Decreasing Surface Coverage Research Articles

Assessing the dynamics of O2 desorption from cobalt phthalocyanine by in situ electrochemical scanning tunneling microscopy

We report here the in situ electrochemical scanning tunneling microscopy (ECSTM) study of cobalt phthalocyanine (CoPc)-catalyzed O2 evolution reaction (OER) and the dynamics of CoPc-O2 dissociation. The self-assembled CoPc monolayer is fabricated on Au(111) substrate and resolved by ECSTM in 0.1 M KOH electrolyte. The OH− adsorption on CoPc prior to OER is observed in ECSTM images. During OER, the generated O2 adsorbed on CoPc is observed in the CoPc monolayer. Potential step experiment is employed to monitor the desorption of OER-generated O2 from CoPc, which results in the decreasing surface coverage of CoPc-O2 with time. The rate constant of O2 desorption is evaluated through data fitting. The insights into the dynamics of Co-O2 dissociation at the molecular level via in situ imaging help understand the role of Co-O2 in oxygen reduction reaction (ORR) and OER.

Charge Transport in Sub‐Monolayer Networks of a Naphthalene‐Diimide‐Based Copolymer

AbstractSub‐monolayer networks of the electron‐transporting semiconducting polymer poly([N,N’‐bis(2‐octyldodecyl)‐naphthalene‐1,4,5,8‐bis(dicarboximide)−2,6‐diyl]‐alt‐5,5′‐(2,2′‐bithiophene)) (P(NDI2OD‐T2)) are realized through judicious choice of spin‐coating solvent and control of solution concentration. These sub‐monolayer networks provide a platform to study the effects of surface coverage, network connectivity, and orientational order on charge transport. It is found that charge transport through semiconducting polymer networks depends not only on the coverage/concentration of the material but also on the nature of the conduction paths themselves. Down to 40% surface coverage, current measured in a field‐effect architecture decreases in line with the decrease in surface coverage, but drops precipitously below a surface coverage of 40%. Below 40% surface coverage, there is a marked decrease in network connectivity (quantified through the branching point density) and a decrease in orientational order. Furthermore, an increase in the density of dangling branches (dead ends) is also seen with decreasing surface coverage. Additional supporting data in the form of grazing incidence wide‐angle X‐ray scattering (GIWAXS), optical spectroscopy, and charge modulation spectroscopy are presented, which help to establish that long‐range interconnectivity rather than local morphology dominates charge transport in P(NDI2OD‐T2) sub‐monolayers as well as thin films.

Modulation of spin and charge currents through functionalized 2D diamond devices

In this study, we explore the potential of functionalized two-dimensional (2D) diamond for spin-dependent electronic devices using first-principles calculations. Specifically, we investigate functionalizations with either hydroxyl (−OH) or fluorine (−F) groups. In the case of an isolated layer, we observe that the quantity and distribution of (−OH) or (−F) on the 2D diamond surface significantly influence the sp 2/sp 3 ratio of the carbon atoms in the layer. As the coverage is reduced, both the band gap and magnetic moment decrease. When the 2D diamond is placed between gold contacts and functionalized with (−OH), it results in a device with lower resistance compared to the (−F) functionalization. We predict that the maximum current achieved in the device increases with decreasing (−OH) surface coverage, while the opposite behavior occurs for (−F). Additionally, the surface coverage alone can alter the direction of current rectification in (−F) functionalized 2D diamonds. For all studied systems, a single spin component contributes to the total current for certain values of applied bias, indicating a spin filter behavior.

Understanding chemical enhancements of surface-enhanced Raman scattering using a Raman bond model for extended systems.

In this work, we extend a previously developed Raman bond model to periodic slab systems for interpreting chemical enhancements of surface-enhanced Raman scattering (SERS). The Raman bond model interprets chemical enhancements as interatomic charge flow modulations termed Raman bonds. Here, we show that the Raman bond model offers a unified interpretation of chemical enhancements for localized and periodic systems. As a demonstration of the Raman bond model, we study model systems consisting of CO and pyridine molecules on Ag clusters and slabs. We find that for both localized and periodic systems, the dominant Raman bonds are distributed near the molecule-metal interface and, therefore, the chemical enhancements are determined by a common Raman bond pattern. The effects of surface coverage, thickness, and roughness on the chemical enhancements have been studied, which shows that decreasing surface coverage or creating surface roughness increases chemical enhancements. In both of these cases, the inter-fragment charge flow connectivity is improved due to more dynamic polarization at the interface. The chemical enhancement is shown to scale with the inter-fragment charge flow to the fourth power. Since the inter-fragment charge flow is determined by the charge transfer excitation energy, the Raman bond model is connected to the transition-based analysis of chemical enhancements. We also show that the SERS spectra of localized and periodic systems normalized by inter-fragment charge flows can be unified. In summary, the Raman bond model offers a unique framework for understanding SERS spectra in terms of Raman bond distributions and offers a connection between localized and periodic model systems of SERS studies.

