To wet or not to wet? Dispersion forces tip the balance for water ice on metals

Despite widespread discussion, the role of van der Waals dispersion forces in wetting remains unclear. Here we show that nonlocal correlations contribute substantially to the water-metal bond and that this is an important factor in governing the relative stabilities of wetting layers and 3D bulk ice. Because of the greater polarizability of the substrate metal atoms, nonlocal correlations between water and the metal exceed those between water molecules within ice. This sheds light on a long-standing problem, wherein common density functional theory exchange-correlation functionals incorrectly predict that none of the low temperature experimentally characterized icelike wetting layers are thermodynamically stable.     

Modelling of spreading process: effect from hydrogen bonds

Lubricant spreading on solid substrates has drawn considerable attention not only for the microscopic wetting theory but also for the dramatic application in head-disk interface of magnetic storage drive systems. Molecular dynamic simulation based on a coarse-grained bead-spring model has been used to study such a spreading process. The spreading profiles indicate that the hydrogen bonds among lubricant molecules and the hydrogen bonds between lubricant molecules and polar atoms of solid substrates will complicate the spreading process in a tremendous degree. The hydrogen bonds among lubricant molecules will strengthen the lubricant combination intensity, which may hinder most molecules from flowing down to the substrates and diffusing along the substrates. And the hydrogen bonds between lubricant molecules and polar atoms of solid substrates will confine the lubricant molecules around polar atoms, which may hinder the molecules from diffusing along the substrates and cause precursor film to vanish.     

正癸烷纳米液滴润湿特性的分子动力学研究

流体液滴在固体表面的浸润性对其润滑性能至关重要.本文利用分子动力学方法研究了正癸烷纳米液滴在铜表面上的润湿特性.结果表明：在平坦光滑表面上,壁面的厚度和分子数目对润湿效果影响不大.随着壁面能量势阱参数εs 增大,接触角线性减小.随着温度升高,液滴的接触角减小.在沟槽粗糙表面上,随着粗糙度因子增大,对于疏液表面,接触角增大到一定值后基本保持不变,符合Cassie理论;中性和亲液表面接触角则会减小,为Wenzel润湿模式.当表面分数增大时,疏液与亲液表面接触角整体呈减小的趋势,对中性表面影响不大.当温度升高时,粗糙疏液表面接触角会增大,润湿效果更差,而粗糙中性和亲液表面液滴润湿性会更好.     

Relationship between Spreading Rate and Wetting Behaviour of Oil on Surface of Surfactant Solution

We report a systematic investigation of the spreading of a polydimethylsiloxane oil layer on fiat surfaces of solution containing anionic surfactant of sodium dodecylsulfate. The experiment reveals that different wetting behaviours of the oil follow different spreading rates. In the case of complete wetting, it obeys a 0.75 power law, while in the pseudopartial wetting it follows a non-power law. The results can well be explained by a new simple theory of spreading. The theory further predicts that for a complete wetting state there exists another spreading rate.     

微液滴在不同能量表面上润湿状态的分子动力学模拟

微小液滴在不同能量表面上的润湿状态对于准确预测非均相核化速率和揭示界面效应影响液滴增长微观机理具有重要意义. 通过分子动力学模拟, 研究了纳米级液滴在不同能量表面上的铺展过程和润湿形态. 结果表明, 固液界面自由能随固液作用强度增加而增加, 并呈现不同液滴铺展速率和润湿特性. 固液作用强度小于1.6的低能表面呈现疏水特征, 继续增强固液作用强度时表面变为亲水, 而固液作用强度大于3.5的高能表面上液体呈完全润湿特征. 受微尺度条件下非连续、非对称作用力影响, 微液滴气液界面存在明显波动, 呈现与宏观液滴不同的界面特征. 统计意义下, 微小液滴在不同能量表面上铺展后仍可以形成特定接触角, 该接触角随固液作用强度增加而线性减小, 模拟结果与经典润湿理论计算获得的结果呈现相似变化趋势. 模拟结果从分子尺度为核化理论中的毛细假设提供了理论支持, 揭示了液滴气液界面和接触角的波动现象, 为核化速率理论预测结果和实验测定结果之间的差异提供了定性解释.     

