl. charge satn. results can be interpreted based on the proposed theory.
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  8. 8
    Sun, Y.; Huang, X.; Soh, S. Using the gravitational energy of water to generate power by separation of charge at interfaces. Chem. Sci. 2015, 6, 3347– 3353,  DOI: 10.1039/C5SC00473J
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    8
    Using the gravitational energy of water to generate power by separation of charge at interfaces
    Sun, Yajuan; Huang, Xu; Soh, Siowling
    Chemical Science (2015), 6 (6), 3347-3353CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
    When a fluid comes into contact with a solid surface, charge separates at the interface. This study describes a method that harvests the gravitational energy of water-available in abundance naturally, such as in rain and rivers-through the sepn. of charge at the interface. Essentially, it is found that water can be charged by flowing it across a solid surface under its own wt.; thus, a continuous flow of water can produce a const. supply of power. After optimizing the system, a power of up to ∼170 μW (per Teflon tube of 2 mm in diam.) can be generated. The efficiency, defined as the energy generated by the system over the gravitational energy that the water losses, can reach up to ∼3-4%. In order to generate a continuous stream of pos.-charged water, there should also be a const. prodn. of neg.-charged species in the system. Exptl. results suggest that the neg. charge transfers constantly to the atm. due to dielec. breakdown of air. With regards to applications related to high elec. potential of water droplets, the amt. of charge generated in a single water droplet is found to be equiv. to that produced by charging the water droplet with a high-voltage power supply operated at ∼5 kV. In general, the energy generated is clean, renewable, and tech. simple and inexpensive to produce.
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  9. 9
    Soh, S.; Kwok, S. W.; Liu, H.; Whitesides, G. M. Contact De-electrification of Electrostatically Charged Polymers. J. Am. Chem. Soc. 2012, 134, 20151– 20159,  DOI: 10.1021/ja309268n
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    9
    Contact De-electrification of Electrostatically Charged Polymers
    Soh, Siowling; Kwok, Sen Wai; Liu, Helena; Whitesides, George M.
    Journal of the American Chemical Society (2012), 134 (49), 20151-20159CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The contact electrification of insulating org. polymers is still incompletely understood, in part because multiple fundamental mechanisms may contribute to the movement of charge. This study describes a mechanism previously unreported in the context of contact electrification: i.e., "contact de-electrification", a process in which polymers charged to the same polarity discharge on contact. Both pos. charged polymeric beads, e.g., polyamide 6/6 (Nylon) and polyoxymethylene (Delrin), and neg. charged polymeric beads, e.g., polytetrafluoroethylene (Teflon) and polyamide-imide (Torlon), discharge when the like-charged beads are brought into contact. The beads (both with charges of ∼±20 μC/m2, or ∼100 charges/μm2) discharge on contact regardless of whether they are made of the same material, or of different materials. Discharge is rapid: discharge of flat slabs of like-charged Nylon and Teflon pieces is completed on a single contact (∼3 s). The charge lost from the polymers during contact de-electrification transfers onto mols. of gas in the atm. When like-charged polymers are brought into contact, the increase in elec. field at the point of contact exceeds the dielec. breakdown strength of the atm. and ionizes mols. of the gas; this ionization thus leads to discharge of the polymers. The detection (using a Faraday cup) of charges transferred to the cup by the ionized gas is compatible with the mechanism. Contact de-electrification occurs for different polymers and in atmospheres with different values of dielec. breakdown strength (helium, argon, oxygen, carbon dioxide, nitrogen, and sulfur hexafluoride): the mechanism thus appears to be general.
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  10. 10
    Lin, S.; Xu, L.; Chi Wang, A.; Wang, Z. L. Quantifying electron-transfer in liquid-solid contact electrification and the formation of electric double-layer. Nat. Commun. 2020, 11, 399,  DOI: 10.1038/s41467-019-14278-9
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    10
    Quantifying electron-transfer in liquid-solid contact electrification and the formation of electric double-layer
    Lin, Shiquan; Xu, Liang; Wang, Aurelia Chi; Wang, Zhong Lin
    Nature Communications (2020), 11 (1), 399CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
    Abstr.: Contact electrification (CE) has been known for more than 2600 years but the nature of charge carriers and their transfer mechanisms still remain poorly understood, esp. for the cases of liq.-solid CE. Here, we study the CE between liqs. and solids and investigate the decay of CE charges on the solid surfaces after liq.-solid CE at different thermal conditions. The contribution of electron transfer is distinguished from that of ion transfer on the charged surfaces by using the theory of electron thermionic emission. Our study shows that there are both electron transfer and ion transfer in the liq.-solid CE. We reveal that solutes in the soln., pH value of the soln. and the hydrophilicity of the solid affect the ratio of electron transfers to ion transfers. Further, we propose a two-step model of electron or/and ion transfer and demonstrate the formation of elec. double-layer in liq.-solid CE.
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  11. 11
    Nie, J.; Ren, Z.; Xu, L.; Lin, S.; Zhan, F.; Chen, X.; Wang, Z. L. Probing Contact-Electrification-Induced Electron and Ion Transfers at a Liquid–Solid Interface. Adv. Mater. 2020, 32, 1905696  DOI: 10.1002/adma.201905696
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    11
    Probing Contact-Electrification-Induced Electron and Ion Transfers at a Liquid-Solid Interface
    Nie, Jinhui; Ren, Zewei; Xu, Liang; Lin, Shiquan; Zhan, Fei; Chen, Xiangyu; Wang, Zhong Lin
    Advanced Materials (Weinheim, Germany) (2020), 32 (2), 1905696CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    As a well-known phenomenon, contact electrification (CE) has been studied for decades. Although recent studies have proven that CE between two solids is primarily due to electron transfer, the mechanism for CE between liq. and solid remains controversial. The CE process between different liqs. and polytetrafluoroethylene (PTFE) film is systematically studied to clarify the electrification mechanism of the solid-liq. interface. The CE between deionized water and PTFE can produce a surface charges d. in the scale of 1 nC cm-2, which is ten times higher than the calcn. based on the pure ion-transfer model. Hence, electron transfer is likely the dominating effect for this liq.-solid electrification process. Meanwhile, as ion concn. increases, the ion adsorption on the PTFE hinders electron transfer and results in the suppression of the transferred charge amt. Furthermore, there is an obvious charge transfer between oil and PTFE, which further confirms the presence of electron transfer between liq. and solid, simply because there are no ions in oil droplets. It is demonstrated that electron transfer plays the dominant role during CE between liqs. and solids, which directly impacts the traditional understanding of the formation of an elec. double layer (EDL) at a liq.-solid interface in phys. chem.
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  12. 12
    Sosa, M. D.; Martínez Ricci, M. L.; Missoni, L. L.; Murgida, D. H.; Cánneva, A.; D’Accorso, N. B.; Negri, R. M. Liquid–polymer triboelectricity: chemical mechanisms in the contact electrification process. Soft Matter 2020, 16, 7040– 7051,  DOI: 10.1039/D0SM00738B
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    12
    Liquid-polymer triboelectricity: chemical mechanisms in the contact electrification process
    Sosa, Mariana D.; Martinez Ricci, M. Luz; Missoni, Leandro L.; Murgida, Daniel H.; Canneva, Antonela; D'Accorso, Norma B.; Negri, R. Martin
    Soft Matter (2020), 16 (30), 7040-7051CODEN: SMOABF; ISSN:1744-6848. (Royal Society of Chemistry)
    Liq.-polymer contact electrification between sliding water drops and the surface of polytetrafluoroethylene (PTFE) was studied as a function of the pH and ionic strength of the drop as well as ambient relative humidity (RH). The PTFE surface was characterized by using SEM, water-contact-angle measurements, FTIR spectroscopy, XPS, and Raman spectroscopy. The charge acquired by the drops was calcd. by detecting the transient voltage induced on a specifically designed capacitive sensor. It is shown that water drops become pos. charged at pH > pHzch (pHzch being the zero charge point of the polymer) while they become neg. charged for pH < pHzch. The addn. of non-hydrolysable salts (NaCl or CaCl2) to water decreases the elec. charge induced in the drop. The charge also decreases with increasing RH. These results suggest proton or hydroxyl transfer from the liq. to the hydrophobic polymer surface. A proposed thermodn. model for the ion transfer process allows explaining the obsd. effects of RH, pH and ionic strength.
