WETTING AND CAPILLARY CONDENSATION OF LATTICE GASES IN THIN-FILM GEOMETRY

Monte Carlo studies of lattice gas models with attractive interactions between nearest neighbors on a simple cubic lattice are carried out for a L x L x D geometry with two hard walls of size L x L and periodic boundary conditions parallel to the wall. Two types of short-range forces at the walls are considered: (i) Both walls are of the same type and exert an attractive force of the same strength (in Ising model terminology, surface fields H(D) = H-1 occur). (ii) The walls differ, one attracts and the other repels particles, again with the same strength (H(D) = -H-1). In the first case, capillary condensation occurs at a chemical potential differing from its value for phase coexistence in the bulk, and the (second-order) wetting transition that occurs for D --> infinity is rounded off. In the second case, an interface parallel to the walls is stabilized and we observe the interface delocalization transition predicted by Parry and Evans. A first attempt to study the nature of this ''quasi wetting transition'' by finite size scaling methods is reported, and discussed in the context of recent theories.