Wetting transitions at triple points; A study based upon the analysis of stepwise adsorption isotherms

This adsorption isotherm study finds that ethylene adsorption on PbI₂ is limited to only a few layers at T<T_3t (the triple point temperature of bulk ethylene) but that almost unlimited adsorption can take place as T_3t, is approached. This behaviour is qualitatively identical to the behaviour of the ethylene/graphite system previously reported. In both cases wetting by ethylene is incomplete below T_3t since the adlayers formed have an orientational order which is in compatible with that which is found in the bulk solid ethylene. However, C₂H₄ shows a strong tendency to wetting somewhat below its bulk triple point temperature because it can form a liquid or at least an orientationally disordered solid film of limited thickness. The similarities and differences between these two systems are discussed. In addition, we discuss in general systems for which wetting is temperature dependent. The first part of this paper is devoted to outlining in detail how stepwise adsorption isotherms on powdered samples can be used to give information about wetting of the exposed basal face. The merits and limitations of this method are discussed in the context of the ethylene/PbI₂ example.     

Liquid nanostripes

Equilibrium wetting of ethanol, a volatile liquid, onto chemically patterned nanostripes has been investigated using noncontact atomic force microscopy (AFM). The chemical patterns, generated by a conducting AFM tip, are composed of COOH terminated "wetting" regions and CH3 terminated "nonwetting" regions. Controlled amounts of ethanol, from the vapor phase, condense on the COOH stripes and their shape is imaged in situ versus their width (70 < w < 300 nm). The measured profile shapes at saturation and their w(1/2) height dependence are well described by density functional theory with dispersive, nonretarded potentials.     

Avoided critical behavior in dynamically forced wetting

A solid object can be coated by a nonwetting liquid since a receding contact line cannot exceed a critical speed. In this Letter we study the dynamical wetting transition at which a liquid film gets deposited by withdrawing a vertical plate out of a liquid reservoir. It has recently been predicted that this wetting transition is critical with diverging time scales and coincides with the disappearance of stationary menisci. We demonstrate experimentally and theoretically that the transition is due to the formation of a solitary wave, well below the critical point. As a consequence, relaxation times remain finite at threshold. The structure of the liquid deposited on the plate involves a capillary ridge that does not trivially match the Landau-Levich film.     

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.     

Observation of a dispersion transition and the stability of a liquid in a nanoporous medium

It has been found that the removal of overpressure is accompanied by a transition of some nonwetting-liquid nanoclusters to the stable state in narrow ranges of the filling factor and temperature. This means that the nonwetting liquid becomes ??wetting.??     

Laboratory observations of altered porous fluid flow behavior in Berea sandstone induced by low-frequency dynamic stress stimulation

It has been observed repeatedly that low-frequency (1–500 Hz) seismic stress waves can enhance oil production from depleted reservoirs and contaminant extraction from groundwater aquifers. The physics coupling stress waves to fluid flow behavior in porous media is not understood, although numerous physical mechanisms have been proposed to explain the observations. To quantify the effects of low-frequency, dynamic-stress stimulation on multiphase fluid flow and in situ particle behavior in porous media, laboratory experiments were conducted with a core flow stimulation apparatus that allows for precise control and measurement of applied stress and strain, static confinement, and fluid flow parameters. Results are reported for experiments on stimulated single-phase and two-phase fluid flow behavior in 2.54-cm-diameter Berea sandstone cores. For all experiments, stimulation was applied to the cores in the form of sinusoidal, axial, mechanical stress coupled to the solid porous matrix at frequencies of 25 to 75 Hz. Applied stress RMS amplitudes ranged from 300 to 1200 kPa and, at these levels, produced coupled, pore-pressure fluctuations of much less than 1.2 to 4.8 kPa, respectively. During single-phase brine flow, stimulation increased the absolute permeability of the rock by 10–20%. This was caused by mobilizing in situ clay particles that were partially plugging the pore throats. During two-phase, steady-state, constant-rate flow of oil-brine and decane-brine mixtures, stimulation caused significant changes in the bulk fluid pressure drop across the core. The pressure changes showed a strong dependence on the viscosity of the nonwetting fluid phase (oil or decane) relative to the wetting phase (brine). This may indicate that relative changes in the mobility of wetting versus nonwetting fluid phases were induced by the dynamic stress. Under the specific experimental conditions used, pore-scale particle perturbation and altered wettability are possible physical mechanisms that can explain the results.     

