Fluid-fluid Potentials Research Articles

The dynamics approach to the wetting transition with short range forces

The generalized Sullivan model, in which the wall and fluid-fluid potentials are of exponential form, but with different renge, allows, within a Van der Walls framework, both continuous and first-order wetting transitions. This model is reconsidered on the basis of a formulation of the problem as one in classical dynamics. Recent non-trivial results on the locus of tricriticality, when the difference in range is small, are rederived from this point of view. The dynamics approach, in addition, allows a discussion of the order of transition when the difference in interaction range is not small. Padé approximants are used for an accurate determination of the locus of tricriticality in the high temperature region.

The interfacial properties of a model of a nematic liquid crystal

We have used the theory developed in the previous paper to study the induced orientational order at simple models of a solid-liquid crystal (LC) interface. The LC is modelled by a Maier-Saupe type of anisotropic pairwise potential while the solid is modelled by an impenetrable wall which exerts an anisotropic external potential on the fluid molecules. Our theory treats both the solid-fluid and fluid-fluid potentials on a microscopic level which allows us to study the role played by these two types of interactions in the alignment of the fluid molecules at the solid surface. In particular, we study in detail the nature of the wetting of the solid-isotropic liquid interface by the nematic phase. We illustrate this by presenting the results of numerical calculations for the density and orientational order parameter profiles at the wetting transition for two different models of the solid substrate. We find that due to the weakness of the first order N/I bulk transition, the surface phase transition for z -3 substrate potentials is also weakly first order, in contrast with the results for isotropic systems. For exponential solid substrates the transition is found to depend on the range of the solid-fluid interaction; for short-ranged potentials the transition is continuous whereas for longer-ranged potentials the transition becomes more strongly first-order as the range of the interaction increases. This type of behaviour is in agreement with the results for isotropic solid-fluid interfaces. We calculate the pre-wetting lines for the two solid-fluid models. We find that the structure of the co-existing films as well as the surface critical point depend rather strongly on the model for the solid-fluid interaction. Finally, we compare the results of our theory with the results obtained from a phenomenological one-order parameter Landau-de Gennes theory and with some recent experimental results. We assess their validity and possible range of application.

Wetting transitions at models of a solid-gas interface

Using mean field free energy functionals of the type employed by Sullivan, we have investigated the nature of the transition from partial to complete wetting that occurs at a solid-gas interface. We show that this wetting transition can be a first or a second order phase transition. If it is first order, as predicted by Cahn, the coverage diverges discontinuously and the temperature derivative of the interfacial tension changes discontinuously at the transition whereas if it is second order these properties change continuously, as found by Sullivan. For a given model system the order of the transition depends strongly on the range of the solid-fluid potential. When the solid-fluid and fluid-fluid potentials are to be taken to be exponentially decaying functions of distance the transition is second order if the range of the solid-fluid potential is smaller or equal to that of the fluid-fluid potential but may be first order when the solid-fluid potential is of longer range. For the more realistic model, a ...

Pairwise correlations and wetting transitions at a solid-gas interface

We summarize the main results of our study of the density-density correlation function for Sullivan's model of a gas adsorbed on a solid substrate. In the approach to complete wetting, when a thick film of liquid density is adsorbed at the substrate, long-ranged transverse (parallel to the surface) correlations develop at the edge of the film where the density profile is similar to that of a liquid-gas interface. For a class I wetting situation the range of the transverse correlations increases and ultimately diverges as the bulk gas pressure approaches the saturated vapour pressure. We comment on other situations where long-ranged correlations arise and mention the possibility of observing these in diffraction experiments and in computer simulations. Sullivan's model always predicts a second-order phase transition between class II and class I wetting. By extending his model and allowing the attractive part of the solid-fluid potential to be longer-ranged than the attractive fluid-fluid potential we find that this wetting transition can become a first-order (Cahn) transition.