Long ranged correlations at a solid-fluid interface A signature of the approach to complete wetting

We have investigated the behaviour of the pairwise distribution function for Sullivan's model of a gas adsorbed on a solid substrate. We show that in the approach to complete wetting, when a thick film of liquid density is adsorbed on the substrate, long ranged transverse correlations (parallel to the surface) develop at the edge of the film where the density profile of the fluid resembles that of a liquid-gas interface. The long ranged correlations can be attributed to damped capillary-wave-like fluctuations; for a class I wetting situation the damping decreases and the range of the correlations increases and ultimately diverges as the bulk gas pressure approaches the saturated vapour pressure.

Our analysis provides a physical explanation of the long ranged transverse correlations calculated by Foiles and Ashcroft in their recent study of a model of argon at a carbon dioxide substrate. We also predict that long range transverse correlations will occur for the case of adsorption from a dense liquid provided the solid-fluid potential is such that a thick film of gas forms between the substrate and the bulk liquid.     

Microscopic density functional theory of wetting and drying of a solid substrate by an explicit solvent model of ionic solutions

Classical density functional theory (DFT) of inhomogeneous fluids is applied to an explicit solvent ‘semi-primitive’ model (SPM) of ionic solutions to investigate the influence of ionic solutes on the wetting behaviour of a solvent in contact with a neutral or charged planar substrate. The SPM is made up of three species of hard sphere particles with different diameters, interacting via an attractive Yukawa potential to model excluded volume and cohesion. The solvent particles are neutral, while the monovalent anions and cations are oppositely charged. The polar nature of the solvent is modelled by a continuum dielectric permittivity linked to the local solvent density. All three species interact with the impenetrable substrate via an attractive external potential. While excluded volume effects are accurately described by a Rosenfeld ‘fundamental measure’ free energy functional, the short range Yukawa attraction and Coulombic interactions are treated within the mean-field approximation. The ionic solutes are found to have a significant impact on the wetting behaviour of the solvent, in particular on the wetting temperature. Strong electric fields, or long-ranged (weakly screened) Coulombic forces are shown to have the propensity to change the wetting transition from second to first order. The cation–anion size asymmetry leads to charge separation on the liquid–vapour interface of the solution, which in turn can induce a drying transition on the liquid side of liquid–vapour coexistence.     

Wetting, spreading, and adsorption on randomly rough surfaces

The wetting properties of solid substrates with customary (i.e., macroscopic) random roughness are considered as a function of the microscopic contact angle of the wetting liquid and its partial pressure in the surrounding gas phase. Analytic expressions are derived which allow for any given lateral correlation function and height distribution of the roughness to calculate the wetting phase diagram, the adsorption isotherms, and to locate the percolation transition in the adsorbed liquid film. Most features turn out to depend only on a few key parameters of the roughness, which can be clearly identified. It is shown that a first-order transition in the adsorbed film thickness, which we term "Wenzel prewetting", occurs generically on typical roughness topographies, but is absent on purely Gaussian roughness. It is thereby shown that even subtle deviations from Gaussian roughness characteristics may be essential for correctly predicting even qualitative aspects of wetting.     

Wetting of a plane with a narrow solvophobic stripe

ABSTRACT

We present a numerical study of a simple density functional theory model of fluid adsorption occurring on a planar wall decorated with a narrow deep stripe of a weaker adsorbing (relatively solvophobic) material, where wall-fluid and fluid-fluid intermolecular forces are considered to be dispersive. Both the stripe and outer substrate exhibit first-order wetting transitions with the wetting temperature of the stripe lying above that of the outer material. This geometry leads to a rich phase diagram due to the interplay between the pre-wetting transition of the outer substrate and an unbending transition corresponding to the local evaporation of liquid near the stripe. Depending on the width of the stripe, the line of unbending transitions merges with the pre-wetting line inducing a two-dimensional wetting transition occurring across the substrate. In turn, this leads to the continuous pre-drying of the thick pre-wetting film as the pre-wetting line is approached from above. Interestingly we find that the merging of the unbending and pre-wetting lines occurs even for the widest stripes considered. This contrasts markedly with the scenario where the outer material has the higher wetting temperature, for which the merging of the unbending and pre-wetting lines only occurs for very narrow stripes.     

Critical point wetting: The effect of hydrodynamical and steric interfacial interactions

It is shown for the first time that the capillary waves and hydrodynamic correlations in an adsorbed film of a simple fluid provide incomplete wetting in some vicinity of the critical point T_c. On approaching T_c the film thickness grows infinitely and the wetting angle tends to zero due to the steric repulsion between the film-bulk fluid interface and the substrate.     