Wetting behaviour of a model symmetric binary mixture with partially miscible components from a density functional approach

We investigate wetting transitions in a binary fluid at a solid surface by means of a density functional approach. For this purpose we use the symmetric binary mixture model, which exhibits a demixing in a bulk phase. We concentrate on the evaluation of the phase diagrams in the case of adsorption from a gas phase at a fixed composition. Our calculations have revealed different scenarios, leading to the change of wettability of the surface. In the case of adsorption from an equimolar bulk gas the wetting transition may be of the first or of the second order. In the case of non-equimolar bulk composition we observe either a transition from partial to complete wetting, or a first-order transition between two partial wetting states.     

Wetting transitions of simple liquid films adsorbed on self-assembled monolayer substrates: an ellipsometric study

We report on an ellipsometric experimental study designed to explore the relevance of the wetting phase diagram predicted by liquid state physics of basic models, to the wide class of simple organic liquid films that adsorb from saturated vapour onto planar substrates at room temperature. The wetting properties are explored by measuring adsorption isotherms in the approach to saturation, in particular, for adsorption of n-hexane on a variety of specially constructed substrates (self-assembled monolayers) spanning a wide range of surface energy, and by carrying out the microscopic equivalent of contact angle experiments at saturation. We locate a wetting transition, which in our case is continuous, and then study its properties in detail. The general prediction of the wetting phase diagram, that wetting transitions should be ubiquitous in nature and readily located via control over the substrate field, is supported by our data, but the quantitative nature of the thick film adsorption regime is not in agreement with Lifshitz theory. This conclusion supports the work of a variety of earlier related studies, but contrasts with recent results for adsorption onto the surface of water. In addition, the correlation length determined from our complete wetting adsorption isotherms is mesoscopic, suggesting that equilibrium statistical mechanics of simple models of inhomogeneous fluids cannot explain the data.     

Macroscopic capillarity without a constitutive capillary pressure function

This paper challenges the foundations of the macroscopic capillary pressure concept. The capillary pressure function, as it is traditionally assumed in the constitutive theory of two-phase immiscible displacement in porous media, relates the pressure difference between nonwetting and wetting fluid to the saturation of the wetting fluid. The traditional capillary pressure function neglects the fundamental difference between percolating and nonpercolating fluid regions as first emphasized in R. Hilfer [Macroscopic equations of motion for two phase flow in porous media, Phys. Rev. E 58 (1998) 2090]. The theoretical approach proposed here starts from residual saturations as the volume fractions of nonpercolating phases. The resulting equations of motion open the possibility to describe flow processes where drainage and imbibition occur simultaneously. The theory predicts hysteresis and process dependence of capillary phenomena. The traditional theory is recovered as a special case in the residual decoupling approximation. Explicit calculations are presented for quasistatic equilibrium profiles near hydrostatic equilibrium. The results are found to agree with experiment.     

Topography driven spreading

Roughening a hydrophobic surface enhances its nonwetting properties into superhydrophobicity. For liquids other than water, roughness can induce a complete rollup of a droplet. However, topographic effects can also enhance partial wetting by a given liquid into complete wetting to create superwetting. In this work, a model system of spreading droplets of a nonvolatile liquid on surfaces having lithographically produced pillars is used to show that superwetting also modifies the dynamics of spreading. The edge speed-dynamic contact angle relation is shown to obey a simple power law, and such power laws are shown to apply to naturally occurring surfaces.     

Adsorption of a flexible self-avoiding polymer chain: Exact results on fractal lattices

The large-scale behavior of surface-interacting self-avoiding polymer chains placed on finitely ramified fractal lattices is studied using exact recursion relations. It is shown how to obtain surface susceptibility critical indices and how to modify a scaling relation for these indices in the case of fractal lattices. We present the exact results for critical exponents at the point of adsorption transition for polymer chains situated on a class of Sierpinski gasket-type fractals. We provide numerical evidence for a critical behavior of the type found recently in the case of bulk self-avoiding random walks at the fractal to Euclidean crossover.     