Effect of selected 3-chloromethylcoumarin derivatives on mild steel corrosion in acidic medium: experimental and computational studies

Two 3-chloromethylcoumarins synthesized via the Baylis-Hillman reaction have been investigated for their ability to inhibit corrosion of mild steel in 2.0 M HCl. The inhibition efficiency was evaluated by gravimetric and electrochemical methods. Inhibition efficiency was found to increase with increase in concentration of the 3-chloromethylcoumarins. Kinetics, thermodynamics and adsorption equilibrium of the corrosion of steel in the acid and acid-containing 3-chloromethylcoumarins media were investigated. The results showed that the inhibitive actions of the compounds occurred as a result of adsorption of 3-chloromethylcoumarins molecules on mild steel surface. 3-Chloromethyl-8-ethoxycoumarin 4a showed higher inhibition efficiencies than 6-chloro-3-chloromethylcoumarin 4b, especially at low concentrations and temperatures. Physical adsorption was predicted based on decreasing surface coverage of the inhibitor molecules with increase in temperature. The studied 3-chloromethylcoumarins behaved as mixed-type corrosion inhibitors for mild steel in 2 M HCl. The higher electron-donating ability and lower energy gap of 4a compared to 4b, as derived from computational studies suggest that 4a has higher potentials to inhibit steel corrosion in HCl than 4b. Similar trend was reflected in the Monte Carlo simulations adsorption energies.

On the link between CO surface coverage and selectivity to CH4 during CO2 hydrogenation over supported cobalt catalysts

An operando IR study was carried out to investigate the loss of selectivity to higher hydrocarbons observed over cobalt-based Fischer-Tropsch catalysts when CO is replaced with CO2. Our data unambiguously show that the structure and oxidation state of the surface of metallic cobalt remains the same whether CO2 or CO are cofed with H2. The selectivity to CH4 appears to increase monotonously with decreasing surface coverage of adsorbed CO. The switch from Fischer-Tropsch synthesis to methanation using CO2-rich feeds over metallic cobalt can therefore be assigned to surface coverage effects only, assuming that most, if not all, hydrocarbons products are formed on metallic cobalt.

Polarization of Macrophages, Cellular Adhesion, and Spreading on Bacterially Contaminated Gold Nanoparticle-Coatings in Vitro.

Biomaterial-associated infections often arise from contaminating bacteria adhering to an implant surface that are introduced during surgical implantation and not effectively eradicated by antibiotic treatment. Whether or not infection develops from contaminating bacteria depends on an interplay between bacteria contaminating the biomaterial surface and tissue cells trying to integrate the surface with the aid of immune cells. The biomaterial surface plays a crucial role in defining the outcome of this race for the surface. Tissue integration is considered the best protection of a biomaterial implant against infectious bacteria. This paper aims to determine whether and how macrophages aid osteoblasts and human mesenchymal stem cells to adhere and spread over gold nanoparticle (GNP)-coatings with different hydrophilicity and roughness in the absence or presence of contaminating, adhering bacteria. All GNP-coatings had identical chemical surface composition, and water contact angles decreased with increasing roughness. Upon increasing the roughness of the GNP-coatings, the presence of contaminating Staphylococcus epidermidis in biculture with cells gradually decreased surface coverage by adhering and spreading cells, as in the absence of staphylococci. More virulent Staphylococcus aureus fully impeded cellular adhesion and spreading on smooth gold- or GNP-coatings, while Escherichia coli allowed minor cellular interaction. Murine macrophages in monoculture tended toward their pro-inflammatory "fighting" M1-phenotype on all coatings to combat the biomaterial, but in bicultures with contaminating, adhering bacteria, macrophages demonstrated Ym1 expression, indicative of polarization toward their anti-inflammatory "fix-and-repair" M2-phenotype. Damage repair of cells by macrophages improved cellular interactions on intermediately hydrophilic/rough (water contact angle 30 deg/surface roughness 118 nm) GNP-coatings in the presence of contaminating, adhering Gram-positive staphylococci but provided little aid in the presence of Gram-negative E. coli. Thus, the merits on GNP-coatings to influence the race for the surface and prevent biomaterial-associated infection critically depend on their hydrophilicity/roughness and the bacterial strain involved in contaminating the biomaterial surface.