Influence of surface temperature and surface modifications on the initial layer growth of para-hexaphenyl on mica (0 0 1)

Ultra-thin films of para-hexaphenyl (6P) were prepared on muscovite mica (0 0 1) utilizing organic molecular beam deposition (OMBD) under well defined ultra high vacuum (UHV) conditions. The 6P growth characteristics were studied as a function of substrate temperature and substrate surface conditions. For the initial state of layer growth, thermal desorption spectroscopy (TDS) was used to verify the existence of a wetting layer. In this monomolecular continuous wetting layer, the molecules lie flat on the surface and are rather strongly bonded. For thicker films, in-situ X-ray photoelectron spectroscopy (XPS) in combination with (TDS) was applied to reveal the kinetics of the layer growth. Ex-situ atomic-force microscopy (AFM) was used to determine the film morphology. In particular, the influence of surface modifications (carbon contamination, sputtering) on 6P layer growth was investigated. XPS and low energy electron diffraction (LEED) were used to characterize the mica surface before the film deposition. TDS and AFM revealed a considerable change in film growth, from a needle-like island growth of flat laying molecules on top of the wetting layer (for the air cleaved mica) to terrace-like film growth of standing molecules, without a wetting layer (after surface modifications).     

The influence of substrate temperature on growth of para-sexiphenyl thin films on Ir{111} supported graphene studied by LEEM

The growth of para-sexiphenyl (6P) thin films as a function of substrate temperature on Ir{111} supported graphene flakes has been studied in real-time with Low Energy Electron Microscopy (LEEM). Micro Low Energy Electron Diffraction (μLEED) has been used to determine the structure of the different 6P features formed on the surface. We observe the nucleation and growth of a wetting layer consisting of lying molecules in the initial stages of growth. Graphene defects – wrinkles – are found to be preferential sites for the nucleation of the wetting layer and of the 6P needles that grow on top of the wetting layer in the later stages of deposition. The molecular structure of the wetting layer and needles is found to be similar. As a result, only a limited number of growth directions are observed for the needles. In contrast, on the bare Ir{111} surface 6P molecules assume an upright orientation. The formation of ramified islands is observed on the bare Ir{111} surface at 320 K and 352 K, whereas at 405 K the formation of a continuous layer of upright standing molecules growing in a step flow like manner is observed.     

Wetting properties of rare gases on weak substrates

Since the original prediction that liquid He does not wet Cs at low temperatures and the soon after experimental observation of a wetting transition on this system, noble gases on alkalis have become model systems for the study of wetting transitions and of their accompanying line of prewetting transitions off coexistence. Here we review very briefly the theory of wetting and prewetting and discuss some results on the properties of rare gases adsorbed on alkali surfaces obtained with the use of the density functional theory and of accurate adsorbate-substrate potentials.     

The nucleation of pentacene thin films

Photoemission Electron Microscopy was used to determine basic factors for nucleation and growth of thin pentacene films. Dependence of both substrate chemistry and deposition rate on the nucleation density was observed. On SiO₂ pentacene shows a high nucleation density and forms small islands consisting of almost vertically oriented molecules. On Si(001) the nucleation density of this thin-film phase is much smaller, but the pentacene film first forms a flat-lying wetting layer. The thin-film phase only forms on top of this wetting layer. Adsorption of a cyclohexene self-assembled monolayer on Si(001) prior to the pentacene growth suppresses the initial pentacene wetting layer but maintains diffusion parameters similar to pentacene on Si(001). The nucleation of pentacene layers on cyclohexene/Si(001) can be described by classical nucleation theory with a critical nucleus size i6. Simple surface modification techniques such as e-beam irradiation of the substrates prior to pentacene adsorption can also have a significant effect on the pentacene nucleation density. PACS 68.37.Nq; 68.43.Fg; 68.47.Fg; 68.55.Ac     

Cross-Over Between First-Order and Critical Wetting at the Liquid-Vapour Interface of n-Alkane/Methanol Mixtures: Tricritical Wetting and Critical Prewetting

