Effect of surface morphology on measurement and interpretation of boundary slip on superhydrophobic surfaces

Highly liquid repellent surfaces have been obtained by the combination of roughness and hydrophobicity. Studies have reported that the flow over such surfaces exhibits larger boundary slip as compared to the smooth hydrophobic surfaces. However, the surface roughness can also lead to apparent slip. Thus, the effect of the two factors, that is, wettability and roughness, needs to be segregated. In this study, we have measured the slippage of water on rough hydrophilic and hydrophobic surfaces using colloidal probe atomic force microscopy technique (CP‐AFM). Results showed that the effect of surface roughness on the measured slip is dominant over that of wettability. It was also found that slip on surfaces with sparsely distributed asperities is highly local and measurements on various locations give dissimilar results. The results suggested that the main reason of the larger slip, on rough hydrophobic surfaces, is likely to be the roughness and not the hydrophobicity. Moreover, it was also found that the slip does not vary considerably with the increase or decrease in the shear rate. Most likely, this kind of slip phenomena is caused by the apparent decrease of the drag force, because the nanoasperities on the surface restrict the probe from reaching the surface properly. Copyright © 2016 John Wiley & Sons, Ltd.     

Ratchetlike slip angle anisotropy on printed superhydrophobic surfaces

The fabrication and properties of superhydrophobic surfaces that exhibit ratchet-like anisotropic slip angle behavior is described. The surface is composed of arrays of poly(dimethylsiloxane) (PDMS) posts fabricated by a type of 3D printing. By controlling the dispense parameters, regular arrays of asymmetric posts were deposited such that the slope of the posts was varied from 0 to 50 relative to the surface normal. Advancing and receding contact angles as well as slip angles were measured as a function of the post slope and droplet volume. Ratchetlike slip angle anisotropy was observed on surfaces composed of sloped features. The maximum slip angle difference (for a 180° tilt angle variation) was 32° for 20 μL droplets on surfaces with posts fabricated with a slope of 50°. This slip angle anisotropy is attributed to an increase in the triple contact line (TCL) length as the droplet is tilted in a direction against the post slope whereas the TCL decreases continuously when the drop travels in a direction parallel to the post slope. The increasing length of the TCL creates an increased energy barrier that accounts for the higher slip angles in this direction.     

Effect of surface roughness on rate-dependent slip in simple fluids

Molecular dynamics simulations are used to investigate the influence of molecular-scale surface roughness on the slip behavior in thin liquid films. The slip length increases almost linearly with the shear rate for atomically smooth rigid walls and incommensurate structures of the liquid/solid interface. The thermal fluctuations of the wall atoms lead to an effective surface roughness, which makes the slip length weakly dependent on the shear rate. With increasing the elastic stiffness of the wall, the surface roughness smoothes out and the strong rate dependence is restored again. Both periodically and randomly corrugated rigid surfaces reduce the slip length and its shear rate dependence.     

Modulation of Hydrophobic Interaction by Mediating Surface Nanoscale Structure and Chemistry,not Monotonically by Hydrophobicity

The hydrophobic (HB) interaction plays a critical role in many colloidal and interfacial phenomena, biophysical and industrial processes. Surface hydrophobicity, characterized by the water contact angle, is generally considered the most dominant parameter determining the HB interaction. Herein, we quantified the HB interactions between air bubbles and a series of hydrophobic surfaces with different nanoscale structures and surface chemistry in aqueous media using a bubble probe atomic force microscopy (AFM). Surprisingly, it is discovered that surfaces of similar hydrophobicity can show different ranges of HB interactions, while surfaces of different hydrophobicity can have similar ranges of HB interaction. The increased heterogeneity of the surface nanoscale structure and chemistry can effectively decrease the decay length of HB interaction from 1.60 nm to 0.35 nm. Our work provides insights into the physical mechanism of HB interaction.     

Modulation of Hydrophobic Interaction by Mediating Surface Nanoscale Structure and Chemistry,not Monotonically by Hydrophobicity

The hydrophobic (HB) interaction plays a critical role in many colloidal and interfacial phenomena, biophysical and industrial processes. Surface hydrophobicity, characterized by the water contact angle, is generally considered the most dominant parameter determining the HB interaction. Herein, we quantified the HB interactions between air bubbles and a series of hydrophobic surfaces with different nanoscale structures and surface chemistry in aqueous media using a bubble probe atomic force microscopy (AFM). Surprisingly, it is discovered that surfaces of similar hydrophobicity can show different ranges of HB interactions, while surfaces of different hydrophobicity can have similar ranges of HB interaction. The increased heterogeneity of the surface nanoscale structure and chemistry can effectively decrease the decay length of HB interaction from 1.60 nm to 0.35 nm. Our work provides insights into the physical mechanism of HB interaction.     

