Rough Solid Surface Research Articles

Using hierarchical memory to calculate friction force between fractal rough solid surface and elastomer with arbitrary linear rheological properties

An algorithm for calculating the force of friction between a fractal rough solid surface and a model elastomer with arbitrary linear rheological properties has been developed based on the method of dimensionality reduction with application of a hierarchically organized memory. Using this method, it is possible to calculate the friction force between an elastomer and a solid surface with experimentally determined topography, taking into account both the broad spectrum of wave vectors (ranging from nanometers to centimeters) for real surfaces and the wide range of relaxation times (from nanoseconds to seconds).

The rose petal effect and the modes of superhydrophobicity

The wetting of rough surfaces remains a subject of active investigation by scientists. The contact angle (CA) is a traditional parameter used to characterize the hydrophobicity/philicity of a solid surface. However, it was found recently that high CAs can coexist with strong adhesion between water and a solid surface in the case of the so-called 'rose petal effect'. Several additional parameters have been proposed to characterize the interaction of water with a rough solid surface, including the CA hysteresis, the ability of water droplets to bounce off a solid surface, the tilt angle needed to initiate the flow of a droplet, and the normal and shear adhesion. It is clear now that wetting is not characterized by a single parameter, since several modes or regimes of wetting of a rough surface can exist, including the Wenzel, Cassie, lotus and petal. Understanding the wetting of rough surfaces is important in order to design non-adhesive surfaces for various applications.

Erratum: “Theoretical model for adhesive friction between elastomers and rough solid surfaces” [J. Chem. Phys. 132, 114105 (2010)

Erratum: “Theoretical model for adhesive friction between elastomers and rough solid surfaces” [J. Chem. Phys. 132, 114105 (2010)

Implications of wettability in biological materials science

Understanding liquid-solid interactions through the behavior of the liquid-solid interface is of paramount interest in many applications ranging from plant surfaces to fabrics, metal casting and biomedical implants. Liquid-solid interactions may or may not include chemical reactions, and the degree of liquid spreading over the solid surface (wetting) may vary based on the chemical properties of the materials involved (surface free energy) and the topography of the solid surface (roughness). The wetting of solid surfaces by biological fluids is often necessary for a chain of biological events to unfurl so that a foreign material may be accepted in vivo and thus become bioactive. We discuss the fundamentals of wettability, and how it pertains to the biological environment. The widespread use of contact angle measurements for the determination of surface free energy is also discussed. The use of contact angle as a general test for biocompatibility has inherent pitfalls as the effects of roughness on contact angle may be significant and misleading about the true chemical nature of the surface. Techniques to characterize the surface free energy are much more reliable, but are not as easily implemented as contact angles.

Theoretical model for adhesive friction between elastomers and rough solid surfaces

A theoretical model for the adhesive friction between elastomers and rough solid surfaces is proposed on the basis of opening crack propagation processes at the boundary of the contact interfaces and the rate processes of formation of molecular bonds on the solid surface. This model, which is expressed as a product of the terms related to the two abovementioned processes, requires some measurable and fitted parameters such as the frictional shear strength expressed as a function of viscoelastic dissipation, rate-dependent elasticity, density of bonded molecular chains at a contact junction, critical velocity related to viscoelastic relaxation, and critical velocity related to the rate process of formation of molecular bonds on the solid surface. The friction-velocity relationship exhibits a remarkable fit to previously obtained experimental results for polymers such as engineering rubber, gels, and plastics (glassy polymers), and all fitting parameters are physically reasonable. The viscoelastic index "n" is also related to the "glass-to-rubber transition" of a nanometer-thick polymer layer for frictional behavior. Thus, from a practical viewpoint, this model can be used effectively for fitting the adhesive friction behavior of polymers.

Electron Spectroscopy of Corrugated Solid Surfaces

This paper reviews work done of the influence of non-ideal surface topography on electron spectral intensities of surface-sensitive electron spectroscopic methods: primarily X-ray induced photoelectron spectroscopy (XPS) and concise Auger electron spectroscopy (AES), electron energy loss spectroscopy in the reflection mode (REELS), and elastic peak electron spectroscopy (EPES). Several attempts to solve the problem are mentioned, where (i) the effect of surface roughness is corrected using a single parameter, (ii) computer simulations based on a model of surface roughness composed of regular geometrical units are used for electron spectral intensity calculations, and finally, (iii) a semi-empirical method based on careful surface mapping of analyzed sample by atomic force microscopy (AFM) is discussed in greater detail. The first approach was found to be rather simple to properly include any complex surface topography. The second technique can help us to understand surface topography related phenomena. The latter method, suitable for arbitrary rough solid surfaces covered by conformal overlayer(s), can be incorporated in current quantitative procedures valid for ideally flat surfaces.