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  13. 13
    Xu, W.; Zheng, H.; Liu, Y.; Zhou, X.; Zhang, C.; Song, Y.; Deng, X.; Leung, M.; Yang, Z.; Xu, R. X.; Wang, Z. L.; Zeng, X. C.; Wang, Z. A droplet-based electricity generator with high instantaneous power density. Nature 2020, 578, 392– 396,  DOI: 10.1038/s41586-020-1985-6
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    13
    A droplet-based electricity generator with high instantaneous power density
    Xu, Wanghuai; Zheng, Huanxi; Liu, Yuan; Zhou, Xiaofeng; Zhang, Chao; Song, Yuxin; Deng, Xu; Leung, Michael; Yang, Zhengbao; Xu, Ronald X.; Wang, Zhong Lin; Zeng, Xiao Cheng; Wang, Zuankai
    Nature (London, United Kingdom) (2020), 578 (7795), 392-396CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
    Extensive efforts have been made to harvest energy from water in the form of raindrops1-6, river and ocean waves7,8, tides9 and others10-17. However, achieving a high d. of elec. power generation is challenging. Traditional hydraulic power generation mainly uses electromagnetic generators that are heavy, bulky, and become inefficient with low water supply. An alternative, the water-droplet/solid-based triboelec. nanogenerator, has so far generated peak power densities of less than one watt per square metre, owing to the limitations imposed by interfacial effects-as seen in characterizations of the charge generation and transfer that occur at solid-liq.1-4 or liq.-liq.5,18 interfaces. Here we develop a device to harvest energy from impinging water droplets by using an architecture that comprises a polytetrafluoroethylene film on an indium tin oxide substrate plus an aluminum electrode. We show that spreading of an impinged water droplet on the device bridges the originally disconnected components into a closed-loop elec. system, transforming the conventional interfacial effect into a bulk effect, and so enhancing the instantaneous power d. by several orders of magnitude over equiv. devices that are limited by interfacial effects.
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  14. 14
    Yatsuzuka, K.; Mizuno, Y.; Asano, K. Electrification phenomena of pure water droplets dripping and sliding on a polymer surface. J. Electrostat. 1994, 32, 157– 171,  DOI: 10.1016/0304-3886(94)90005-1
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  15. 15
    Sun, Q.; Wang, D.; Li, Y.; Zhang, J.; Ye, S.; Cui, J.; Chen, L.; Wang, Z.; Butt, H.-J.; Vollmer, D.; Deng, X. Surface charge printing for programmed droplet transport. Nat. Mater. 2019, 18, 936– 941,  DOI: 10.1038/s41563-019-0440-2
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    15
    Surface charge printing for programmed droplet transport
    Sun, Qiangqiang; Wang, Dehui; Li, Yanan; Zhang, Jiahui; Ye, Shuji; Cui, Jiaxi; Chen, Longquan; Wang, Zuankai; Butt, Hans-Jurgen; Vollmer, Doris; Deng, Xu
    Nature Materials (2019), 18 (9), 936-941CODEN: NMAACR; ISSN:1476-1122. (Nature Research)
    The directed, long-range and self-propelled transport of droplets on solid surfaces is crucial for many applications from water harvesting to bio-anal.1-9. Typically, preferential transport is achieved by topog. or chem. modulation of surface wetting gradients that break the asym. contact line and overcome the resistance force to move droplets along a particular direction10-16. Nonetheless, despite extensive progress, directional droplet transport is limited to low transport velocity or short transport distance. Here we report the high-velocity and ultralong transport of droplets elicited by surface charge d. gradients printed on diverse substrates. We leverage the facile water droplet printing on superamphiphobic surfaces to create rewritable surface charge d. gradients that stimulate droplet propulsion under ambient conditions17 and without the need for addnl. energy input. Our strategy provides a platform for programming the transport of droplets on flat, flexible and vertical surfaces that may be valuable for applications requiring a controlled movement of droplets17-19.
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  16. 16
    Lin, S.; Zheng, M.; Luo, J.; Wang, Z. L. Effects of Surface Functional Groups on Electron Transfer at Liquid–Solid Interfacial Contact Electrification. ACS Nano 2020, 14, 10733– 10741,  DOI: 10.1021/acsnano.0c06075
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    16
    Effects of Surface Functional Groups on Electron Transfer at Liquid-Solid Interfacial Contact Electrification
    Lin, Shiquan; Zheng, Mingli; Luo, Jianjun; Wang, Zhong Lin
    ACS Nano (2020), 14 (8), 10733-10741CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Contact electrification (CE) at interfaces is sensitive to the functional groups on the solid surface, but its mechanism is poorly understood, esp. for the liq.-solid cases. A core controversy is the identity of the charge carriers (electrons or/and ions) in the CE between liqs. and solids. Here, the CE between SiO2 surfaces with different functional groups and different liqs., including DI water and org. solns., is systematically studied, and the contribution of electron transfer is distinguished from that of ion transfer according to the charge decay behavior at surfaces at specific temp., because electron release follows the thermionic emission theory. It is revealed that electron transfer plays an important role in the CE between liqs. and functional group modified SiO2. Moreover, the electron transfer between the DI water and the SiO2 is found highly related to the electron affinity of the functional groups on the SiO2 surfaces, while the electron transfer between org. solns. and the SiO2 is independent of the functional groups, due to the limited ability of org. solns. to donate or gain electrons. An energy band model for the electron transfer between liqs. and solids is further proposed, in which the effects of functional groups are considered. The discoveries in this work support the "two-step" model about the formation of an elec. double-layer (Wang model), in which the electron transfer occurs first when the liqs. contact the solids for the very first time.
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  17. 17
    Zhang, J.; Lin, S.; Zheng, M.; Wang, Z. L. Triboelectric Nanogenerator as a Probe for Measuring the Charge Transfer between Liquid and Solid Surfaces. ACS Nano 2021, 15, 14830– 14837,  DOI: 10.1021/acsnano.1c04903
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    17
    Triboelectric Nanogenerator as a Probe for Measuring the Charge Transfer between Liquid and Solid Surfaces
    Zhang, Jinyang; Lin, Shiquan; Zheng, Mingli; Wang, Zhong Lin
    ACS Nano (2021), 15 (9), 14830-14837CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    The phenomenon of triboelectricity involves the flow of charged species across an interface, but conclusively establishing the nature of the charge transfer has proven extremely difficult, esp. for the liq.-solid cases. Herein, we developed a self-powered droplet triboelec. nanogenerator (droplet-TENG) with spatially arranged electrodes as a probe for measuring the charge transfer process between liq. and solid interfaces. The information on the elec. signal on spatially arranged electrodes shows that the charge transfer between droplets and the solid is an accumulation process during the dropping and that the electron is the dominant charge-transfer species. Such a droplet-TENG showed a high sensitivity to the ratio of solvents in the mixed org. soln., and we postulated this is due to the possibility of generation of a hydrogen bond, affecting the elec. signal on the spatially arranged electrodes. This work demonstrated a chem. sensing application based on the self-powered droplet triboelec. nanogenerator.
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  18. 18
    Zhu, G.; Su, Y.; Bai, P.; Chen, J.; Jing, Q.; Yang, W.; Wang, Z. L. Harvesting Water Wave Energy by Asymmetric Screening of Electrostatic Charges on a Nanostructured Hydrophobic Thin-Film Surface. ACS Nano 2014, 8, 6031– 6037,  DOI: 10.1021/nn5012732
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    18
    Harvesting Water Wave Energy by Asymmetric Screening of Electrostatic Charges on a Nanostructured Hydrophobic Thin-Film Surface
    Zhu, Guang; Su, Yuanjie; Bai, Peng; Chen, Jun; Jing, Qingshen; Yang, Weiqing; Wang, Zhong Lin
    ACS Nano (2014), 8 (6), 6031-6037CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Energy harvesting from ambient H2O motions is a desirable but underexplored soln. to on-site energy demand for self-powered electronics. Here the authors report a liq.-solid electrification-enabled generator based on a fluorinated ethylene propylene thin film, below which an array of electrodes are fabricated. The surface of the thin film is charged 1st due to the H2O-solid contact electrification. Aligned nanowires created on the thin film make it hydrophobic and also increase the surface area. Then the asym. screening to the surface charges by the waving H2O during emerging and submerging processes causes the free electrons on the electrodes to flow through an external load, resulting in power generation. The generator produces sufficient output power for driving an array of small electronics during direct interaction with H2O bodies, including surface waves and falling drops. Polymer-nanowire-based surface modification increases the contact area at the liq.-solid interface, leading to enhanced surface charging d. and thus elec. output at an efficiency of 7.7%. The planar-structured generator features an all-in-one design without sep. and movable components for capturing and transmitting mech. energy. It has extremely lightwt. and small vol., making it a portable, flexible, and convenient power soln. that can be applied on the ocean/river surface, at coastal/offshore areas, and even in rainy places. Considering the demonstrated scalability, it can also be possibly used in large-scale energy generation if layers of planar sheets are connected into a network.