Hydrophobic anatase TiO(2)-based thin films modified with Al, Cr derivatives to reach reversible wettability control

Polycrystalline anatase TiO(2)-based thin films with surface roughness were fabricated using a sublimation method. Their surfaces showed hydrophobicity with a water contact angle (CA) higher than 130 degrees when stored in the dark. For the films, the hydrophobic conversion behavior and critical CA depended on surface morphologies. The higher hydrophobic conversion rate and higher critical CA were explained by the smaller r in the Wenzel equation and the smaller f (f = 0.56, 0.30) in the Cassie equation, respectively. Moreover, good reversibility between hydrophobicity (CA = 130 degrees ) and high hydrophilicity (CA = 0 degrees ) was observed by alternating between UV light irradiation and dark storage. The combination of chromium oxide with anatase TiO(2) markedly shortens the period of hydrophobic conversion from 12 to 5 days.     

Effect of wall slip on the viscoelastic particle ordering in a microfluidic channel

The formation of a line of equally spaced particles at the centerline of a microchannel, referred as “particle ordering,” is desired in several microfluidic applications. Recent experiments and simulations highlighted the capability of viscoelastic fluids to form a row of particles characterized by a preferential spacing. When dealing with non-Newtonian fluids in microfluidics, the adherence condition of the liquid at the channel wall may be violated and the liquid can slip over the surface, possibly affecting the ordering efficiency. In this work, we investigate the effect of wall slip on the ordering of particles suspended in a viscoelastic liquid by numerical simulations. The dynamics of a triplet of particles in an infinite cylindrical channel is first addressed by solving the fluid and particle governing equations. The relative velocities computed for the three-particle system are used to predict the dynamics of a train of particles flowing in a long microchannel. The distributions of the interparticle spacing evaluated at different slip coefficients, linear particle concentrations, and distances from the channel inlet show that wall slip slows down the self-assembly mechanism. For strong slipping surfaces, no significant change of the initial microstructure is observed at low particle concentrations, whereas strings of particles in contact form at higher concentrations. The detrimental effect of wall slip on viscoelastic ordering suggests care when designing microdevices, especially in case of hydrophobic surfaces that may enhance the slipping phenomenon.     

Wettability of tri-block copolymer coated hydrophobic surfaces Predictions and measurements

Hydrophobic surfaces with adsorbed tri-block copolymers are wetted by oil in spite of the hydrophilic buoy groups of the block copolymer that are present near the surface. The effect of the buoy group length of the adsorbed molecules on the wettability of hydrophobic surfaces is studied by contact angle measurements and by computer modelling.

The computer model predicts an increase in interfacial free energy with increasing buoy group length for equilibrium adsorption of block copolymer from water. Molecules with large buoy groups occupy more lateral space; therefore the “bare” surface gets more exposed and the anchor groups contribute less to the interfacial free energy which thus increases with the buoy group length.

The calculations showed that the variation of the interaction parameter between solvent and buoy group hardly influences the interfacial free energy. In contrast the interaction parameter between solvent and surface influences the interfacial free energy to a large extent because the oil/surface interactions have a lower energetic value as compared to water/surface interactions and therefore the interfacial free energy is lower than in water. The interfacial free energy varies slightly with increasing buoy group length, depending on the value chosen for the solvent/surface interaction parameter.

Advancing and receding contact angles of hexadecane, sunflower oil and hydrolysate (partly hydrolysed sunflower oil) were measured on hydrophobic surfaces. All oil/water contact angles were small, indicating a hydrophobic apolar surface character. It was found that, for oils with a “good” interaction with the surface (hexadecane and sunflower oil), the contact angle has a minimum value at a certain buoy group length. For hydrolysate (less-strong interaction with the surface) the contact angle decreases monotonically with increasing buoy group length. The results for hexadecane, sunflower oil and hydrolysate are in reasonable agreement with the model predictions. The effect of increasing buoy group length is weak; both decreasing and increasing angles are found, depending on the type of oil used.     