Bacteria mediated silver nanoparticles: comparison as potent antibiofilm agents

Introduction: Biosynthesized silver nanoparticles (AgNPs) have good antimicrobial properties comparable to chemically synthesized silver nanoparticles. Further, they have good potential as antibiofilm agents. Methods: AgNPs were synthesized from Pseudomonas aeruginosa (ATCC 27853), Escherichia coli (ATCC 25922), Staphylococcus aureus (ATCC 25923) and Acinetobacter baumannii (confirmed clinical isolate) and physically characterized by several techniques. The antibiofilm activity of the AgNPs against biofilms of P. aeruginosa, S. aureus and C. albicans was studied using crystal violet assay. Biofilms were formed in 96-well polystyrene plates and treated with biosynthesized AgNPs for 24 and 48 h. Results: AgNPs synthesized by all bacteria except S. aureus mediated AgNPs displayed 50% biofilm inhibition at AgNP concentrations between 1.98 - 0.225 mg/ml. S. aureus mediated AgNPs showed 50% biofilm inhibition only against S. aureus biofilm. Scanning Electron microscopic images indicated that biosynthesized AgNPs were able to decrease surface coverage of biofilms and to reduce the extracellular matrix causing morphological changes in biofilms noticeably. Conclusion: This study reports the antibiofilm activity of bacteria mediated AgNPs. This is the first report on antibiofilm activity of AgNPs synthesized by Acinetobacter baumannii and also as a comparison of antibiofilm activity of several bacteria mediated AgNPs. According to the results, low dosages of green AgNPs can be applied in treating drug-resistant microbial infections in a cost effective manner. In conclusion, the bacterial synthesized AgNPs have antibiofilm activity and good stability suggesting its usefulness as economic and environmental friendly antibiofilm agents.

A surface plasmon resonance based inhibition immunoassay for measurement of steroid hormones

Quantitative measurement of small-molecule metabolites is now emerging as an effective way to link the metabolite profile to disease state. Surface plasmon resonance (SPR) is a sensing platform that has demonstrated applicability for a large range of biomolecules. However, direct detection of small molecules with SPR challenges the refractive index based detection mechanism. Herein, we utilized an indirect detection format and developed an inhibition immunoassay for the quantitative measurement of 17β-estradiol (E2) using SPR. One competitor, BSA-E2 conjugate, was immobilized to the SPR chip via the reaction between the primary amino group of the conjugate and the succinimide group (NHS) introduced by the formation of a thiol-NHS monolayer on gold surface. Free E2 molecules compete with BSA-E2 on chip surface for binding sites provided by a monoclonal anti-E2 antibody. It was found the binding affinity of the antibody to BSA-E2 conjugate increases with decreasing surface coverage of BSA-E2 conjugate. Under optimal conditions, a sigmoidal calibration curve with a negative slope and a dynamic range from 10 pM to 2 nM was generated. The detection limit of the immunoassay is estimated to be 0.3 pM. Moreover, the immunoassay exhibits high specificity for E2 detection using estrone (E1) as a potential interference.

Thixotropy of SCC—A model describing the effect of particle packing and superplasticizer adsorption on thixotropic structural build-up of the mortar phase based on interparticle interactions

This article considers the thixotropy of SCC mortars and the responsible mechanisms. The objective is a model describing the thixotropic structural build-up based on interparticle interactions and hydration kinetics of the fresh binder paste. In the experimental studies, the formation of thixotropic structure in SCC mortars was varied by changing the composition of the binder paste and quantified rheologically. At the same time, the surface coverage of particles by superplasticizer polymers and the particle packing in the suspension were determined to characterize interparticle interactions. Based on the results, a qualitative model as well as a calculation method were developed to describe the thixotropic behaviour. It is shown that the contribution of colloidal surface interactions and hydration reactions to thixotropy both increase with decreasing surface coverage and therefore decreasing particle separation. In addition, thixotropy increases with increasing contact interactions resulting from a higher solid volume fraction or a lower maximum packing density.