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  19. 19
    Lin, Z.-H.; Cheng, G.; Lin, L.; Lee, S.; Wang, Z. L. Water–Solid Surface Contact Electrification and its Use for Harvesting Liquid-Wave Energy. Angew. Chem., Int. Ed. 2013, 52, 12545– 12549,  DOI: 10.1002/anie.201307249
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    19
    Water-Solid Surface Contact Electrification and its Use for Harvesting Liquid Wave Energy
    Lin, Zong-Hong; Cheng, Gang; Lin, Long; Lee, Sangmin; Wang, Zhong Lin
    Angewandte Chemie, International Edition (2013), 52 (48), 12545-12549CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
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    Langmuir (2020), 36 (26), 7236-7245CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    To better understand the wetting of crosslinked polydimethylsiloxane (PDMS), we measured advancing and receding contact angles of sessile water drops on crosslinked PDMS as a function of contact line velocity (up to 100μm/s). Three types of samples were investigated: pristine PDMS, PDMS where oligomers were removed by toluene treatment, and PDMS with an enriched concn. of oligomers. Depending on the velocity of advancing contact lines and the contact time with water, different modes of wetting were obsd.: one with a relatively low contact angle hysteresis (Δθ ≈ 10°) and one with a larger hysteresis. We attribute the low hysteresis state, called the lubricated state, to the enrichment of free oligomers at the water-PDMS interface. The enrichment of oligomers is induced by drop contact. The kinetics of the transition to the lubricated state can be described by adaptation theory. PDMS adapts to the presence of water by an enrichment of free oligomers at the interface and a correlated redn. in interfacial tension.
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    Adaptation of a Styrene-Acrylic Acid Copolymer Surface to Water
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    Langmuir (2021), 37 (4), 1571-1577CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    Solid surfaces, in particular polymer surfaces, are able to adapt upon contact with a liq. Adaptation results in an increase in contact angle hysteresis and influences the mobility of sliding drops on surfaces. To study adaptation and its kinetics, we synthesized a random copolymer composed of styrene and 11-25 mol% acrylic acid (PS/PAA). We measured the dynamic advancing (θA) and receding (θR) contact angles of water drops sliding down a tilted plate coated with this polymer. We measured θA ≈ 87° for velocities of the contact line <20μm/s. At higher velocities, θA gradually increased to ~ 98°. This value is similar to θA of a pure polystyrene (PS) film, which we studied for comparison. We assoc. the gradual increase in θA to the adaptation process to water. The presence of water leads to swelling and/or an enrichment of acid groups at the water/polymer interface. By applying the latest adaptation theory, we estd. the time const. of this adaptation process to be «1 s. For sliding water drops, θR is ~ 10° lower compared to the ref. PS surface for all tested velocities. Thus, at the receding side of a sliding drop, the surface is already enriched by acid groups. For a water drop with a width of 5 mm, the increase in contact angle hysteresis corresponds to an increase in capillary force in the range of 45-60μN, depending on sliding velocity.
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    Many surfaces reversibly change their structure and interfacial energy upon being in contact with a liq. Such surfaces adapt to a specific liq. We propose the first order kinetic model to describe dynamic contact angles of such adaptive surfaces. The model is general and does not refer to a particular adaptation process. The aim of the proposed model is to provide a quant. description of adaptive wetting and to link changes in contact angles to microscopic adaptation processes. By introducing exponentially relaxing interfacial energies and applying Young's equation locally, we predict a change of advancing and receding contact angles depending on the velocity of the contact line. Even for perfectly homogeneous and smooth surfaces, a dynamic contact angle hysteresis is obtained. As possible adaptations, we discuss changes and reconstruction of polymer surfaces or monolayers, diffusion and swelling, adsorption of surfactants, replacement of contaminants, reorientation of liq. mols., or formation of an elec. double layer.
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    Nano Letters (2019), 19 (3), 1892-1901CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Super-hydrophobic, super-oleo(amphi)phobic, and super-omniphobic materials are universally important in the fields of science and engineering. Despite rapid advancements, gaps of understanding still exist between each distinctive wetting state. The transition of super-hydrophobicity to super-(oleo-, amphi-, and omni-)phobicity typically requires the use of re-entrant features. Today, re-entrant geometry induced super-(amphi- and omni-)phobicity is well-supported by both expts. and theory. However, owing to geometrical complexities, the concept of re-entrant geometry forms a dogma that limits the industrial progress of these unique states of wettability. Moreover, a key fundamental question remains unanswered: are extreme surface chem. enhancements able to influence super-liq. repellency. Here, this was rigorously tested via an alternative pathway that does not require explicit designer re-entrant features. Highly controllable and tunable vertical network polymn. and functionalization were used to achieve fluoroalkyl densification on nanoparticles. For the first time, relative fluoro-functionalization densities are quant. tuned and correlated to super-liq. repellency performance. Step-wise tunable super-amphiphobic nanoparticle films with a Cassie-Baxter state (contact angle of >150° and sliding angle of <10°) against various liqs. is demonstrated. This was tested down to very low surface tension liqs. to a min. of ca. 23.8 mN/m. Such findings could eventually lead to the future development of super-(amphi)omniphobic materials that transcend the sole use of re-entrant geometry.
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    Langmuir (2011), 27 (16), 10206-10215CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    Fluoroalkyl-functionalized SiO2 particles for use in nonwetting surfaces were prepd. by treatment of SiO2 particles with fluoroalkyl-functional chlorosilanes. Both fumed and pptd. SiO2 were studied, as well as the efficiency of surface coverage using mono-, di-, and trifunctional chlorosilanes. The most effective surface treatment was accomplished via the surface grafting of monofunctional chlorosilanes in the presence of preadsorbed dimethylamine under anhyd. conditions at room temp. Confirmation of covalent attachment was accomplished via FTIR spectroscopy, while elemental anal., TGA, and nitrogen adsorption isotherms were used to det. grafting densities and addnl. key geometric characteristics of the grafted layer. The effect of residual silanol content on the moisture uptake properties of the modified SiO2 particles was detd. by measuring the H2O uptake of unbound particles, while liq. wetting properties were detd. by dynamic contact angle anal. of elastomeric composites. Although residual silanol content was shown to effect wetting properties, results suggest that surface geometry dominates the performance of liq.-repellent surfaces. The potential use of fluoroalkyl-functionalized SiO2 particles for hydrophobic and oleophobic applications is discussed.
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    Langmuir (2000), 16 (18), 7268-7274CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
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This article is cited by 9 publications.