Lattice calculations of the photoionization of Li

Calculations are presented for the double photoionization (with excitation) and triple photoionization of the Li atom. The motion of all three electrons is treated equally by solving the time-dependent Schr?dinger equation in nine dimensions. A radial lattice is used to represent three of the nine dimensions, while a coupled channel expansion is used to represent the other six dimensions. Probabilities for photoionization are obtained by t--> infinity projection onto fully antisymmetric spatial and spin functions. Double photoionization cross sections for lithium leaving the ion in the 1s, 2s, and 2p states are presented. Good agreement is found with the measurements of Huang et al. [Phys. Rev. A 59, 3397 (1999)]] for the total double photoionization cross section and with the measurements of Wehlitz et al. [Phys. Rev. Lett. 81, 1813 (1998)]] for the triple photoionization cross section.     

Multilayer adsorption and wetting of acetone on graphite

Adsorption/desorption isotherms of acetone on highly oriented pyrolytic graphite have been measured by ellipsometry for temperatures above the bulk triple point. The behavior in the monolayer and submonolayer regime is conventional, with 2D gas-liquid and 2D liquid-solid coexistence regions. Further liquid monolayers grow on top of the completed monolayer. The growth is basically layer-by-layer. For temperatures between 190 K and the triple point a prewetting-type transition occurs with a thin-thick jump of the layer thickness on adsorption but a layer-wise removal of the film on desorption. In this temperature regime the first monolayer is solid and its molecules are oriented perpendicular to the substrate whereas the higher layers are orientationally disordered polar liquid.     

Wetting-Induced Aggregation of Colloids

More than two decades ago, in a seminal paper John Cahn proposed scaling arguments for the possibility of a wetting transition in two coexisting fluid phases near the critical point. Since then, Cahn's model has been tested in many fluid systems and further refined by including the real interactions between the fluid and the solid wall. A fascinating consequence of the existence of a wetting transition is the possibility for a transition from weak to strong adsorption in the homogeneous phase. The situation is further enriched in nonstandard geometries having special geometrical constraints. The subject of this review concerns one such situation, where charge-stabilized colloidal particles are suspended in the homogeneous region of a binary liquid mixture. In this case, the preferential adsorption of one of the liquid components on to the colloid surface completely modifies the stability of the particles leading to an aggregation process. Although the exact mechanism underlying the adsorption phenomenon is still debated, it is closely related to the wetting transition. Recent experimental developments concerning the static and dynamic aspects of this phenomenon are reviewed. In addition, the main findings of a theoretical model based on the adsorption-modified electrostatic interactions between the colloidal particles are discussed.     

Entropy-driven triple point wetting in hard-rod mixtures

We theoretically study binary mixtures of thin and thick hard rods with diameter ratio more extreme than 1:4. The bulk phase diagram of these systems exhibits a triple point, where an isotropic (I) phase coexists with two nematic phases ( N1 and N2) of different composition. Using density functional theory, we predict that the I-N2 interface is completely wet by N1 upon approach of the the I-N1-N2 triple point. This entropic triple point wetting should be experimentally observable in colloidal suspensions of rodlike particles.     

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.     

Lattice Boltzmann simulations of drops colliding with solid surfaces

Video images of drops colliding with solid surfaces shown by Rioboo et al. (2002) reveal that, for large drop velocities, the drops flatten and form a ring structure before receding and, in some cases, rebounding from the surface. They described the sequence of events in terms of four distinct regimes. During the initial kinematic phase, the dimensionless wetting radius of the drop follows a universal form if the drop Weber and Reynolds numbers are sufficiently large. In the second phase, the drop becomes highly flattened and the values of the Weber and Reynolds numbers influence the time evolution of the dimensionless wetting radius and its maximum value. This is followed by a third phase in which the wetting radius begins to decrease with time and the wettability of the surface influences the dynamics. This paper presents simulation results for the early stages of drop impact and spreading on a partially wetting solid surface. The simulations were performed with a modified version of the lattice Boltzmann method (LBM) developed by Inamuro et al. (2004) for a liquid-gas density ratio of 1000. The Inamuro et al. version of the LBM was modified by incorporating rigid, no-slip boundary conditions and incorporating a boundary condition on the normal derivative of the order parameter to impose the desired equilibrium contact angle.     

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.