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.     

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.     

Entropic wetting and many-body induced layering in a model colloid-polymer mixture

We develop an efficient simulation scheme to study a model suspension of equally sized colloidal hard spheres and nonadsorbing ideal polymer coils, both in bulk and adsorbed against a planar hard wall. The many-body character of the polymer-mediated effective interactions between the colloids yields a bulk phase diagram and adsorption phenomena that differ substantially from those found for pairwise simple fluids; e.g., we find an anomalously large bulk liquid regime and, far from the bulk triple point, three layering transitions in the partial wetting regime prior to a transition to complete wetting by colloidal liquid.     

Phase transitions in physisorbed films on cadmium (0001): comparison with graphite (0001)

Physical adsorption of simple molecules has been studied experimentally on cadmium (0001). Adsorption isotherms of Kr and CF₄ at 77.3 K show a first-order phase transition in the first layer, characteristic of adsorption on uniform substrates.

The nature of the phase transition on cadmium (0001) in the first layer is investigated for Kr and CF₄ by a thermodynamic approach in comparison with results obtained earlier on graphite (0001).

From the comparison between the two uniform substrates, we show that (1) cadmium (0001) has a less attractive force field than graphite (0001); and (2) like graphite (0001), cadmium (0001) is a very uniform surface suitable for studying two-dimensional phases according to the Gibbsian formulation.     

New insights on contact angle/roughness dependence on high surface energy materials

The relationship between wettability and roughness has been studied on micro-roughened titanium surface after different cleaning procedures. Whereas most studies addressing (super)-hydrophobic behaviors have so far dealt with the wetting of low surface energy and textured substrates in air environment, we here report on a totally novel system and configuration involving the wetting of highly hydrophilic, textured metallic materials in liquid alkane medium, the so-called two liquid phase method. Roughness characterization showed that substrates were isotropic (2D), at a lengthscale much smaller than the size of the drop, with a heterogeneous (vertical) distribution of peaks and valleys. Depending on whether the alkane that initially penetrates and resides in the pores is displaced or not by the water drop (as for air pockets in air environment), we show that different wetting regimes may appear, depending on the cleaning procedure. To our knowledge, this is the first systematic study dealing with the interplay between surface roughness, the wetting behavior and in particular the (super)-hydrophilicity of high surface energy substrates, in non water miscible liquid environments. Whenever competitive processes of liquid/liquid displacement are involved at such high surface energy and textured substrates, such as titanium implant in bone tissue, these results may contribute understanding and predicting their wetting behavior.     

Elucidating alkane adsorption in sodium-exchanged zeolites from molecular simulations to empirical equations

Configurational-bias Monte Carlo (CBMC) simulations provide adsorption isotherms, Henry coefficients and heats of adsorption of linear alkanes in sodium-exchanged MFI- and FAU-type zeolites. These simulations were carried out using our newly developed force field that reproduces experimental sodium positions in the dehydrated zeolites, and successfully predicts alkane adsorption properties over a wide range of sodium cation densities, temperatures, and pressures. We derived empirical expressions from the simulation data to describe the adsorption of linear alkanes in MFI- and FAU-type zeolites. These expressions afford a suitable substitute for complex CBMC simulations. In the low coverage regime we provide simple expressions that adequately describe the Henry coefficient and adsorption enthalpy of n-alkanes as a function of sodium density and temperature. The predicted Henry coefficients and heats of adsorption compare extremely well to available experimental data. In the high coverage regime we provide an expression for saturation capacities of linear alkanes in the zeolite. This expression, combined with the expression for the Henry coefficients, provides of the complete adsorption isotherms of pure adsorbents and mixtures, in good agreement with the adsorption isotherms obtained from CBMC.     

Thermodynamics and surface properties of liquid Cu–B alloys

The study of the thermodynamic and the surface properties of liquid Cu–B alloys can help better understanding of a complex interfacial chemistry related to liquid Cu–brazes in contact with boride substrates. Despite a simplicity of the Cu–B phase diagram, only a few thermodynamic data are available in the literature, while in the case of the surface properties a complete lack of data is evident. The quasi-chemical approximation (QCA) for the regular solution has been applied to describe the mixing behaviour of liquid Cu–B alloys in terms of their thermodynamic and surface properties as well as the microscopic functions. In view of joining processes related to liquid Cu–brazes/solid boride systems a particular attention is paid to the surface properties of the Cu-rich part of the system and the calculated values are substantiated by the new surface tension experimental data of liquid Cu and Cu–10 at.% B alloy. The tests have been performed by the sessile-drop method under the same experimental conditions. Considering the experimental uncertainties, the effect of oxygen on the surface tension of liquid Cu and Cu–10 at.% B alloy has been analysed by simple model that combines the physical property data included in Butler’s equation with the oxygen solubility data and it gives the same results as Belton’s adsorption equation.     

