Wetting transitions at fluid-fluid interfaces

Abstract

Using a simple model free energy functional we have shown that the transition from partial to complete wetting which occurs at the αγ interface for a binary mixture of phase separated liquids (β and γ) in equilibrium with their vapour (α) can be a first order surface phase transition, as predicted by Cahn, or a second order surface phase transition. For the single bulk phase diagram that we consider the order of the transition depends on the particular model which is assumed for the attractive part of the intermolecular pairwise potentials. If these potentials decay exponentially with distance the transition is second order and the density profiles at the αγ interface change continuously, i.e. the wetting (β) film grows continuously, at the wetting transition. For inverse sixth power law attractive potentials the transition is first order and the structure of the αγ interface changes discontinuously at the transition—provided the range of the potential between unlike species is longer than that between like species, otherwise it is second order. The results of our calculations of the various interfacial tensions suggest that it would be difficult to determine the order of the wetting transition in real fluids from surface tension or contact angle measurements. It might be more practicable to focus experimental attention on the relative adsorption Γ_1,2 at the αγ interface in the partial wetting regime. Γ_1,2 will diverge continuously in the approach to the transition if this is second order whereas it will exhibit a discontinuous divergence for a first order wetting transition.