Optimization of antifouling coatings incorporating butenolide, a potent antifouling agent via field and laboratory tests

Rosin-based antifouling paint with the incorporation of butenolide, a promising antifoulant, possesses the potential to deter the settlement of marine organisms on submerged surfaces. With the purpose to extend the antifouling duration, this research investigated the respective contribution of paint ingredients, including butenolide concentrations (5%, 10% and 15%), pigment choices (TiO2, Fe2O3, Cu2O and ZnO) and binder compositions (acrylic copolymer to rosin at 1: 2.5, 1.5: 2 and 2.5: 1), to the field antifouling performance of butenolide. A raft trial was carried out at Yung Shue 0, Hong Kong after the application of antifouling paints on PVC panels. Biofouling dynamics on panel surfaces, such as coverage percentage and biomass accumulation, were monitored until submersion for 6 months to allow for the assessment of antifouling efficiency. Field results showed that butenolide incorporation generally inhibited the settlement of fouling species on the coated panels as demonstrated by the decreased surface coverage and biomass weight. Coatings with 1: 2.5 paints containing 10% butenolide exhibited the best antifouling performance with only 34% of the surface covered by fouling organisms, which mainly consisted of algae and slime. The smallest biomass increase of the fouling community was also observed for 1: 2.5 coatings. An increased proportion of rosin in binder compositions yielded better antifouling performance following the order of 1: 2.5 > 1.5: 2 > 2.5: 1. Laboratory experiments were also conducted to examine the behavior of paint coatings in stirring artificial seawater. Butenolide addition decreased the film hardness and inhibited water uptake, but resulted in weight loss of paint coatings. Along with the gradual release of butenolide, the hardness of paint films increased gradually. Overall, a service life of 6 months, while eliminating the use of heavy metals, highlights the effectiveness of butenolide-incorporated paint formulation, especially 1: 2.5 paint, as an environmentally benign and fouling-resistant candidate for future antifouling application.

Impact of Gold Nanoparticle Stabilizing Ligands on the Colloidal Catalytic Reduction of 4-Nitrophenol

Gold nanoparticles (AuNPs) have received considerable interest owing to their unique properties and applications in catalysis. One of the major challenges for colloidal nanoparticles in catalysis is the limited stability and resulting aggregation. Nanoparticle functionalization with ligands or polymers is a common strategy to improve the colloidal stability, which in turn blocks the reactive surface sites and eliminates catalytic activity. Here, we investigate thiolated polyethylene glycol (HS-PEG) as a stabilizing ligand during AuNP catalytic reduction of 4-nitrophenol. We show a direct relationship between the chain length and packing density of HS-PEG with respect to AuNP catalytic activity. High surface coverage of low molecular weight HS-PEG (1 kDa) completely inhibited the catalytic activity of AuNPs. Increasing HS-PEG molecular weight and decreasing surface coverage was found to correlate directly with increasing rate constants and decreasing induction time. Time-resolved UV–vis absorbance spectros...

Adatom Interactions on GaN(0001) Surface I: Coverage-Dependent Adsorption

The adsorption of an adatom on the surface of a crystal is affected by the surface coverage due to adatom–adatom and adatom–substrate interactions. We study the dependence of adsorption energy of Ga and N adatoms on wurtzite GaN(0001) as a function of decreasing surface coverage from 0.25 to 0.04 monolayers through first-principles calculations. The adsorption energies of Ga and N adatoms on the flat, clean GaN(0001) substrate do not converge with decreasing coverage. Further, it appears that the Ga and N adatoms significantly distort the substrate lattice from the flat configuration. We found that these distortions increase with increase in system size (or, equivalently, with reducing coverage). This observation is counterintuitive since it is expected that lattice distortions should decrease with decreasing surface coverage. We separate the different contributions to the adsorption energy and identify the part of adsorption energy that arises due to lattice distortions. This contribution appears to be m...

