Wetting transitions at models of a solid-gas interface

Abstract

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 Lennard-Jones 12–6 fluid at a Lennard-Jones 9–3 substrate, the results of numerical calculations and of an approximate analytical theory indicate that the wetting transition is always first order.