Water Structure at Superhydrophobic Quartz/Water Interfaces: A Vibrational Sum Frequency Generation Spectroscopy Study

The water structure at superhydrophobic solid/bulk water interfaces has been investigated by vibrational sum frequency generation (SFG) spectroscopy. We have adapted a simple and facile chemical modification procedure (methyltrichlorosilane/toluene treatment followed by extraction with ethanol) to prepare transparent quartz crystal surfaces with varied wetting properties, from hydrophilic to hydrophobic and superhydrophobic. In comparison with those obtained on bare quartz, the SFG spectra on polymethylsiloxane-modified surfaces showed significant changes of the relative intensities of the two broad OH stretching modes of hydrogen-bonded water at ∼3200 and ∼3450 cm−1, which can be correlated with surface morphology and molecular variations. Intriguingly, on superhydrophobic quartz these bands are very weak and replaced by a characteristic “free OH” (not hydrogen-bonded) stretching (>3600 cm−1) band that is typically observed at water/air interfaces. These results suggest that hydrogen bonding between water molecules weakens as hydrophobicity increases on rough heterogeneous solid surfaces. More importantly, this study provides direct evidence for the existence of stable solid/air/water three-phase interfaces when a superhydrophobic solid is in contact with bulk water.

Justification of biexponential rate law of spreading over heterogeneous and rough surfaces

Both surface roughness and chemical heterogeneity can be viewed as surface energy fluctuations acting as energy barriers for the three phase contact (TPC) line motion. This viewpoint provides a way to obtain the TPC line propagation rate as a function of the TPC contact angle in the frame of either nucleation rate theory or Eyring's absolute rate theory. In this study, a general biexponential rate law is deduced, and the wetting kinetic characteristics are related to statistical characteristics of the surface chemical heterogeneity and roughness, experimentally available from surface image analysis. A step mechanism for TPC line propagation is proposed; the barrier for a step is due to solid surface roughness and liquid surface corrugation, and the absolute frequency of steps is estimated through the dispersion relation of the capillary waves at the liquid surface. The free energy vs. TPC contact angle diagram of a randomly rough surface is deduced and found to be very similar to Dettre–Johnson's diagrams of periodically rough surfaces. The TPC propagation rate law is generally asymmetric, in accordance with the available experimental data. A comparison with experimental dynamic contact angle hysteretic curves gives reasonable values of the model's parameters.

Molecular momentum transport at fluid-solid interfaces in MEMS/NEMS: a review.

This review is focused on molecular momentum transport at fluid-solid interfaces mainly related to microfluidics and nanofluidics in micro-/nano-electro-mechanical systems (MEMS/NEMS). This broad subject covers molecular dynamics behaviors, boundary conditions, molecular momentum accommodations, theoretical and phenomenological models in terms of gas-solid and liquid-solid interfaces affected by various physical factors, such as fluid and solid species, surface roughness, surface patterns, wettability, temperature, pressure, fluid viscosity and polarity. This review offers an overview of the major achievements, including experiments, theories and molecular dynamics simulations, in the field with particular emphasis on the effects on microfluidics and nanofluidics in nanoscience and nanotechnology. In Section 1 we present a brief introduction on the backgrounds, history and concepts. Sections 2 and 3 are focused on molecular momentum transport at gas-solid and liquid-solid interfaces, respectively. Summary and conclusions are finally presented in Section 4.

A predictive model for the evolution of the thermal conductance at the casting–die interfaces in high pressure die casting

An analytical model is proposed to predict the time varying thermal conductance at the casting–die interface during solidification of light alloys during High pressure Die Casting. Details of the topography of the interface between the casting and the die are included in the model through the inclusion of solid surface roughness parameters and the mean trapped air layer at the interface. The transitory phase of the interfacial thermal conductance has been related to the degradation of contact as solidification progresses through the casting thickness. The modelled time varying thermal conductance showed very good agreement with experimentally determined values for different alloy compositions and casting geometries. The analysis shows that the parameters that govern the thermal conductance are different for the first stage of contact compared to the second stage of contact when the alloy begins to solidify.