The low-temperature properties of the one-dimensional fluid

The low-temperature properties of a one-dimensional fluid with hard core attractive nearest-neighbor interactions have been investigated. The fluid exhibits a critical behavior nearT=0 which is, in some respects, analogous to that of the one-dimensional Ising models. With the proper choice of scaling variables the singular part of the appropriate thermodynamic potential has the same homogeneous scaling form as the Ising model. The correlations in the scaling region have a more complex structure than in the Ising model but do have a long-ranged part of scaling form. TheT=0 limit in the scaling region gives states of low density and zero pressure whose correlations are those of a two-phase state, of which one component is a perfect crystal phase and the other is a zero density phase. The positive pressureT=0 states are single-phase perfect crystal states whose long-range order develops continuously asT approaches zero. Those Fourier components of the correlations, which correspond to reciprocal lattice vectors, diverge asT approaches zero; hence, the transition is second-order, unlike higher-dimensional systems.     

Vortices in weakly coupled layered superconductors

The vortex system in high-temperature layered superconductors exhibits a rich phase diagram with many proposals of phase transitions modifying the correlations both within and between the layers. We focus on the limit where the magnetic coupling between “pancake” vortices dominates over the interlayer Josephson coupling. The weak, but long-ranged nature of this magnetic interaction allows for an accurate “mean-field” treatment where the pancakes in each layer move independently in a self-consistent substrate potential. We calculate the form of the two relevant phase transitions in this system. First, we determine when the substrate potential is too weak to stabilize the two-dimensional (2D) fluctuations and the lattice evaporates to a pancake gas. Second, within the lattice we find a Kosterlitz–Thouless unbinding transition of vacancies and interstitials. For a small but finite Josephson term, this is identified with the phase-decoupling transition.     

Crossover between complete wetting and critical adsorption

The effects of bulk critical fluctuations on the adsorption of a fluid on a planar attractive substrate are studied as a function of the temperature and chemical potential in the presence of long-ranged forces. The latter are explicitly included in a phenomenological free energy functional constructed to give non-classical critical exponents. The crossover between the complete wetting and the critical adsorption regime is studied along different paths in thermodynamic space and a comparison is made with the results of recent experiments.     

Éléments d'un modèle d'interactions oxygène sous basse pression-métaux de transition à haute température

This paper presents a general model for the interaction of low pressure oxygen with transition metals at high temperatures, when all products are volatile. It is based on the following major hypotheses: (a) adsorption of molecular oxygen is the only direct process between the gas phase and the surface; (b) all chemical reactions occur in the adsorbed layer. This model is in agreement not only with our experimental results on platinum and rhenium but also with those of other authors on different metals (W, Mo. Ta, Nb). It becomes identical to Stickney's quasi-equilibrium theory when assuming that desorption activated complexes are identical to molecules moving freely in a plane parallel to the surface.     

The multilayer melting transition in methane adsorbed on graphite

High resolution heat capacity measurements of multilayer methane adsorbed on graphite are presented and analyzed. The evidence indicates the presence of two wetting transitions: a first-order dewetting transition at T_w = 90.48 K, and a continuous wetting transition at the triple point, T_t = 90.66 K. This behavior is to be expected in connection with the melting transition in any system where both solid and liquid wet the surface. Heat capacity measurements can provide a valuable diagnostic tool for the wetting behavior of films too thick to be investigated by other means. In the thin film limit, we find that the latent heat of melting vanishes at about 4 layers.     

Critical point wetting in NbH0.31

The local α-α' phase distribution in a thin Nb single crystal loaded with the critical H-concentration was investigated with X-rays. The sample used in the experiment showed a macroscopic hydrogen density mode, leading to a curvature of the whole crystal plate with the α-phase on the concave side and the α'-phase on the convex side. In addition an α-phase wetting layer was found on top of the α'-phase separated from the bulk α'-phase by a mixed α-α' phase region. In a simple model their thickness were determined to about $\text{\$}\text{?}$1.0 and 7?0μm, resp. This thin wetting layer leads to a new interpretation of the phase transition of H and Nb, involving the familiar long-range elastic interaction as well as a short-range surface free energy contribution.     

Percolation transition in an irreversible-surface reaction model

Monte Carlo simulation studies of percolation transition in a surface reaction model describing the oxidation of carbon mono-oxide on a catalytic surface are presented. The percolation transition for adsorbed oxygen atoms occurs below the poisoning transition where carbon mono-oxide completely covers the surface of the catalyst and takes place for an oxygen coverage of about 0.525 which is close to the percolation transition in an Ising lattice gas with nearest-neighbor attractive interactions. In several respects the oxygen clusters near the percolation threshold resemble those of the Ising lattice gas near its critical point.     