  1. María del Mar Cammarata, Luciano Rey, Vanesa Torres, Ricardo Kindsvater, Antonela Cánneva, Mariana D. Sosa, Mirta Fascio, Norma B. D′Accorso, Mario D. Contin, R. Martín Negri. Water–Polymer Slide Electrification in Polyethylene Films Coated with Amphiphilic Compounds and Its Connection to Surface Properties Dependent on Water–Polymer Interactions. Langmuir 2024, 40 (41) , 21741-21757. https://doi.org/10.1021/acs.langmuir.4c02856
  2. William S. Y. Wong, Mariia S. Kiseleva, Abhinav Naga. Polarity-Induced Reactive Wetting: Spreading and Retracting Sessile Water Drops. Langmuir 2024, 40 (26) , 13562-13572. https://doi.org/10.1021/acs.langmuir.4c01085
  3. Benjamin Leibauer, Ognen Pop-Georgievski, Mariana D. Sosa, Yun Dong, Wolfgang Tremel, Hans-Jürgen Butt, Werner Steffen. How Surface and Substrate Chemistry Affect Slide Electrification. Journal of the American Chemical Society 2024, 146 (14) , 10073-10083. https://doi.org/10.1021/jacs.4c01015
  4. Lars Egil Helseth. Charge Transfer Quenching and Maximum of a Liquid–Air Contact Line Moving over a Hydrophobic Surface. Langmuir 2024, 40 (8) , 4340-4349. https://doi.org/10.1021/acs.langmuir.3c03605
  5. A. Kumar, S. Gautam, S. Atri, H. V. Tafreshi, B. Pourdeyhimi. Importance of Dipole Orientation in Electrostatic Aerosol Filtration. Langmuir 2023, 39 (49) , 17653-17663. https://doi.org/10.1021/acs.langmuir.3c02016
  6. Vasily Artemov, Laura Frank, Roman Doronin, Philipp Stärk, Alexander Schlaich, Anton Andreev, Thomas Leisner, Aleksandra Radenovic, Alexei Kiselev. The Three-Phase Contact Potential Difference Modulates the Water Surface Charge. The Journal of Physical Chemistry Letters 2023, 14 (20) , 4796-4802. https://doi.org/10.1021/acs.jpclett.3c00479
  7. L. E. Helseth. Ion Concentration Influences the Charge Transfer Due to a Water–Air Contact Line Moving over a Hydrophobic Surface: Charge Measurements and Theoretical Models. Langmuir 2023, 39 (5) , 1826-1837. https://doi.org/10.1021/acs.langmuir.2c02716
  8. Shuai Li, Pravash Bista, Stefan A. L. Weber, Michael Kappl, Hans-Jürgen Butt. Spontaneous Charging of Drops on Lubricant-Infused Surfaces. Langmuir 2022, 38 (41) , 12610-12616. https://doi.org/10.1021/acs.langmuir.2c02085
  9. Mariana D. Sosa, Norma B. D’Accorso, M. Luz Martínez Ricci, R. Martín Negri. Liquid–Polymer Contact Electrification: Modeling the Dependence of Surface Charges and ξ-Potential on pH and Added-Salt Concentration. Langmuir 2022, 38 (29) , 8817-8828. https://doi.org/10.1021/acs.langmuir.2c00813
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  • Abstract

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    Figure 1

    Figure 1. Concept: bi-layered surface functionalization. To inhibit wetting, (a) plain soda lime glass was functionalized with a primary layer of (b) hydrocarbon (trichloro(propyl)silane, TCPS) or (c) fluorocarbon (trichloro(1H,1H,2H,2H-perfluorooctyl)silane, PFOTS). Thereafter, secondary layers of (d,e) (3-aminopropyl)triethoxysilane, APTES, or (f) N-(3-trimethoxysilylpropyl)diethylenetriamine, NTDET, were added. The relevant functional molecular groups are hydrocarbon: green, fluorocarbon: purple, amine: blue, and unreacted hydroxyls: orange, respectively. The (+) and (−) symbols indicate the nature of polarity during slide electrification.

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    Figure 2

    Figure 2. Tunable slide electrification. Slide electrification is achieved by sequentially dropping-and-sliding liquid water drops on tilted, functionalized glass substrates. (a) The drop current is measured using an electrode, which is then amplified for analysis. (b) Depending on the surface, the charges collected can be positive (hydro/fluorocarbon, as shown: PFOTS-functionalized) or negative (amines, as shown: PFOTS-APTES-functionalized). (c) The drop charge saturation for all variants is achieved within 500 drops (10 repeat runs, presented as average ± standard deviation). Both pure hydrophobic variants (PFOTS, purple, and TCPS, green) show positive initial and saturation charges. Secondary functionalization of the mono-amine (APTES) with both hydrophobic variants (PFOTS-APTES, blue, and TCPS-APTES, cyan) results in negative drop charging, showing both negative initial and saturation charges.

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    Figure 3

    Figure 3. Adaptive charging phenomenon (PFOTS-NTDET). Sequential drop deposition (45 μL, 2 s interval) shows an (a) initial positive drop charge but adapts and saturates at a negative drop charge equilibrium. (b) The positive charge persists for ca. 3 drops before flipping toward a negative charge. The drop charge saturation is achieved, albeit first showing a minor overshoot, within 50 drops (10 repeat runs, presented as average ± standard deviation). (c) The positive-to-negative drop charging adaptation in PFOTS-NTDET is reversible (dark blue), with a longer drop interval (top x-axis, in seconds) enabling reversal back to a positive charging behavior. (d) Drop sliding velocity was analyzed using a conductive silicon wafer substrate that removes the influence of charging on drop mobility. Without the influence of electrostatics, and over the course of 1000 sequential drops (33 μL), drop mobility adapts (i.e., changes), indicative strongly of surface chemistry adaptation. (c, d) PFOTS (purple) is included as a control for comparative purposes.

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    4. 4
      Kudin, K. N.; Car, R. Why Are Water–Hydrophobic Interfaces Charged?. J. Am. Chem. Soc. 2008, 130, 3915– 3919,  DOI: 10.1021/ja077205t
      4
      Why Are Water-Hydrophobic Interfaces Charged?
      Kudin, Konstantin N.; Car, Roberto
      Journal of the American Chemical Society (2008), 130 (12), 3915-3919CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      We report ab initio mol. dynamics simulations of hydroxide and hydronium ions near a hydrophobic interface, indicating that both ions behave like amphiphilic surfactants that stick to a hydrophobic hydrocarbon surface with their hydrophobic side. We show that this behavior originates from the asymmetry of the mol. charge distribution which makes one end of the ions strongly hydrophobic while the other end is even more hydrophilic than the regular water (H2O) mols. The effect is more pronounced for the hydroxide than for the hydronium. Our results are consistent with several exptl. observations and explain why hydrophobic surfaces in contact with water acquire a net neg. charge, a phenomenon that has important implications for biol. and polymer science.
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    5. 5
      Zimmermann, R.; Rein, N.; Werner, C. Water ion adsorption dominates charging at nonpolar polymer surfaces in multivalent electrolytes. Phys. Chem. Chem. Phys. 2009, 11, 4360– 4364,  DOI: 10.1039/b900755e
      5
      Water ion adsorption dominates charging at nonpolar polymer surfaces in multivalent electrolytes
      Zimmermann Ralf; Rein Nelly; Werner Carsten
      Physical chemistry chemical physics : PCCP (2009), 11 (21), 4360-4 ISSN:1463-9076.
      We systematically applied microslit electrokinetic experiments to explore the role of various electrolytes (KCl, CaCl(2), K(2)SO(4) and LaCl(3)) in the pH dependent charging of nonpolar poly(tetrafluoroethylene-co-2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole) (Teflon(R) AF) thin films. The results impressively reveal the dominating role of hydroxide and hydronium ions for the charging of water/hydrophobic wall interfaces even in the presence multivalent electrolyte ions. In any system the charging primarily reflected the preferential adsorption of hydroxide over hydronium ions while the electrolyte ions were concluded to determine the magnitude of the surface charge according to their hydration characteristics.
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    6. 6
      McCarty, L. S.; Whitesides, G. M. Electrostatic Charging Due to Separation of Ions at Interfaces: Contact Electrification of Ionic Electrets. Angew. Chem., Int. Ed. 2008, 47, 2188– 2207,  DOI: 10.1002/anie.200701812
      6
      Electrostatic charging due to separation of ions at interfaces: contact electrification of ionic electrets
      McCarty, Logan S.; Whitesides, George M.
      Angewandte Chemie, International Edition (2008), 47 (12), 2188-2207CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. This Review discusses ionic electrets: their prepn., their mechanisms of formation, tools for their characterization, and their applications. An electret is a material that has a permanent, macroscopic elec. field at its surface; this field can arise from a net orientation of polar groups in the material, or from a net, macroscopic electrostatic charge on the material. An ionic electret is a material that has a net electrostatic charge due to a difference in the no. of cationic and anionic charges in the material. Any material that has ions at its surface, or accessible in its interior, has the potential to become an ionic electret. When such a material is brought into contact with some other material, ions can transfer between them. If the anions and cations have different propensities to transfer, the unequal transfer of these ions can result in a net transfer of charge between the two materials. This Review focuses on the exptl. evidence and theor. models for the formation of ionic electrets through this ion-transfer mechanism, and proposes-as a still-unproved hypothesis- that this ion-transfer mechanism may also explain the ubiquitous contact electrification ("static electricity") of materials, such as org. polymers, that do not explicitly have ions at their surface.
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    7. 7
      Stetten, A. Z.; Golovko, D. S.; Weber, S. A. L.; Butt, H.-J. Slide electrification: charging of surfaces by moving water drops. Soft Matter 2019, 15, 8667– 8679,  DOI: 10.1039/C9SM01348B
      7
      Slide electrification: charging of surfaces by moving water drops
      Stetten, Amy Z.; Golovko, Dmytro S.; Weber, Stefan A. L.; Butt, Hans-Juergen
      Soft Matter (2019), 15 (43), 8667-8679CODEN: SMOABF; ISSN:1744-6848. (Royal Society of Chemistry)
      We investigate the charge sepn. caused by the motion of a water drop across a hydrophobic, insulating solid surface. Although the phenomenon of liq. charging has been consistently reported, these reports are primarily observational, results are difficult to reproduce, and no quant. theory has been developed. In this work, we address both the exptl. and theor. sides of this problem. We reproducibly measure the charge gained by water drops sliding down a substrate, and we outline an anal. theory to describe this charging process. As an exptl. system, we choose water drops moving down an inclined plane of glass hydrophobized with perfluoro octadecyltrichlorosilane (PFOTS). On this surface, sliding drops gain a pos. charge. We observe charge satn. in three variables: increasing drop no., increasing interval between drops, and increasing drop-sliding length. These charge saturations indicate a limited "storage capacity" of the system, as well as a gradual discharging of the surface. To explain these results, we theorize that some fraction of the charge in the Debye layer is transferred to the surface rather than being neutralized as the drop passes. This fraction, or "transfer coeff.", is dependent on the elec. potentials of surface and drop. All of our exptl. charge satn. results can be interpreted based on the proposed theory.