Crystal nucleation and detachment from a chilling metal surface with vibration

A novel method of crystal nucleation and detachment from a vibrating chilling solid surface has been proposed to further increase the proportion of equiaxed grains in the solidification microstructure. Using a transparent NH 4Cl–H 2O alloy, the surface nucleation and evolution behaviors of dendrites were in situ observed. The effects of vibration frequency as well as amplitude on the equiaxed crystallographic morphology were experimentally studied, and the effects of the roughness of solid surface were also considered. The results show that exerting vibration to a chilling metal surface is an effective way to produce a lot of nuclei for forming equiaxed grains microstructure, and with higher vibration frequency and amplitude, much finer equiaxed grains are obtained. Moreover, nucleation on the chilling surface is a much localized behavior, some fixed positions on the chilling surface act as active nucleation sites, of which the number increases with a lowering chilling temperature.

Modeling and simulation of the fluid–solid interaction in wetting

In this work, a lattice-Boltzmann method based on field mediators is proposed for thesimulation of wetting in a liquid–vapor system. The method includes the effect oflong-range interactions between the particles and solid walls, and is implementedtogether with a liquid–vapor model which is known from the literature. The resultsthat were obtained in the simulations show that the contact angle is stronglydependent on the long-range interactions. The initial formation of a film ahead of themacroscopic bulk liquid is observed below a specific contact angle. When roughsolid surfaces were used, the simulations exhibit the contact angle hysteresis.The spreading dynamics on solid surfaces was studied and showed a considerablediscrepancy from the expected value, which was attributed to the high resolution of thenumerical simulations. It is shown that the initial conditions have a strong influenceon the first steps of the droplet spreading, but they become negligible at longtimes.

Contact Angles of Nanodrops on Chemically Rough Surfaces

The experimental observations of Gao and McCarthy [Gao, L.; McCarthy, T. Langmuir, 2007, 23, 3762] that only the interfacial area near the leading edges of the drop on physically smooth but chemically rough solid surfaces affects the contact angle and that most of the contact area has no effect is checked for nanodrops on the basis of a density functional theory. The contact angle was calculated for three cases: (i) the leading edges of the drops are located on much higher or (ii) much lower hydrophobic surfaces than the remaining surface beneath the drop; (iii) the surface is composed of a periodic array of two kinds of stripelike solid plates. In the first two cases, if the distance between the leading edges and the region which has higher or lower hydrophobicity is sufficiently large, there is agreement with the experiments mentioned. However, when those distances are sufficiently small, the internal part affects the value of the angle. In the third case, we found that the internal part always affects the wetting angle. All these peculiarities, as well as the contact angle hysteresis, can be explained by accounting for the local conditions in the vicinity of the leading edges of the drop.

Drainage of a Wetting Liquid: Effective Slippage or Polymer Depletion?

A surface force apparatus has been used to investigate the drainage of a viscosity improver lubricant. The characterization of the VI confined interface has been performed. Surprisingly, dynamical measurements enlighten a significant negative value of the immobile layer thickness. This result is discussed in terms of occurrence of slip at the wall, liquid–solid interface wettability, surface roughness and cleanliness, and friction experiments. Consequently, an interface molecular organization modelling is proposed based on polymer depletion near the solid–liquid interface.

Microscopic calculation of the sticking force for nanodrops on an inclined surface

A two-dimensional nanodrop on a vertical rough solid surface is examined using a nonlocal density functional theory in the presence of gravity. The roughness is modeled either as a chemical inhomogeneity of the solid or as a result of the decoration with pillars of a smooth homogeneous surface. From the obtained fluid density distribution, the sticking force, which opposes the drop motion along an inclined surface, and the contact angles on the lower and upper leading edges of the drop are calculated. On the basis of these results, it is shown that the macroscopically derived equation for a drop in equilibrium on an inclined surface is also applicable to nanodrops. The liquid-vapor surface tension involved in this equation was calculated for various specific cases, and the values obtained are of the same order of magnitude as those obtained in macroscopic experiments.

Wettability control of photocatalytic crystal layers by hydrophobic coating and subsequent UV light irradiation

Wettability of solid surfaces with water is governed by surface morphology and composition. In this study, a surface finishing technique consisting of three processes, that is, nanotexturing by nanosized photocatalytic crystal coating, hydrophobic coating by chemical vapor deposition and subsequent ultraviolet light photolithography, was employed to form ultrahydrophobic/ultrahydrophilic solid surfaces. Flux grown nanosized K 4Nb 6O 17 crystals were selected as a photocatalyst. A proper nanotexure of crystal layer surfaces enhanced their hydrophobicity. The rough solid surfaces modified by alkyl- and fluoro-alkyl silane were converted from ultrahydrophobic (> 150°) to ultrahydrophilic (< 10°) by photocatalytic oxidation using the K 4Nb 6O 17 crystal layer. Fine pitch ultrahydrophobic/ultrahydrophilic patterns were readily obtained by photocatalytic lithography by use of area-selective 254 nm-ultraviolet light irradiation.