A simple mean-field theory is presented which describes the basic observations of recent experiments revealing rich wetting behaviour of n-alkane/methanol mixtures at the liquid-vapour interface. The theory, qualitative and in part heuristic, is based on a microscopic lattice-gas model from which a Cahn–Landau approach is distilled. Besides the physics associated with the short-range components of the intermolecular interactions, effects of the long-range tails of the net van der Waals forces between interfaces are also taken into account. Further, gravitational thinning of the wetting phase is incorporated. The calculation of the spreading coefficient S is extended to the experimentally relevant situation in which the bulk adsorbate is slightly away from two-phase coexistence due to gravity. Analysis of this novel approximation to S for systems with short-range forces leads to the conclusion that the surface specific heat exponents _s =1,1/2, and 0, for first-order wetting, tricritical wetting and critical wetting, respectively, are robust with respect to (weak) gravitational thinning, consistently with experiment. For three different systems the adsorption is calculated as a function of temperature and compared with the experimentally measured ellipticity. Including weak long-range forces which favour wetting in the theory does not visibly alter the critical wetting transition for the nonane/methanol mixture, in contrast with the generic expectation of first-order wetting for such systems, but in good agreement with experiment. For decane/methanol weak long-range forces bring the transition very close to the prewetting critical point, leading to an adsorption behaviour closely reminiscent of short-range tricritical wetting, observed experimentally for alkane chain length between 9.6 and 10. Finally, for undecane/methanol the transition is clearly of first order. First-order wetting is also seen in the experiment.     

Solute transport and interface evolution in dissolutive wetting

Dissolutive wetting, i.e., droplet wetting on dissolvable surfaces, is essential for various natural phenomena and industrial applications such as the formation of sinkholes, enhancing shale gas recovery, drug design, MEMS, and so on. It is difficult to predict the evolution of concentration field and solid-liquid interface owing to the coupled effects of wetting, diffusion, and convection. This study makes substantial progress by proposing a new theory based on Onsager's variational principle and finding two modes of solute transport, i.e., shifting and lifting modes. Furthermore, we investigate the influence of wetting and dissolution coupling on the interface shape using a phase diagram. Using our theory, we can predict and inversely predict the interface evolution.     

Quartic coupling and its effect on wetting behaviors in nematic liquid crystals

Based on the fact that rubbed groove patterns also affect the anchoring of liquid crystals at substrates,a quartic coupling is included in constructing the surface energy for a liquid crystal cell.The phase diagram and the wetting behaviors of the liquid crystal cell,bounded by surfactant-laden interfaces in a magnetic field perpendicular to the substrate are discussed by taking the quartic coupling into account.The nematic order increases at the surface while it decreases in the bulk as a result of the introduction of quartic substrate-liquid crystal coupling,indicating that the groove anchoring makes the liquid crystal molecules align more orderly near the substrate than away from it.This causes a different wetting behavior:complete wetting.     

Tailoring the structure of water at a metal surface: a structural analysis of the water bilayer formed on an alloy template

Recent studies show that structures based on the traditional "icelike" water bilayer are not stable on flat transition metal surfaces and, instead, more complex wetting layers are formed. Here we show that an ordered bilayer can be formed on a SnPt(111) alloy template and determine the structure of the water layer by low energy electron diffraction. Close agreement is found between experiment and the structure calculated by density functional theory. Corrugation of the alloy surface allows only alternate water molecules to chemisorb, stabilizing the H-down water bilayer by reducing the metal-hydrogen repulsion compared to a flat surface.     

Diffusion in thin films on the surface of a porous solid.

A water-wet mono-dispersed glass bead system is saturated with two phases, a wetting phase of water and a non-wetting phase of tetrachloroethylene (no 1H signal). Pulsed field gradient NMR measurements of the one-dimensional probability density distribution P(Delta) (X) for the diffusive displacements of water molecules in times, Delta, are presented for the whole accessible water saturation range. At lower water contents the distributions show a distinctive shape, which is attributed to the distribution of the aqueous phase in thin surface wetting films connecting pendular rings where the beads are in contact. The data are reproduced well by a computer simulation of a random walk model based on diffusion of molecules within such a structure, allowing determination of the surface film thickness.     

Triple-point wetting on rough substrates

The influence of substrate roughness on the wetting scenario of adsorbed van der Waals films is investigated by theory and experiment. Calculating the bending free energy penalty of a solid sheet picking up the substrate roughness, we show that a finite roughness always leads to triple-point wetting reducing the widths of the adsorbed solid films considerably as compared to that of smooth substrates. Testing the theory against our experimental data for molecular hydrogen adsorbed on gold, we find quantitative agreement.     