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    8. 8
      Sun, Y.; Huang, X.; Soh, S. Using the gravitational energy of water to generate power by separation of charge at interfaces. Chem. Sci. 2015, 6, 3347– 3353,  DOI: 10.1039/C5SC00473J
      8
      Using the gravitational energy of water to generate power by separation of charge at interfaces
      Sun, Yajuan; Huang, Xu; Soh, Siowling
      Chemical Science (2015), 6 (6), 3347-3353CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      When a fluid comes into contact with a solid surface, charge separates at the interface. This study describes a method that harvests the gravitational energy of water-available in abundance naturally, such as in rain and rivers-through the sepn. of charge at the interface. Essentially, it is found that water can be charged by flowing it across a solid surface under its own wt.; thus, a continuous flow of water can produce a const. supply of power. After optimizing the system, a power of up to ∼170 μW (per Teflon tube of 2 mm in diam.) can be generated. The efficiency, defined as the energy generated by the system over the gravitational energy that the water losses, can reach up to ∼3-4%. In order to generate a continuous stream of pos.-charged water, there should also be a const. prodn. of neg.-charged species in the system. Exptl. results suggest that the neg. charge transfers constantly to the atm. due to dielec. breakdown of air. With regards to applications related to high elec. potential of water droplets, the amt. of charge generated in a single water droplet is found to be equiv. to that produced by charging the water droplet with a high-voltage power supply operated at ∼5 kV. In general, the energy generated is clean, renewable, and tech. simple and inexpensive to produce.
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    9. 9
      Soh, S.; Kwok, S. W.; Liu, H.; Whitesides, G. M. Contact De-electrification of Electrostatically Charged Polymers. J. Am. Chem. Soc. 2012, 134, 20151– 20159,  DOI: 10.1021/ja309268n
      9
      Contact De-electrification of Electrostatically Charged Polymers
      Soh, Siowling; Kwok, Sen Wai; Liu, Helena; Whitesides, George M.
      Journal of the American Chemical Society (2012), 134 (49), 20151-20159CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The contact electrification of insulating org. polymers is still incompletely understood, in part because multiple fundamental mechanisms may contribute to the movement of charge. This study describes a mechanism previously unreported in the context of contact electrification: i.e., "contact de-electrification", a process in which polymers charged to the same polarity discharge on contact. Both pos. charged polymeric beads, e.g., polyamide 6/6 (Nylon) and polyoxymethylene (Delrin), and neg. charged polymeric beads, e.g., polytetrafluoroethylene (Teflon) and polyamide-imide (Torlon), discharge when the like-charged beads are brought into contact. The beads (both with charges of ∼±20 μC/m2, or ∼100 charges/μm2) discharge on contact regardless of whether they are made of the same material, or of different materials. Discharge is rapid: discharge of flat slabs of like-charged Nylon and Teflon pieces is completed on a single contact (∼3 s). The charge lost from the polymers during contact de-electrification transfers onto mols. of gas in the atm. When like-charged polymers are brought into contact, the increase in elec. field at the point of contact exceeds the dielec. breakdown strength of the atm. and ionizes mols. of the gas; this ionization thus leads to discharge of the polymers. The detection (using a Faraday cup) of charges transferred to the cup by the ionized gas is compatible with the mechanism. Contact de-electrification occurs for different polymers and in atmospheres with different values of dielec. breakdown strength (helium, argon, oxygen, carbon dioxide, nitrogen, and sulfur hexafluoride): the mechanism thus appears to be general.
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    10. 10
      Lin, S.; Xu, L.; Chi Wang, A.; Wang, Z. L. Quantifying electron-transfer in liquid-solid contact electrification and the formation of electric double-layer. Nat. Commun. 2020, 11, 399,  DOI: 10.1038/s41467-019-14278-9
      10
      Quantifying electron-transfer in liquid-solid contact electrification and the formation of electric double-layer
      Lin, Shiquan; Xu, Liang; Wang, Aurelia Chi; Wang, Zhong Lin
      Nature Communications (2020), 11 (1), 399CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
      Abstr.: Contact electrification (CE) has been known for more than 2600 years but the nature of charge carriers and their transfer mechanisms still remain poorly understood, esp. for the cases of liq.-solid CE. Here, we study the CE between liqs. and solids and investigate the decay of CE charges on the solid surfaces after liq.-solid CE at different thermal conditions. The contribution of electron transfer is distinguished from that of ion transfer on the charged surfaces by using the theory of electron thermionic emission. Our study shows that there are both electron transfer and ion transfer in the liq.-solid CE. We reveal that solutes in the soln., pH value of the soln. and the hydrophilicity of the solid affect the ratio of electron transfers to ion transfers. Further, we propose a two-step model of electron or/and ion transfer and demonstrate the formation of elec. double-layer in liq.-solid CE.
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    11. 11
      Nie, J.; Ren, Z.; Xu, L.; Lin, S.; Zhan, F.; Chen, X.; Wang, Z. L. Probing Contact-Electrification-Induced Electron and Ion Transfers at a Liquid–Solid Interface. Adv. Mater. 2020, 32, 1905696  DOI: 10.1002/adma.201905696
      11
      Probing Contact-Electrification-Induced Electron and Ion Transfers at a Liquid-Solid Interface
      Nie, Jinhui; Ren, Zewei; Xu, Liang; Lin, Shiquan; Zhan, Fei; Chen, Xiangyu; Wang, Zhong Lin
      Advanced Materials (Weinheim, Germany) (2020), 32 (2), 1905696CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      As a well-known phenomenon, contact electrification (CE) has been studied for decades. Although recent studies have proven that CE between two solids is primarily due to electron transfer, the mechanism for CE between liq. and solid remains controversial. The CE process between different liqs. and polytetrafluoroethylene (PTFE) film is systematically studied to clarify the electrification mechanism of the solid-liq. interface. The CE between deionized water and PTFE can produce a surface charges d. in the scale of 1 nC cm-2, which is ten times higher than the calcn. based on the pure ion-transfer model. Hence, electron transfer is likely the dominating effect for this liq.-solid electrification process. Meanwhile, as ion concn. increases, the ion adsorption on the PTFE hinders electron transfer and results in the suppression of the transferred charge amt. Furthermore, there is an obvious charge transfer between oil and PTFE, which further confirms the presence of electron transfer between liq. and solid, simply because there are no ions in oil droplets. It is demonstrated that electron transfer plays the dominant role during CE between liqs. and solids, which directly impacts the traditional understanding of the formation of an elec. double layer (EDL) at a liq.-solid interface in phys. chem.
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    12. 12
      Sosa, M. D.; Martínez Ricci, M. L.; Missoni, L. L.; Murgida, D. H.; Cánneva, A.; D’Accorso, N. B.; Negri, R. M. Liquid–polymer triboelectricity: chemical mechanisms in the contact electrification process. Soft Matter 2020, 16, 7040– 7051,  DOI: 10.1039/D0SM00738B
      12
      Liquid-polymer triboelectricity: chemical mechanisms in the contact electrification process
      Sosa, Mariana D.; Martinez Ricci, M. Luz; Missoni, Leandro L.; Murgida, Daniel H.; Canneva, Antonela; D'Accorso, Norma B.; Negri, R. Martin
      Soft Matter (2020), 16 (30), 7040-7051CODEN: SMOABF; ISSN:1744-6848. (Royal Society of Chemistry)
      Liq.-polymer contact electrification between sliding water drops and the surface of polytetrafluoroethylene (PTFE) was studied as a function of the pH and ionic strength of the drop as well as ambient relative humidity (RH). The PTFE surface was characterized by using SEM, water-contact-angle measurements, FTIR spectroscopy, XPS, and Raman spectroscopy. The charge acquired by the drops was calcd. by detecting the transient voltage induced on a specifically designed capacitive sensor. It is shown that water drops become pos. charged at pH > pHzch (pHzch being the zero charge point of the polymer) while they become neg. charged for pH < pHzch. The addn. of non-hydrolysable salts (NaCl or CaCl2) to water decreases the elec. charge induced in the drop. The charge also decreases with increasing RH. These results suggest proton or hydroxyl transfer from the liq. to the hydrophobic polymer surface. A proposed thermodn. model for the ion transfer process allows explaining the obsd. effects of RH, pH and ionic strength.