Grain Boundary Phase Transitions and their Influence on Properties of Polycrystals

Grain boundary (GB) phase transitions can change drastically the properties of polycrystals. The GB wetting phase transition can occur in the two-phase area of the bulk phase diagram where the liquid (L) and solid (S) phases are in equlibrium. Above the temperature of the GB wetting phase transition a GB cannot exist in equlibrium contact with the liquid phase. The experimental data on GB wetting phase transitions in numerous systems are analysed. The GB wetting tie-line can continue in the one-phase area of the bulk phase diagram as a GB solidus line. This line represents the GB premelting or prewetting phase transitions. The GB properties change drastically when GB solidus line is crossed by a change in the temperature or concentration. The experimental data on GB segregation, energy, mobility and diffusivity obtained in various systems both in polycrystals and bicrystals are analysed. In case if two solid phases are in equilibrium, the GB “solid state wetting” can occur. In this case the layer of the solid phase 2 has to substitute GBs in the solid phase 1. Such GB phase transition occurs if the energy of two interphase boundaries is lower than the GB energy in the phase 1.     

Effect of oxygen surface groups on adsorption of benzene derivatives from aqueous solutions onto active carbon samples

The process of adsorption of selected benzene derivatives from aqueous solution is investigated on two carbonaceous materials of differentiated surface properties - quantity of oxygen functional groups. Carbon samples were prepared by removal of external layers from granules of unmodified and oxidized active carbon. The surface and structure characteristics of carbon samples were estimated by various methods. The experimental isotherms of organics adsorption from liquid phase were measured and interpreted in terms of the theory of adsorption on heterogeneous solid surfaces. The influence of differences in adsorbate and adsorbent properties on adsorption uptake was analyzed. The adsorption effectiveness was regarded as a result of the differences in adsorbate hydrophobicities and the effect of specific interactions of its functional groups with active sites on carbon surface.     

Wetting by solid helium,a model system

This is a review of the wetting properties of solid helium on various solid substrates. Due to its extreme purity and to its very fast growth dynamics, solid helium 4 is often considered as a model system in materials science. Several wetting phenomena have been studied with helium 4 crystals, namely contact angles on solid substrates with variable roughness, wetting on graphite where epitaxial growth takes place, the roughening transition as a function of film thickness, the wetting of grain boundaries by the liquid phase.     

Condensation of gold vapour on silicone oil – effect of the liquid substrate

When gold vapour condenses onto a liquid substrate, the inherent structure of the liquid could influence the condensate growth and coverage. A thorough comparison between the liquid (silicone oil) and solid (amorphous carbon) substrates is reported by analysing the changes in their condensate growth. Low condensate coverage with large areas of empty regions is observed on the liquid surface in contrast to the solid carbon surface displaying uniformly distributed gold clusters at all times. This is deduced to be caused by the incoming gold atoms restricting the liquid molecules’ degrees of freedom upon binding. This effect could perturb the entire liquid structure, causing the liquid to collectively act against the adsorption of gold atoms. This could lead to differences in growth kinetics on the liquid substrate and can account for the observed dissimilarity in condensate coverage. The substrate structure effect discussed here serves as a step forward for utilizing liquid substrates for a variety of potential applications.     

Monte Carlo study of binary mixtures adsorbed on square lattices

The monolayer adsorption of interacting binary mixtures on 2D square lattices has been studied through grand canonical Monte Carlo simulation in the framework of the lattice-gas model. The process has been monitored through total and partial isotherms and differential heats of adsorption corresponding to both species of the mixture. Repulsive lateral interactions between the adsorbed particles have been considered, resulting in a rich variety of structural orderings in the adlayer. At the end of this work, the phase diagram characterizing the transitions occurring in the system has been determined. A nontrivial interdependence between the partial surface coverage of both species was observed and discussed.     

Wetting transition of a nematic liquid crystal on a periodic wedge-structured substrate

It is known that the wetting behaviour of a fluid is deeply altered by the presence of rough or structured substrates. We first review some simple considerations about isotropic fluids and rough substrates, and then we generalize Wenzel's law, which assigns an effective contact angle to a droplet on a rough substrate, when the wetting layer has an ordered phase, like a nematic. We estimate the conditions for which the wetting behavior of an ordered fluid can be qualitatively different from that usually found in a simple fluid. To support our general considerations, we use the Landau-de Gennes mean field approach to investigate theoretically and numerically the wetting transition of a nematic phase on a periodic triangular structured substrate.     

The mean-field adsorption on a sinusoidally corrugated substrate revised

The fluid system at the bulk liquid–gas coexistence in a presence of a sinusoidally corrugated substrate exhibits not only the wetting transition, but additionally a first-order, thin–thick transition. The mean-field analysis of this transition based on a simple effective Hamiltonian is valid only in long wavelength limit. In this case the filling transition occurs so close to the wetting temperature, that the behavior of the interface is dominated by fluctuations, therefore the mean-field approach breaks down. We analyze the filling transition with the help of Hamiltonian evaluated from Landau theory. The applicability of our Hamiltonian is not restricted only to the vicinity of the wetting point. We obtain the phase diagram valid beyond the temperature range corresponding to the strong fluctuations regime. It displays more complex dependence on different length scales of the system and includes the old one as a particular case.     