Hydrogen and helium films as model systems of wetting

Optical experiments on the wetting properties of liquid ⁴He and molecular hydrogen are reviewed. Hydrogen films on noble metal surfaces serve as model systems for studying triple point wetting, a continuous transition between wetting and non-wetting. By means of optically excited surface plasmons, the adsorbed film thickness for temperatures around, and far below, the bulk melting temperature is measured, and the physical mechanisms responsible for the transition are elucidated. Possible applications for other experiments in pure and applied research are discussed. Thin films and droplets of liquid helium are studied on cesium surfaces, on which there is a first order wetting transition. Our studies concentrate on dynamical observations via surface plasmon microscopy, which provide insight into the morphology of liquid helium droplets spreading at different temperatures. Features corresponding to pinning forces, the prewetting line, and the Kosterlitz-Thouless transition are clearly observed.     

Transition from van der Waals to H bond dominated interaction in n-propanol physisorbed on graphite

Multilayer sorption isotherms of 1-propanol on graphite have been measured by means of high-resolution ellipsometry within the liquid regime of the adsorbed film for temperatures ranging from 180 to 260 K. In the first three monolayers the molecules are oriented parallel to the substrate and the growth is roughly consistent with the Frenkel-Halsey-Hill (FHH) model that is obeyed in van der Waals systems on strong substrates. The condensation of the fourth and higher layers is delayed with respect to the FHH model. The fourth layer is actually a bilayer. Furthermore, there is indication of a wetting transition. The results are interpreted in terms of hydrogen-bridge bonding within and between the layers.     

Diffuse LEED intensities of disordered crystal surfaces: II. Multiple scattering on disordered overlayers

The diffraction of low energy electrons from disordered overlayers adsorbed on ordered substrates is treated theoretically by an extension of Beeby's multiple scattering method. A lattice gas model is assumed for the disordered adsorbate layer. Multiple scattering within a certain area around each atom — each atom of the overlayer and within the ordered substrate — is treated self-consistently, the remaining contributions to the total scattering amplitude being averaged. The theory can be used in the limiting cases of random distribution and of long range order within the adsorbate layer.     

Influence of surface ordering on the wetting of structured liquids

The substrate is shown to induce substantial ordering in diblock copolymer thin films above the bulk order-disorder transition (ODT) where, thermodynamically, a phase mixed state is favored. Initially, uniform films reorganize to form a hierarchy of transient surface patterns and stable film thicknesses that depend on the initial film thickness and on the substrate. Self-consistent field calculations of the free energy of the system for different situations, depending on the relative tendency for the different block components to be attracted to the substrate and/or free surface, provide an explanation of the formation of the stable film thicknesses. A continuum picture proposed earlier by Brochard et al.rovides an explanation of the wetting characteristics of this system. In some cases the ordering destabilizes the film so that dewetting occurs (wetting autophobicity), whereas in other cases the surface ordering results in a kinetic stabilization of a film that would otherwise dewet. Received 3 August 2001 and Received in final form 1 November 2001     

A new approach to the calculation of tight-binding surface density of states in thin films

We present a new approach to the calculation of planar electronic density of states of thin films within the tight-binding scheme. The diagonal elements of the electronic Green's function are obtained by an iteration procedure derived from the transfer-matrix approach for semi-infinite crystals. An application is made to the study of the (100) planes of a model transition metal film with a simple cubic structure.     

Strong suppression of the Kosterlitz-Thouless transition in a 4He film under high pressure

We have found that the surface specularity for 3He quasiparticle scattering is closely related to the superfluidity and the Kosterlitz-Thouless (KT) transition of 4He film adsorbed on the surface. The specularity is determined by measurements of the transverse acoustic impedance of bulk liquid 3He. The unique point of our system is that we can control the correlation among 4He atoms in the film by changing the pressure of the bulk 3He. The observed KT transition temperature is significantly suppressed by increasing the pressure, which suggests a strong correlation effect on KT transition.     

Stability of thermoresponsive methylcellulose thin film: x‐ray reflectivity study

Methylcellulose (MC), a thermoreversible polymer, was fabricated as thin films into silicon substrates and characterized by x‐ray reflectivity (XRR) measurements for its stability with time and heating. XRR data from the as‐is thin films showed good agreement with the single‐layer model on top of a substrate from Parratt's formalism. Data fitting showed that the density of the thin films is slightly higher than the reported value by manufacturers. Interface roughness values indicate good wetting of the polymer onto the substrate. Heating the thin films at the phase transition temperatures and quenching them to room temperature showed no significant changes in the thin film parameters before and after heating. This showed the thermal stability and/or thermoreversibility of the film. Diffuse scattering measurements also showed no significant changes in the lateral structure of the film with heating and quenching. XRR measurements done on fabricated thin films stored for a month showed a slight increase in the film thickness which could be due to the hygroscopic nature of the polymer. Vacuum heating of the stored thin films at 100 °C for 1 h slightly decreased the thickness, but it has been found that other parameters such as density and surface/interface roughness show good thermal stability. Copyright © 2009 John Wiley & Sons, Ltd.