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    13. 13
      Xu, W.; Zheng, H.; Liu, Y.; Zhou, X.; Zhang, C.; Song, Y.; Deng, X.; Leung, M.; Yang, Z.; Xu, R. X.; Wang, Z. L.; Zeng, X. C.; Wang, Z. A droplet-based electricity generator with high instantaneous power density. Nature 2020, 578, 392– 396,  DOI: 10.1038/s41586-020-1985-6
      13
      A droplet-based electricity generator with high instantaneous power density
      Xu, Wanghuai; Zheng, Huanxi; Liu, Yuan; Zhou, Xiaofeng; Zhang, Chao; Song, Yuxin; Deng, Xu; Leung, Michael; Yang, Zhengbao; Xu, Ronald X.; Wang, Zhong Lin; Zeng, Xiao Cheng; Wang, Zuankai
      Nature (London, United Kingdom) (2020), 578 (7795), 392-396CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
      Extensive efforts have been made to harvest energy from water in the form of raindrops1-6, river and ocean waves7,8, tides9 and others10-17. However, achieving a high d. of elec. power generation is challenging. Traditional hydraulic power generation mainly uses electromagnetic generators that are heavy, bulky, and become inefficient with low water supply. An alternative, the water-droplet/solid-based triboelec. nanogenerator, has so far generated peak power densities of less than one watt per square metre, owing to the limitations imposed by interfacial effects-as seen in characterizations of the charge generation and transfer that occur at solid-liq.1-4 or liq.-liq.5,18 interfaces. Here we develop a device to harvest energy from impinging water droplets by using an architecture that comprises a polytetrafluoroethylene film on an indium tin oxide substrate plus an aluminum electrode. We show that spreading of an impinged water droplet on the device bridges the originally disconnected components into a closed-loop elec. system, transforming the conventional interfacial effect into a bulk effect, and so enhancing the instantaneous power d. by several orders of magnitude over equiv. devices that are limited by interfacial effects.
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    14. 14
      Yatsuzuka, K.; Mizuno, Y.; Asano, K. Electrification phenomena of pure water droplets dripping and sliding on a polymer surface. J. Electrostat. 1994, 32, 157– 171,  DOI: 10.1016/0304-3886(94)90005-1
      There is no corresponding record for this reference.
    15. 15
      Sun, Q.; Wang, D.; Li, Y.; Zhang, J.; Ye, S.; Cui, J.; Chen, L.; Wang, Z.; Butt, H.-J.; Vollmer, D.; Deng, X. Surface charge printing for programmed droplet transport. Nat. Mater. 2019, 18, 936– 941,  DOI: 10.1038/s41563-019-0440-2
      15
      Surface charge printing for programmed droplet transport
      Sun, Qiangqiang; Wang, Dehui; Li, Yanan; Zhang, Jiahui; Ye, Shuji; Cui, Jiaxi; Chen, Longquan; Wang, Zuankai; Butt, Hans-Jurgen; Vollmer, Doris; Deng, Xu
      Nature Materials (2019), 18 (9), 936-941CODEN: NMAACR; ISSN:1476-1122. (Nature Research)
      The directed, long-range and self-propelled transport of droplets on solid surfaces is crucial for many applications from water harvesting to bio-anal.1-9. Typically, preferential transport is achieved by topog. or chem. modulation of surface wetting gradients that break the asym. contact line and overcome the resistance force to move droplets along a particular direction10-16. Nonetheless, despite extensive progress, directional droplet transport is limited to low transport velocity or short transport distance. Here we report the high-velocity and ultralong transport of droplets elicited by surface charge d. gradients printed on diverse substrates. We leverage the facile water droplet printing on superamphiphobic surfaces to create rewritable surface charge d. gradients that stimulate droplet propulsion under ambient conditions17 and without the need for addnl. energy input. Our strategy provides a platform for programming the transport of droplets on flat, flexible and vertical surfaces that may be valuable for applications requiring a controlled movement of droplets17-19.
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    16. 16
      Lin, S.; Zheng, M.; Luo, J.; Wang, Z. L. Effects of Surface Functional Groups on Electron Transfer at Liquid–Solid Interfacial Contact Electrification. ACS Nano 2020, 14, 10733– 10741,  DOI: 10.1021/acsnano.0c06075
      16
      Effects of Surface Functional Groups on Electron Transfer at Liquid-Solid Interfacial Contact Electrification
      Lin, Shiquan; Zheng, Mingli; Luo, Jianjun; Wang, Zhong Lin
      ACS Nano (2020), 14 (8), 10733-10741CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Contact electrification (CE) at interfaces is sensitive to the functional groups on the solid surface, but its mechanism is poorly understood, esp. for the liq.-solid cases. A core controversy is the identity of the charge carriers (electrons or/and ions) in the CE between liqs. and solids. Here, the CE between SiO2 surfaces with different functional groups and different liqs., including DI water and org. solns., is systematically studied, and the contribution of electron transfer is distinguished from that of ion transfer according to the charge decay behavior at surfaces at specific temp., because electron release follows the thermionic emission theory. It is revealed that electron transfer plays an important role in the CE between liqs. and functional group modified SiO2. Moreover, the electron transfer between the DI water and the SiO2 is found highly related to the electron affinity of the functional groups on the SiO2 surfaces, while the electron transfer between org. solns. and the SiO2 is independent of the functional groups, due to the limited ability of org. solns. to donate or gain electrons. An energy band model for the electron transfer between liqs. and solids is further proposed, in which the effects of functional groups are considered. The discoveries in this work support the "two-step" model about the formation of an elec. double-layer (Wang model), in which the electron transfer occurs first when the liqs. contact the solids for the very first time.
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    17. 17
      Zhang, J.; Lin, S.; Zheng, M.; Wang, Z. L. Triboelectric Nanogenerator as a Probe for Measuring the Charge Transfer between Liquid and Solid Surfaces. ACS Nano 2021, 15, 14830– 14837,  DOI: 10.1021/acsnano.1c04903
      17
      Triboelectric Nanogenerator as a Probe for Measuring the Charge Transfer between Liquid and Solid Surfaces
      Zhang, Jinyang; Lin, Shiquan; Zheng, Mingli; Wang, Zhong Lin
      ACS Nano (2021), 15 (9), 14830-14837CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      The phenomenon of triboelectricity involves the flow of charged species across an interface, but conclusively establishing the nature of the charge transfer has proven extremely difficult, esp. for the liq.-solid cases. Herein, we developed a self-powered droplet triboelec. nanogenerator (droplet-TENG) with spatially arranged electrodes as a probe for measuring the charge transfer process between liq. and solid interfaces. The information on the elec. signal on spatially arranged electrodes shows that the charge transfer between droplets and the solid is an accumulation process during the dropping and that the electron is the dominant charge-transfer species. Such a droplet-TENG showed a high sensitivity to the ratio of solvents in the mixed org. soln., and we postulated this is due to the possibility of generation of a hydrogen bond, affecting the elec. signal on the spatially arranged electrodes. This work demonstrated a chem. sensing application based on the self-powered droplet triboelec. nanogenerator.
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    18. 18
      Zhu, G.; Su, Y.; Bai, P.; Chen, J.; Jing, Q.; Yang, W.; Wang, Z. L. Harvesting Water Wave Energy by Asymmetric Screening of Electrostatic Charges on a Nanostructured Hydrophobic Thin-Film Surface. ACS Nano 2014, 8, 6031– 6037,  DOI: 10.1021/nn5012732
      18
      Harvesting Water Wave Energy by Asymmetric Screening of Electrostatic Charges on a Nanostructured Hydrophobic Thin-Film Surface
      Zhu, Guang; Su, Yuanjie; Bai, Peng; Chen, Jun; Jing, Qingshen; Yang, Weiqing; Wang, Zhong Lin
      ACS Nano (2014), 8 (6), 6031-6037CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Energy harvesting from ambient H2O motions is a desirable but underexplored soln. to on-site energy demand for self-powered electronics. Here the authors report a liq.-solid electrification-enabled generator based on a fluorinated ethylene propylene thin film, below which an array of electrodes are fabricated. The surface of the thin film is charged 1st due to the H2O-solid contact electrification. Aligned nanowires created on the thin film make it hydrophobic and also increase the surface area. Then the asym. screening to the surface charges by the waving H2O during emerging and submerging processes causes the free electrons on the electrodes to flow through an external load, resulting in power generation. The generator produces sufficient output power for driving an array of small electronics during direct interaction with H2O bodies, including surface waves and falling drops. Polymer-nanowire-based surface modification increases the contact area at the liq.-solid interface, leading to enhanced surface charging d. and thus elec. output at an efficiency of 7.7%. The planar-structured generator features an all-in-one design without sep. and movable components for capturing and transmitting mech. energy. It has extremely lightwt. and small vol., making it a portable, flexible, and convenient power soln. that can be applied on the ocean/river surface, at coastal/offshore areas, and even in rainy places. Considering the demonstrated scalability, it can also be possibly used in large-scale energy generation if layers of planar sheets are connected into a network.