Hydrophobization of glass surface by adsorption of poly(dimethylsiloxane)

Silica or glass particles are introduced in a poly(dimethylsiloxane) (PDMS) matrix for various applications. A particular feature of these systems is that PDMS adsorbs on the surface of the dispersed particles, thus rendering them more hydrophobic with time. The mechanism of this process of in situ hydrophobization is still poorly understood. The major aims of the present study are (1) to quantify the rate of surface hydrophobization by PDMS and, on this basis, to discuss the mechanism of the process; (2) to compare the contact angles of surfaces that are hydrophobized by different procedures and are placed in contact with different fluid interfaces-PDMS-water, hexadecane-water, and air-water; and (3) to check how the type of surfactant affects the contact angles, viz., the effective hydrophobicity of the surface. We present experimental results for the kinetics of hydrophobization of glass surfaces, which are characterized by measuring the three-phase contact angle of glass-surfactant solution-PDMS. The data reveal two consecutive stages in the hydrophobization process: The first stage is relatively fast and the contact angle increases from 0 degrees to about 90 degrees within several minutes. This stage is explained with the physical adsorption of the PDMS chains, as a result of hydrogen-bond formation with the surface silanol groups. The second stage is much slower and hours or days are required at room temperature to reach the final contact angle (typically, 150-160 degrees). This stage is explained as grafting of the PDMS molecules on the surface by chemical reaction with the surface silanol groups. If the glass surface had been pretreated by hexamethyldisilazane (HMDS), so that CH(3) groups had blocked most of the surface silanol groups, the first stage in the hydrophobization process is almost missing-the contact angle slowly changes at room temperature from about 90 degrees up to 120 degrees. The experiments aimed to compare several hydrophobization procedures showed that PDMS ensures larger contact angle (more hydrophobic surface) than grafted alkyl chains. The contact angles at the PDMS-water and hexadecane-water interfaces were found to be very similar to each other, and much larger than that at the air-water interface. Interestingly, we found that the ionic surfactants practically do not affect the contact angle of PDMS-hydrophobized surface, whereas the nonionic surfactants reduce this angle. Similar trends are expected with silica surfaces, as well.     

Effect of surface polarity on water contact angle and interfacial hydration structure

We perform molecular dynamics simulations of water in the presence of hydrophobic/hydrophilic walls at T = 300 K and P = 0 GPa. For the hydrophilic walls, we use a hydroxylated silica model introduced in previous simulations [Lee, S. H.; Rossky, P. J. J. Chem. Phys. 1994, 100, 3334. Giovambattista, N.; Rossky, P. J.; Debenedetti, P. G.; Phys. Rev. E 2006, 73, 041604.]. By rescaling the physical partial atomic charges by a parameter 0 相似文献   

Influence of surface hierarchy of superhydrophobic surfaces on liquid slip

We investigated how the surface hierarchy of superhydrophobic (SHPo) surfaces influences liquid slip by testing well-defined microposts that have nanoposts only on their top. Contrary to the commonly held belief, our results show that such hierarchical surfaces do not always lead to an increase of slip length despite their reduced solid fraction and enhanced hydrophobicity compared to single-scale surfaces. Adding nanoposts on top of the microposts resulted in an increase of slip length only if the original microposts had a solid fraction above a threshold value. For solid fractions below this threshold, adding nanoposts decreased the slip length. We propose that there were not enough nanoposts on the top surface of very thin microposts to support the liquid pressure, allowing the liquid to intrude down to the top corners of the microposts.     

Preparation and Characterisation of Super‐Hydrophobic Surfaces

The interest in highly water‐repellent surfaces has grown in recent years due to the desire for self‐cleaning surfaces. A super‐hydrophobic surface is one that achieves a water contact angle of 150° or greater. This article explores the different approaches used to construct super‐hydrophobic surfaces and identifies the key properties of each surface that contribute to its hydrophobicity. The models used to describe surface interaction with water are considered, with attention directed to the methods of contact angle analysis. A summary describing the different routes to hydrophobicity is also given.     