Threshold criterion for wetting at the triple point

Grand canonical simulations are used to calculate adsorption isotherms of various classical gases on alkali metal and Mg surfaces. Ab initio adsorption potentials and Lennard-Jones gas-gas interactions are used. Depending on the system, the resulting behavior can be nonwetting for all temperatures studied, complete wetting, or (in the intermediate case) exhibit a wetting transition. An unusual variety of wetting transitions at the triple point is found in the case of a specific adsorption potential of intermediate strength. The general threshold for wetting near the triple point is found to be close to that predicted with a heuristic model of Cheng et al. This same conclusion was drawn in a recent experimental and simulation study of Ar on CO2 by Mistura et al. These results imply that a dimensionless wetting parameter w is useful for predicting whether wetting behavior is present at and above the triple temperature. The nonwetting/wetting crossover value found here is w approximately 3.3.     

Planar nematic anchoring on SiO-coated substrates

Summary The nematic planar anchoring is usually explained by using simple elastic models: the surface easy axis corresponds to the surface direction that minimizes the excess of nematic elastic energy. When anisotropic rough substrates are used to align nematic liquid crystals, due to the complex surface morphology, usual elastic models are not directly applicable. This paper presents quantitative topographical data of rough substrates, obtained with oblique SiO evaporation under vacuum for nematic planar anchoring. Experimental data are obtained by means of Atomic Force Microscopy and they are used to demonstrate the self-affine nature of these substrates and to relate the nematic anchoring with the anisotropy of the local fractal properties of the substrate itself. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Phase equilibria in III–V quaternary systems—application to Al-Ga-P-As

A method is given to calculate liquidus-solidus phase diagrams involving quaternary III–V solid solutions of the type A_xB_{1 ? x}C_yD_{1 ? y}. The calculation treats the mixture of A and B atoms on the group III sublattice and C and D atoms on the group V sublattice as strictly regular and the quaternary liquid solution as simple. The input parameters for the calculation are obtained from existing analyses of the binary and ternary boundaries of the phase diagram.Experimental liquidus and solidus data along several Al isoconcentration curves on the 900 ° and 1000 °C isotherm surface are presented for the Al-Ga-P-As system. The liquidus data are obtained by liquid observation and saturation techniques and solidus compositions determined on self-nucleated cyrstals and on mixed crystal layers grown on GaP substrates. The experimental results are consistent with the calculated phase diagram for this system.     

Mean-field phenomenology of wetting in nanogrooves

ABSTRACT

In this special issue article, we bring together our recent research on wetting in confinement, in particular planar walls, wedges, capillary grooves and slit pores, with emphasis on phase transitions and competition between wetting, filling and condensation, and highlight their similarities and disparities. The results presented are obtained with the classical density functional theory (DFT) for fluids, which is a mean-field statistical mechanical framework for including the spatial variations of the fluid density into the thermodynamic equation of state. For wetting in sculpted substrates, we solve numerically the DFT equations to obtain the fluid density profiles, wetting isotherms and phase diagrams. This allows us to contrast the wetting phenomenology of grooves, planar walls, slit and wedge-shaped pores. Of particular interest are the transitions associated with capillary condensation, planar pre-wetting and mean-field wedge pre-filling lines.     

Stability of thin nematic films

We discuss the peculiarity of thin nematic films on solid substrates with a free surface, underlining the differences with what is usually seen in dewetting. We review the thermodynamic basis of the coupled phase/thickness separation that has previously been shown experimentally. We give new experimental evidences for the origin of the coupling force chosen in our previous theoretical model. This additional information contributes to the discussion raised by the article of Ziherl and Zumer in this issue [19].Received: 3 December 2003PACS: 68.15. + e Liquid thin films - 64.70.Md Transitions in liquid crystals - 61.30 Surface phenomena in liquid crystals including anchoring, anchoring transitions, surface-induced layering, surface-induced ordering, wetting, prewetting transitions and wetting transitions     

Wetting of grain boundaries by the second solid phase in Al-based alloys

This work presents data on changes in the structure and properties of aluminum-based alloys subjected to severe plastic deformation by high-pressure torsion. Microstructure and the composition and change in temperature for phases are studied for double (Al-Zn, Al-Mg) and ternary (Al-Mg-Zn) alloys. Tie-lines of the liquid phase wetting of grain boundaries and grain boundary solvus lines are constructed on phase diagrams of our Al-Zn and Al-Mg systems. The shifting of the phase composition in the bulk of aluminum grains from the Al + τ region to the Al + η region of the Al-Mg-Zn phase diagram with an increase in the specific area of grain boundaries is described.