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    19. 19
      Lin, Z.-H.; Cheng, G.; Lin, L.; Lee, S.; Wang, Z. L. Water–Solid Surface Contact Electrification and its Use for Harvesting Liquid-Wave Energy. Angew. Chem., Int. Ed. 2013, 52, 12545– 12549,  DOI: 10.1002/anie.201307249
      19
      Water-Solid Surface Contact Electrification and its Use for Harvesting Liquid Wave Energy
      Lin, Zong-Hong; Cheng, Gang; Lin, Long; Lee, Sangmin; Wang, Zhong Lin
      Angewandte Chemie, International Edition (2013), 52 (48), 12545-12549CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Contact electrification, also called triboelectrification, is an old but well-known phenomenon in which surface charge transfer occurs when two materials are brought into contact.The triboelec. nanogenerator (TENG), which is the first invention utilizing contact electrification to efficiently convert mech. energy into electricity, has been systematically studied to instantaneously drive hundreds of light- emitting diodes (LEDs) and charge a lithium-ion battery for powering a wireless sensor and a com. cell phone. We explore the opportunity to use water contact as one type of "material" choice for TENG. We demonstrate that the contact electrification between water and insulating polymer films can also be useful for TENG, which can derive a new application of TENG esp. in liq. environments for sensing.
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    20. 20
      Park, J.; Song, S.; Shin, C.; Yang, Y.; Weber, S. A. L.; Sim, E.; Kim, Y. S. Ion Specificity on Electric Energy Generated by Flowing Water Droplets. Angew. Chem., Int. Ed. 2018, 57, 2091– 2095,  DOI: 10.1002/anie.201711505
      20
      Ion Specificity on Electric Energy Generated by Flowing Water Droplets
      Park, Junwoo; Song, Suhwan; Shin, ChaeHo; Yang, YoungJun; Weber, Stefan A. L.; Sim, Eunji; Kim, Youn Sang
      Angewandte Chemie, International Edition (2018), 57 (8), 2091-2095CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The development of energy-conversion devices using water movement has actively progressed. Ionovoltaic devices, which are driven by ion dynamics, show ion specificity by which different ions with identical charges show different output performance. However, the ion specificity remains poorly understood because the influence of the ion species on generated elec. signals is not elucidated. The ion specificity in elec. signals induced by flowing water droplet was investigated in terms of its relationship with the potential profile across the solid-liq. interface.
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    21. 21
      Engelnkemper, S.; Wilczek, M.; Gurevich, S. V.; Thiele, U. Morphological transitions of sliding drops: Dynamics and bifurcations. Phys. Rev. Fluids 2016, 1, 073901  DOI: 10.1103/PhysRevFluids.1.073901
      There is no corresponding record for this reference.
    22. 22
      Beattie, J. K. The intrinsic charge on hydrophobic microfluidic substrates. Lab Chip 2006, 6, 1409– 1411,  DOI: 10.1039/b610537h
      22
      The intrinsic charge on hydrophobic microfluidic substrates
      Beattie, James K.
      Lab on a Chip (2006), 6 (11), 1409-1411CODEN: LCAHAM; ISSN:1473-0197. (Royal Society of Chemistry)
      A synthesis of the literature provides an explanation for the hitherto unexplained charge on hydrophobic microfluidic substrates. It is due to the adsorption of hydroxide ions created by the enhanced autolysis of H2O at these surfaces.
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    23. 23
      Wong, W. S. Y.; Hauer, L.; Naga, A.; Kaltbeitzel, A.; Baumli, P.; Berger, R.; D‘Acunzi, M.; Vollmer, D.; Butt, H.-J. Adaptive Wetting of Polydimethylsiloxane. Langmuir 2020, 36, 7236– 7245,  DOI: 10.1021/acs.langmuir.0c00538
      23
      Adaptive Wetting of Polydimethylsiloxane
      Wong, William S. Y.; Hauer, Lukas; Naga, Abhinav; Kaltbeitzel, Anke; Baumli, Philipp; Berger, Ruediger; D'Acunzi, Maria; Vollmer, Doris; Butt, Hans-Juergen
      Langmuir (2020), 36 (26), 7236-7245CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      To better understand the wetting of crosslinked polydimethylsiloxane (PDMS), we measured advancing and receding contact angles of sessile water drops on crosslinked PDMS as a function of contact line velocity (up to 100μm/s). Three types of samples were investigated: pristine PDMS, PDMS where oligomers were removed by toluene treatment, and PDMS with an enriched concn. of oligomers. Depending on the velocity of advancing contact lines and the contact time with water, different modes of wetting were obsd.: one with a relatively low contact angle hysteresis (Δθ ≈ 10°) and one with a larger hysteresis. We attribute the low hysteresis state, called the lubricated state, to the enrichment of free oligomers at the water-PDMS interface. The enrichment of oligomers is induced by drop contact. The kinetics of the transition to the lubricated state can be described by adaptation theory. PDMS adapts to the presence of water by an enrichment of free oligomers at the interface and a correlated redn. in interfacial tension.
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    24. 24
      Li, X.; Silge, S.; Saal, A.; Kircher, G.; Koynov, K.; Berger, R.; Butt, H.-J. Adaptation of a Styrene–Acrylic Acid Copolymer Surface to Water. Langmuir 2021, 37, 1571– 1577,  DOI: 10.1021/acs.langmuir.0c03226
      24
      Adaptation of a Styrene-Acrylic Acid Copolymer Surface to Water
      Li, Xiaomei; Silge, Simon; Saal, Alexander; Kircher, Gunnar; Koynov, Kaloian; Berger, Ruediger; Butt, Hans-Juergen
      Langmuir (2021), 37 (4), 1571-1577CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      Solid surfaces, in particular polymer surfaces, are able to adapt upon contact with a liq. Adaptation results in an increase in contact angle hysteresis and influences the mobility of sliding drops on surfaces. To study adaptation and its kinetics, we synthesized a random copolymer composed of styrene and 11-25 mol% acrylic acid (PS/PAA). We measured the dynamic advancing (θA) and receding (θR) contact angles of water drops sliding down a tilted plate coated with this polymer. We measured θA ≈ 87° for velocities of the contact line <20μm/s. At higher velocities, θA gradually increased to ~ 98°. This value is similar to θA of a pure polystyrene (PS) film, which we studied for comparison. We assoc. the gradual increase in θA to the adaptation process to water. The presence of water leads to swelling and/or an enrichment of acid groups at the water/polymer interface. By applying the latest adaptation theory, we estd. the time const. of this adaptation process to be «1 s. For sliding water drops, θR is ~ 10° lower compared to the ref. PS surface for all tested velocities. Thus, at the receding side of a sliding drop, the surface is already enriched by acid groups. For a water drop with a width of 5 mm, the increase in contact angle hysteresis corresponds to an increase in capillary force in the range of 45-60μN, depending on sliding velocity.
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    25. 25
      Butt, H.-J.; Berger, R.; Steffen, W.; Vollmer, D.; Weber, S. A. L. Adaptive Wetting─Adaptation in Wetting. Langmuir 2018, 34, 11292– 11304,  DOI: 10.1021/acs.langmuir.8b01783
      25
      Adaptive Wetting-Adaptation in Wetting
      Butt, Hans-Juergen; Berger, Ruediger; Steffen, Werner; Vollmer, Doris; Weber, Stefan A. L.