Desorption mechanisms of phosphate from ferrihydrite and goethite surfaces

The fate of phosphate in the environment is governed by reactions at particle surfaces. These adsorption and desorption reactions display biphasic kinetics involving an initial rapid reaction followed by a substantially slower one extending over long time periods. In this study we have investigated the molecular mechanisms of desorption kinetics of phosphate from ferrihydrite and goethite nanoparticles in the absence of competing ligands. Desorption was studied by means of in-situ infrared (IR) spectroscopy over a wide pH range and a time period of 24h. The spectroscopic data sets were subjected to multivariate curve resolution alternating least squares (MCR-ALS), which enabled the resolution of surface species characterized by unique IR spectra together with their corresponding kinetic profiles. The desorption results showed the typical biphasic behavior and that increasing positive surface charge of ferrihydrite and goethite slowed down desorption of the negatively charged phosphate ions. Moreover, diprotonated phosphate desorbed faster than monoprotonated phosphate at a given pH. At circumneutral pH values desorption from ferrihydrite was substantially faster as compared to goethite, and this could be ascribed to electrostatic effects and differences in charging between ferrihydrite and goethite. The collective desorption results were explained by a model, consisting of a series monodentate phosphate surface complexes in different protonation states, in conjunction with a description that accounts for the electrostatic effects on desorption kinetics at charged mineral-water interfaces. The fast and slow desorption followed directly from this model and indicated that biphasic kinetics can be caused by a single phosphate surface complex as a result of decreasing surface coverage along with the lateral repulsive interactions between adsorbed phosphate groups. Hence, in contrast to previous models our study has shown that biphasic desorption kinetics do not have to involve several different structural complexes related to either weak and strong sites or a distribution of phosphate between external surfaces and mineral pores.

Highly sensitive Raman spectroscopy using a gap mode plasmon under an attenuated total reflection geometry

We investigated a gap mode plasmon under an attenuated total reflection (ATR) configuration toward realization of near-field Raman spectroscopy with a single molecule sensitivity and spatial resolution. Additional enhancement in Raman scattering at a nanogap was obtained by a coupling of a propagating surface plasmon (PSP) of Ag films on a prism, and a gap mode between Ag films and Ag nanoparticles (AgNPs). Immobilization of AgNPs on Ag films through thiol-SAM slightly up-shifted the resonance angle of a PSP, which broadened the reflectivity dip owing to an increased out-coupling of a PSP. Raman enhancement factor at a nanogap increased with decreasing surface coverage of AgNPs, albeit the enhancement factor averaged over illuminated area in Ag films decreased, ensuring the largest enhancement factor in tip-enhanced Raman scattering. This is due to increased efficiency for a PSP excitation at lower coverage of AgNPs in consistent with that in theoretical evaluation using finite difference time domain calculations. A gap mode under an ATR configuration was applied to elucidate a plausible photochemical reaction of p-amino thiophenol (PATP) adsorbed on Ag films on a prism. Spectral changes in Raman scattering under laser illumination were observed for PATP with a deuterated amino group, but suppressed by a dimethyl amino group owing to steric hindrance, supporting the photochemical dimerization.

Structure and Friction of Stearic Acid and Oleic Acid Films Adsorbed on Iron Oxide Surfaces in Squalane

The structure and friction of fatty acid surfactant films adsorbed on iron oxide surfaces lubricated by squalane are examined using large-scale molecular dynamics simulations. The structures of stearic acid and oleic acid films under static and shear conditions, and at various surface coverages, are described in detail, and the effects of unsaturation in the tail group are highlighted. At high surface coverage, the measured properties of stearic acid and oleic acid films are seen to be very similar. At low and intermediate surface coverages, the presence of a double bond, as in oleic acid, is seen to give rise to less penetration of lubricant in to the surfactant film and less layering of the lubricant near to the film. The kinetic friction coefficient is measured as a function of shear rate within the hydrodynamic (high shear rate) lubrication regime. Lubricant penetration and layering are observed to be correlated with friction coefficient. The friction coefficient with oleic acid depends only weakly on surface coverage, while stearic acid admits more lubricant penetration, and its friction coefficient increases significantly with decreasing surface coverage. Connections between film structure and friction are discussed.