Surface hydrophobicity modulates the operation of actomyosin-based dynamic nanodevices

We studied the impact of surface hydrophobicity on the motility of actin filaments moving on heavy-meromyosin (HMM)-coated surfaces. Apart from nitrocellulose (NC), which is the current standard for motility assays, all materials tested are good candidates for microfabrication: hydrophilic and hydrophobic glass, poly(methyl methacrylate) (PMMA), poly(tert-butyl methacrylate) (PtBuMA), and a copolymer of O-acryloyl acetophenone oxime with a 4-acryloyloxybenzophenone (AAPO). The most hydrophilic (hydrophilic glass, contact angle 35 degrees) and the most hydrophobic (PtBuMA, contact angle 78 degrees) surfaces do not maintain the motility of actin filaments, presumably because of the low density of adsorbed HMM protein or its high levels of denaturation, respectively. The velocity of actin filaments presents higher values in the middle of this "surface hydrophobicity motility window" (NC, PMMA), and a bimodal distribution, which is more apparent at the edges of this motility window (hydrophobic glass and AAPO). A molecular surface analysis of HMM and its S1 units suggests that the two very different, temporally separated conformations of the HMM heads could exacerbate the surface-modulated protein behavior, which is common to all microdevices using surface-immobilized proteins. An explanation for the above behavior proposes that the motility of actin filaments on HMM-functionalized surfaces is the result of the action of three populations of motors, each in a different surface-protein conformation, that is, HMM with both heads working (high velocities), working with one head (low velocities), and fully denatured HMM (no motility). It is also proposed that the molecularly dynamic nature of polymer surfaces amplifies the impact of surface hydrophobicity on protein behavior. The study demonstrates that PMMA is a good candidate for the fabrication of future actomyosin-driven dynamic nanodevices because it induces the smoothest motility of individual nano-objects with velocities comparable with those obtained on NC.     

To adjust wetting properties of organic surface by in situ photoreaction of aromatic azide

This article describes development of a simple and convenient method to provide stable low-surface-energy coatings on organic surfaces, by designing and synthesizing a surface-reactive molecule 4-azido-N-dodecylbenzamide, which bears an azide group as the reactive surface anchor and an alkyl chain as the hydrophobic tail. After the hydrophobic modification, rough organic surfaces with contact angle of about 0 degrees can change their surface wetting properties from superhydrophilicity to superhydrophobicity, whose contact angles are above 152 degrees and tilt angles lower than 5 degrees. Moreover, by changing the alkyl chain to a PEO segment, a similar concept can be used to adjust the surface wetting properties from hydrophobic (contact angle approximately 130 degrees) to superhydrophilic (contact angle approximately 0 degrees).     

Friction and adsorption of aqueous polyoxyethylene (Tween) surfactants at hydrophobic surfaces

The nanotribological responses of a series of nonionic polyoxyethylene surfactants (Tween 20, Tween 40, Tween 60, and Tween 80) were investigated after they were adsorbed from aqueous solution onto atomically smooth hydrophobic substrates. The hydrophobic surfaces were composed of a condensed monolayer of octadecyltriethoxysilane (OTE; contact angle theta>110 degrees ). The nanorheological measurements were performed using a modified surface forces apparatus after coating atomically smooth mica with these OTE monolayers, while adsorption measurements were performed using phase-modulated ellipsometry on silicon wafers coated with these same monolayers. The minimum surface-surface separation observed under high load in friction studies agreed quantitatively with the thickness obtained from ellipsometry. For Tweens 20, 40, and 60, the thickness of the adsorbed film increases with increasing alkyl chain length. Systematic investigations of the nanorheological response showed that there is a "solid-like" elastic response from confined surfactant layers, which is the case for the smallest separations to separations up to slightly larger than twice the adsorbed film thickness. In kinetic friction, these confined layers are characterized by a shear stress of approximately 3 MPa with minimal dependence on shear rate. The magnitude of the sliding shear stress is the same as the apparent yield stress at approximately 3 MPa; it is independent of alkyl chain length within the Tween family of surfactants and corresponds to a nominal friction coefficient of mu approximately 1. A similar friction coefficient is observed for boundary lubrication on the macroscopic scale in a tribometer utilizing hydrophobic surfaces and mu approximately 1.1 for Tweens 20, 40, and 60. These results suggest that while Tween molecules adsorb onto hydrophobic surfaces to form a robust separating layer, the lubricating properties of these layers are dominated by a highly dissipative slip plane, the same for all alkyl chain lengths.     

UV and thermally stable superhydrophobic coatings from sol-gel processing

A method for the preparation of inorganic superhydrophobic silica coatings using sol-gel processing with tetramethoxysilane and isobutyltrimethoxysilane as precursors is described. Incorporation of isobutyltrimethoxysilane into silica layers resulted in the existence of hydrophobic isobutyl surface groups, thereby generating surface hydrophobicity. When combined with the surface roughness that resulted from sol-gel processing, a superhydrophobic surface was achieved. This surface showed improved UV and thermal stability compared to superhydrophobic surfaces generated from polybutadiene by plasma etching. Under prolonged UV tests (ASTM D 4329), these surfaces gradually lost superhydrophobic character. However, when the as-prepared superhydrophobic surface was treated at 500 degrees C to remove the organic moieties and covered with a fluoroalkyl layer by a perfluorooctylsilane treatment, the surface regained superhydrophobicity. The UV and thermal stability of these surfaces was maintained upon exposure to temperatures up to 400 degrees C and UV testing times of 5500 h. Contact angles remained >160 degrees with contact angle hysteresis approximately 2 degrees.     