      Langmuir (2018), 34 (38), 11292-11304CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      Many surfaces reversibly change their structure and interfacial energy upon being in contact with a liq. Such surfaces adapt to a specific liq. We propose the first order kinetic model to describe dynamic contact angles of such adaptive surfaces. The model is general and does not refer to a particular adaptation process. The aim of the proposed model is to provide a quant. description of adaptive wetting and to link changes in contact angles to microscopic adaptation processes. By introducing exponentially relaxing interfacial energies and applying Young's equation locally, we predict a change of advancing and receding contact angles depending on the velocity of the contact line. Even for perfectly homogeneous and smooth surfaces, a dynamic contact angle hysteresis is obtained. As possible adaptations, we discuss changes and reconstruction of polymer surfaces or monolayers, diffusion and swelling, adsorption of surfactants, replacement of contaminants, reorientation of liq. mols., or formation of an elec. double layer.
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    26. 26
      Li, X.; Bista, P.; Stetten, A.; Bonart, H.; Schür, M.; Hardt, S.; Bodziony, F.; Marschall, H.; Saal, A.; Deng, X.; Berger, R.; Weber, S.; Butt, H.-J. Drop race: How electrostatic forces influence drop motion. Nat. Phys. 2022,  DOI: 10.21203/rs.3.rs-737950/v1
      There is no corresponding record for this reference.
    27. 27
      Silverstein, T. P. The Real Reason Why Oil and Water Don’t Mix. J. Chem. Educ. 1998, 75, 116,  DOI: 10.1021/ed075p116
      27
      The real reason why oil and water do not mix
      Silverstein, Todd P.
      Journal of Chemical Education (1998), 75 (1), 116-118CODEN: JCEDA8; ISSN:0021-9584. (Division of Chemical Education of the American Chemical Society)
      In this paper I describe the scope of the title problem, present thermodn. data along with a generally accepted model that explains the hydrophobic effect, and recommend how textbook authors should approach the problem.
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    28. 28
      Clarke, S. The hydrophobic effect: Formation of micelles and biological membranes, 2nd edition (Tanford, Charles). J. Chem. Educ. 1981, 58, A246,  DOI: 10.1021/ed058pA246.1
      There is no corresponding record for this reference.
    29. 29
      Wong, W. S. Y. Surface Chemistry Enhancements for the Tunable Super-Liquid Repellency of Low-Surface-Tension Liquids. Nano Lett. 2019, 19, 1892– 1901,  DOI: 10.1021/acs.nanolett.8b04972
      29
      Surface Chemistry Enhancements for the Tunable Super-Liquid Repellency of Low-Surface-Tension Liquids
      Wong, William S. Y.
      Nano Letters (2019), 19 (3), 1892-1901CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Super-hydrophobic, super-oleo(amphi)phobic, and super-omniphobic materials are universally important in the fields of science and engineering. Despite rapid advancements, gaps of understanding still exist between each distinctive wetting state. The transition of super-hydrophobicity to super-(oleo-, amphi-, and omni-)phobicity typically requires the use of re-entrant features. Today, re-entrant geometry induced super-(amphi- and omni-)phobicity is well-supported by both expts. and theory. However, owing to geometrical complexities, the concept of re-entrant geometry forms a dogma that limits the industrial progress of these unique states of wettability. Moreover, a key fundamental question remains unanswered: are extreme surface chem. enhancements able to influence super-liq. repellency. Here, this was rigorously tested via an alternative pathway that does not require explicit designer re-entrant features. Highly controllable and tunable vertical network polymn. and functionalization were used to achieve fluoroalkyl densification on nanoparticles. For the first time, relative fluoro-functionalization densities are quant. tuned and correlated to super-liq. repellency performance. Step-wise tunable super-amphiphobic nanoparticle films with a Cassie-Baxter state (contact angle of >150° and sliding angle of <10°) against various liqs. is demonstrated. This was tested down to very low surface tension liqs. to a min. of ca. 23.8 mN/m. Such findings could eventually lead to the future development of super-(amphi)omniphobic materials that transcend the sole use of re-entrant geometry.
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    30. 30
      Campos, R.; Guenthner, A. J.; Haddad, T. S.; Mabry, J. M. Fluoroalkyl-functionalized silica particles: synthesis, characterization, and wetting characteristics. Langmuir 2011, 27, 10206– 10215,  DOI: 10.1021/la201545a
      30
      Fluoroalkyl-Functionalized Silica Particles: Synthesis, Characterization, and Wetting Characteristics
      Campos, Raymond; Guenthner, Andrew J.; Haddad, Timothy S.; Mabry, Joseph M.
      Langmuir (2011), 27 (16), 10206-10215CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      Fluoroalkyl-functionalized SiO2 particles for use in nonwetting surfaces were prepd. by treatment of SiO2 particles with fluoroalkyl-functional chlorosilanes. Both fumed and pptd. SiO2 were studied, as well as the efficiency of surface coverage using mono-, di-, and trifunctional chlorosilanes. The most effective surface treatment was accomplished via the surface grafting of monofunctional chlorosilanes in the presence of preadsorbed dimethylamine under anhyd. conditions at room temp. Confirmation of covalent attachment was accomplished via FTIR spectroscopy, while elemental anal., TGA, and nitrogen adsorption isotherms were used to det. grafting densities and addnl. key geometric characteristics of the grafted layer. The effect of residual silanol content on the moisture uptake properties of the modified SiO2 particles was detd. by measuring the H2O uptake of unbound particles, while liq. wetting properties were detd. by dynamic contact angle anal. of elastomeric composites. Although residual silanol content was shown to effect wetting properties, results suggest that surface geometry dominates the performance of liq.-repellent surfaces. The potential use of fluoroalkyl-functionalized SiO2 particles for hydrophobic and oleophobic applications is discussed.
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    31. 31
      Fadeev, A. Y.; McCarthy, T. J. Self-Assembly Is Not the Only Reaction Possible between Alkyltrichlorosilanes and Surfaces: Monomolecular and Oligomeric Covalently Attached Layers of Dichloro- and Trichloroalkylsilanes on Silicon. Langmuir 2000, 16, 7268– 7274,  DOI: 10.1021/la000471z
      31
      Self-Assembly Is Not the Only Reaction Possible between Alkyltrichlorosilanes and Surfaces: Monomolecular and Oligomeric Covalently Attached Layers of Dichloro- and Trichloroalkylsilanes on Silicon
      Fadeev, Alexander Y.; McCarthy, Thomas J.
      Langmuir (2000), 16 (18), 7268-7274CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      Si-supported alkylsiloxane layers were prepd. by reaction of alkylmethyldichlorosilanes and alkyltrichlorosilanes with Si wafers under two conditions: (1) in the vapor phase and (2) in toluene in the presence of ethyldiisopropylamine. Covalent attachment of di- and trichlorosilanes to the surface of Si/Si oxide through SiS-O-Si bonds occurs for the amine-catalyzed reactions. This sets apart this reaction from the self-assembly process that occurs in the reaction between certain trichlorosilanes and hydrated SiO2 with no amine present. The thickness of the layers formed from dichloro- and trichlorosilanes (as assessed by ellipsometry) is on the order of the single mol. sizes and increases gradually with alkyl chain length. The thickness values are considerably smaller (by a factor of ∼0.75) than the length of the fully stretched alkyl chain, which argues for disordered structures of the monolayers. Dynamic advancing and receding contact angles for H2O, methylene iodide, and hexadecane argue for interaction between the probe fluids and accessible silanol groups (Si-OH) on the surface. H2O contact angles increase with alkyl chain length and level at θA/θR ≃103°/∼90° for relatively long alkyl chains (approx. C6 and longer), indicating that these surfaces project disordered Me and methylene groups toward the probe fluid. N-Hexadecane and methylene iodide contact angles show more complex behavior, which is discussed. The vapor-phase reaction of di- and trichlorosilanes with Si wafers yields surfaces that depend dramatically on the alkyl chain of the silane. Alkylsilanes with short and medium chains form polymeric grafted layers with thicknesses ranging from a few nanometers for dichlorosilanes up to tens of nanometers for trichlorosilanes. The authors suggest a mechanism that involves polycondensation of chlorosilanes into 3-dimensional alkylsiloxanes in the presence of adsorbed H2O. Dynamic advancing and receding contact angles of H2O, methylene iodide, and hexadecane on these surfaces are consistently higher than for surfaces prepd. in the liq. phase. Alkylsilanes with long alkyl moieties yield approx. monomol. layers that exhibit wettabilities similar to those for surfaces prepd. in the liq. phase.
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  • Supporting Information

    Supporting Information

    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.langmuir.2c00941.

    • (1) Descriptions of supplementary experiments, (2) supplementary discussion on TOF-SIMS analysis of surface chemical compositions, (3) AFM analysis of surfaces, and (4) supplementary drop mobility experiments (PDF)


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