A new surface phase in liquid normal-alkanes

A new surface phase is found in liquid normal-alkanes. X-ray and surface tension measurements show the formation of a crystalline monolayer on the surface of the liquid at temperatures up to 3°C above the bulk solidification temperature, T_f. The molecules in the monolayer are hexagonally packed and oriented normal to the surface. A single solid monolayer persists down to T_f, thus exhibiting a very limited partial wetting. The new surface phase is obtained only for chain lengths 14<n50. Its vanishing for n14 is interpreted as a possible transition from surface freezing to surface melting behaviour. The measurements are satisfactory accounted for within a simple theory based on surface energy cosiderations.     

Asymmetric wetting hysteresis on chemical defects

Using the Wilhelmy plate technique, the role of chemical defects in hysteretic wetting behavior was investigated. The wetting and dewetting work differ significantly, depending on the defect energy (i.e., high or low energy with respect to the matrix). For one, or an array of high-energy defects, advancing measurements departed from equilibrium theory, while the receding data were in close agreement. Conversely for low-energy defects, only the receding measurements showed significant departure from theory. We propose that distinct wetting mechanisms for high- and low-energy defects explain the phenomenon of asymmetric hysteresis, where the advancing or receding contact angle deviates more strongly from the equilibrium angle.     

Effect of long-range forces on surface freezing

We examine the effect of long-range van der Waals interactions on surface freezing (SF) in linear hydrocarbon chain molecules, and the wetting criteria of the bulk liquid by the crystalline surface phase. We find that although the effect of van der Waals interactions is small for SF of normal alkanes, it is important for SF of dry and hydrated alcohols. We also find that the long-range interactions should not be ignored in the interpretation of wetting phenomena in recent experimental results. The results are in good agreement with recent experiments.     

Atomistic hybrid DSMC/NEMD method for nonequilibrium multiscale simulations

A multiscale hybrid method for coupling the direct simulation Monte Carlo (DSMC) method to the nonequilibrium molecular dynamics (NEMD) method is introduced. The method addresses Knudsen layer type gas flows within a few mean free paths of an interface or about an object with dimensions of the order of a few mean free paths. It employs the NEMD method to resolve nanoscale phenomena closest to the interface along with coupled DSMC simulation of the remainder of the Knudsen layer. The hybrid DSMC/NEMD method is a particle based algorithm without a buffer zone. It incorporates a new, modified generalized soft sphere (MGSS) molecular collision model to improve the poor computational efficiency of the traditional generalized soft sphere GSS model and to achieve DSMC compatibility with Lennard-Jones NEMD molecular interactions. An equilibrium gas, a Fourier thermal flow, and an oscillatory Couette flow, are simulated to validate the method. The method shows good agreement with Maxwell–Boltzmann theory for the equilibrium system, Chapman–Enskog theory for Fourier flow, and pure DSMC simulations for oscillatory Couette flow. Speedup in CPU time of the hybrid solver is benchmarked against a pure NEMD solver baseline for different system sizes and solver domain partitions. Finally, the hybrid method is applied to investigate interaction of argon gas with solid surface molecules in a parametric study of the influence of wetting effects and solid molecular mass on energy transfer and thermal accommodation coefficients. It is determined that wetting effect strength and solid molecular mass have a significant impact on the energy transfer between gas and solid phases and thermal accommodation coefficient.     

Fluctuation forces and wetting layers in colloid-polymer mixtures

We present confocal microscopy experiments on the wetting of phase-separated colloid-polymer mixtures. We observe that an unusually thick wetting layer of the colloid-rich phase forms at the walls of the glass container that holds the mixture. Because of the ultralow interfacial tension between the colloid-rich and the polymer-rich phases, the thermally activated roughness of the interfaces becomes very big and measurable. We observe that close to the critical point the roughness of the interface between the wetting layer and the polymer-rich phase decreases with decreasing layer thickness: large excursions of the interface are confined in the wetting layer. The measured relationship between the roughness and the thickness of the wetting layer is in qualitative agreement with the predictions of renormalization group theory for short-range forces and complete wetting.