Surfactant–DNA interactions at the liquid crystal–aqueous interface

The presence of single-stranded (ssDNA) vs. double-stranded (dsDNA) DNA at a surfactant-laden aqueous–nematic liquid crystal (LC) interface results in distinctly different orientations of the LC molecular axis; this is of practical interest as a method to detect DNA hybridization. Results presented here provide new insights into the molecular-level mechanisms of these phenomena. The adsorption of ssDNA to a cationic surfactant-laden aqueous–LC interface caused LC reorientation, leading to coexistence between homeotropic and planar (birefringent) oriented regions. Fluorescence microscopy revealed that ssDNA preferentially partitioned into the birefingent regions, presumably causing a decreased surface coverage of surfactant and the resultant planar LC orientation. Both electrostatic and hydrophobic effects were found to be critical to inducing LC reorientation. In particular, insufficient ssDNA adsorption occurred in the absence of a cationic surfactant (e.g. with no surfactant or with a non-ionic surfactant), demonstrating the importance of electrostatic interactions with the polyanionic ssDNA. Even in the presence of a cationic surfactant, however, polyanions without hydrophobic side-group moieties (poly[acrylic acid] and dsDNA) caused no LC reorientation, while polyanions with hydrophobic side groups (polystyrene sulfonate and ssDNA) initiated the desired LC reorientation. These observations are consistent with the fact that interfacial hybridization of adsorbed probe ssDNA to complementary target ssDNA caused a reorientation from planar back to homeotropic. We propose that ssDNA forms an electrostatic interfacial complex with cationic surfactant where the hydrophobic nucleobases associate directly with the LC phase, effectively competing with surfactant molecules for interfacial sites. Upon hybridization, the hydrophobic character of the ssDNA is lost and the nucleobases no longer associate directly with the LC phase, allowing the surfactant molecules to pack more closely at the interface.

Torsion Stiffness of a Protein Pair Determined by Magnetic Particles

We demonstrate the ability to measure torsion stiffness of a protein complex by applying a controlled torque on a magnetic particle. As a model system we use protein G bound to an IgG antibody. The protein pair is held between a magnetic particle and a polystyrene substrate. The angular orientation of the magnetic particle shows an oscillating behavior upon application of a rotating magnetic field. The amplitude of the oscillation increases with a decreasing surface coverage of antibodies on the substrate and with an increasing magnitude of the applied field. For decreasing antibody coverage, the torsion spring constant converges to a minimum value of 1.5 × 10 3 pN·nm/rad that corresponds to a torsion modulus of 4.5 × 10 4 pN·nm 2. This torsion stiffness is an upper limit for the molecular bond between the particle and the surface that is tentatively assigned to a single protein G–IgG protein pair. This assignment is supported by interpreting the measured stiffness with a simple mechanical model that predicts a two orders of magnitude larger stiffness for the protein G–IgG complex than values found for micrometer length dsDNA. This we understand from the structural properties of the molecules, i.e ., DNA is a long and flexible chain-like molecule, whereas the antibody-antigen couple is orders of magnitude smaller and more globular in shape due to the folding of the molecules.

Conformational changes and catalytic competency of hydrolases adsorbing on fumed silica nanoparticles: I. Tertiary structure

We have recently introduced an immobilization protocol for preparations of enzymes on fumed silica for catalysis in organic solvents. The observation of a maximum in apparent catalytic activity at intermediate surface coverage for one enzyme while another enzyme showed continuously increasing apparent catalytic activity with decreasing surface coverage led to speculation on the impact of surface coverage on apparent catalytic activity through different relative surface–protein and protein–protein interactions, combined with different “hardness” or resistance towards unfolding by the enzymes. The kinetics of tertiary unfolding of Candida antarctica Lipase B (CALB), subtilisin Carlsberg, and the Lipase from Thermomyces lanuginosus (TLL) adsorbing on fumed silica nanoparticles were inferred here from tryptophan fluorescence for 2 to 1250%SC, 0.5–4.70 mg/mL enzyme concentration in aqueous buffer solution, and in the presence of the structural modifiers 2,2,2-trifluoroethanol (TFE) and dithiothreitol (DTT). The results shown here confirm the earlier speculation that “hard” enzymes can perform well at low and intermediate surface coverage of the solid fumed silica particles until multi-layer packing imposes mass-transfer limitations, while “soft” enzymes unfold at low surface coverage and therefore show a maximum in catalytic competency at intermediate surface coverage before declining apparent activity is caused by multi-layer packing.

Sliding Behavior of Water Droplets on Smooth Hydrophobic Fluoroalkylsilane Coatings with Different Surface Coverage Ratio

Smooth fluoroalkylsilane (FAS17) coatings (Ra<0.2 nm) with different surface coverage ratios were prepared on a Si substrate by changing the deposition time. Then the internal fluidity of water droplets sliding on the coatings was evaluated using particle image velocimetry (PIV). The water droplets slid downward with acceleration by a caterpillar-like rotation with slippage on the solid-liquid interface. The sliding acceleration of the advancing contact line decreased concomitantly with the decreasing surface coverage ratio. However, the slipping/rolling ratio to the overall sliding acceleration was almost constant against surface coverage ratio. This result suggests that physical roughness and chemical composition contributed differently to slipping and rolling motions of the droplets.