Nanohydrodynamics: the intrinsic flow boundary condition on smooth surfaces

A dynamic surface force apparatus is used to determine the intrinsic flow boundary condition of two simple liquids, water and dodecane, on various smooth surfaces. We demonstrate the impact of experimental errors and data analysis on the accuracy of slip length determination. In all systems investigated, the dissipation is described by a well-defined boundary condition accounting for a whole range of separation, film thickness, and shear rate. A no-slip boundary condition is found in all wetting situations. On strongly hydrophobic surfaces, water undergoes finite slippage that increases with hydrophobicity. We also compare the relative influence of hydrophobicity and liquid viscosity on boundary flow by using water-glycerol mixtures with similar wetting properties.     

Nanoparticle flotation collectors II: the role of nanoparticle hydrophobicity

The ability of polystyrene nanoparticles to facilitate the froth flotation of glass beads was correlated to the hydrophobicity of the nanoparticles. Contact angle measurements were used to probe the hydrophobicity of hydrophilic glass surfaces decorated with hydrophobic nanoparticles. Both sessile water drop advancing angles, θ(a), and attached air bubble receding angle measurements, θ(r), were performed. For glass surfaces saturated with adsorbed nanoparticles, flotation recovery, a measure of flotation efficiency, increased with increasing values of each type of contact angle. As expected, the advancing water contact angle on nanoparticle-decorated, dry glass surfaces increased with surface coverage, the area fraction of glass covered with nanoparticles. However, the nanoparticles were far more effective at raising the contact angle than the Cassie-Baxter prediction, suggesting that with higher nanoparticle coverages the water did not completely wet the glass surfaces between the nanoparticles. A series of polystyrene nanoparticles was prepared to cover a range of surface energies. Water contact angle measurements, θ(np), on smooth polymer films formed from organic solutions of dissolved nanoparticles were used to rank the nanoparticles in terms of hydrophobicity. Glass spheres were saturated with adsorbed nanoparticles and were isolated by flotation. The minimum nanoparticle water contact angle to give high flotation recovery was in the range of 51° < θ(np(min)) ≤ 85°.     

Development and characterization of poly ethyl metha acrylate–iron oxide(III) based hydrophobic liquid nanocomposite films

Select applications of hydrophobic nanocomposites include preparation of robust self-cleaning surfaces, water-repellent glass surfaces, and waterproofing textiles. Various nanocomposites have been reported in the literature; however, the relationship between the nanocomposite surface morphology and its hydrophobicity needs to be understood better. In the present work Fe₂O₃ nanoparticles and poly ethyl metha acrylate (PEMA) were used in varying proportions to obtain a series of model hydrophobic surfaces (spin-coated on glass substrate). The hydrophobicity of these surfaces was measured by static contact angle; a maximum of 103° was obtained at highest loading of iron oxide nanoparticles. These surfaces were also characterized using AFM. The contact angle and characterization data were used to test some of the models which have been proposed in the recent literature on prediction of contact angle for composite surfaces. It is proposed that the hydrophobicity of the iron oxide–PEMA surface is due to the physical roughness causing air entrapment as well as the chemical heterogeneity. Based on the experimental studies and the simulations using the recent models on contact angle, some general features of relationship between a composite surface morphology and its hydrophobicity is proposed.     

Molecular dynamics study of the effect of atomic roughness on the slip length at the fluid-solid boundary during shear flow

A systematic study into the effect of solid roughness on the slip boundary condition during shear flow is presented. Atomic roughness is modeled by varying the size and spacing between solid atoms at constant packing fraction while the interaction parameters and the thermodynamic state of the fluid are kept constant. It is shown that the fluid structure as manifest in the amplitude of the density oscillations increases with increasing smoothness of the surfaces. The fluid-solid slip length is shown to exhibit nonmonotonic behavior as the solid structure is varied from smooth to rough. Slip occurs for both smooth and rough surfaces, and stick occurs only for surfaces commensurate with the fluid.