Capillary effects and instabilities in nanocontacts

Several examples of instabilities related to the capillary effects during nanocontacts are studied. For the single-asperity contact, there is a significant negative Laplace pressure inside water capillary bridges and the bridge may become unstable with respect to the phase transition, leading to an unstable capillary force. For the contact of two rough solid surfaces, the capillary force is also unstable with respect to small variations of roughness. For liquid in contact with a rough solid surface, the composite interface, which is required for the superhydrophobicity, can be destabilized and transformed into the homogeneous interface. Numerous examples show that the instabilities play an important role in nanocontact problems that involve capillarity.

Colloid Transport and Retention in Unsaturated Porous Media: A Review of Interface‐, Collector‐, and Pore‐Scale Processes and Models

Our ability to accurately simulate the transport and retention of colloids in the vadose zone is currently limited by our lack of basic understanding of colloid retention processes that occur at the pore scale. This review discusses our current knowledge of physical and chemical mechanisms, factors, and models of colloid transport and retention at the interface, collector, and pore scales. The interface scale is well suited for studying the interaction energy and hydrodynamic forces and torques that act on colloids near interfaces. Solid surface roughness is reported to have a significant influence on both adhesive and applied hydrodynamic forces and torques, whereas non‐Derjaguin–Landau–Verwey–Overbeek (DLVO) forces such as hydrophobic and capillary forces are likely to play a significant role in colloid interactions with the air–water interface. The flow field can be solved and mass transfer processes can be quantified at the collector scale. Here the potential for colloid attachment in the presence of hydrodynamic forces is determined from a balance of applied and adhesive torques. The fraction of the collector surface that contributes to attachment has been demonstrated to depend on both physical and chemical conditions. Processes of colloid mass transfer and retention can also be calculated at the pore scale. Differences in collector‐ and pore‐scale studies occur as a result of the presence of small pore spaces that are associated with multiple interfaces and zones of relative flow stagnation. Here a variety of straining processes may occur in saturated and unsaturated systems, as well as colloid size exclusion. Our current knowledge of straining processes is still incomplete, but recent research indicates a strong coupling of hydrodynamics, solution chemistry, and colloid concentration on these processes, as well as a dependency on the size of the colloid, the solid grain, and the water content.

Nanodroplets on rough hydrophilic and hydrophobic surfaces

We present results of Molecular Dynamics (MD) calculations on the behavior of liquid nanodroplets on rough hydrophobic and hydrophilic solid surfaces. On hydrophobic surfaces, the contact angle for nanodroplets depends strongly on the root-mean-square roughness amplitude, but it is nearly independent of the fractal dimension of the surface. Since increasing the fractal dimension increases the short-wavelength roughness, while the long-wavelength roughness is almost unchanged, we conclude that for hydrophobic interactions the short-wavelength (atomistic) roughness is not very important. We show that the nanodroplet is in a Cassie-like state. For rough hydrophobic surfaces, there is no contact angle hysteresis due to strong thermal fluctuations, which occur at the liquid-solid interface on the nanoscale. On hydrophilic surfaces, however, there is strong contact angle hysteresis due to higher energy barrier. These findings may be very important for the development of artificially biomimetic superhydrophobic surfaces.

Dry and Wet Contact Modeling of Multilayered Rough Solid Surfaces

Adhesion, friction/stiction, and wear are among the main issues in various commercial applications, including magnetic storage devices, microelectromechanical systems, and nanoelectromechanical systems, having contact interfaces with normal or tangential motion. The contact analysis of multilayered structures under both dry and wet conditions with and without relative motion, which simulates the actual contact situations of the devices, is needed to obtain optimum design parameters, including materials with desired mechanical properties and layer thicknesses. The contact analyses have been used to predict the contact pressure profile on the interface and contact statistics, namely, fractional contact area, the [value of contact pressure, von Mises, principal tensile and shear stresses, and relative meniscus force. The early work does not consider surface roughness and this has been carried out in the later studies for single and multilayered solid surfaces. A numerical three-dimensional multilayered rough contact model is presented in detail to investigate the effects of roughness, stiffness, hardness, layer thicknesses, load, coefficient of friction, and meniscus contribution. Applications of the model to magnetic storage devices are presented.