NOVEL PHASE-BEHAVIOR OF A CONFINED FLUID OR ISING MAGNET

The phase behaviour of a simple fluid or Ising magnet (at temperatures above its roughening transition) confined between parallel walls that exert opposing surface fields h2 = -h1 is found to be markedly different from that which arises for h2 = h1. Whereas critical wetting plays little role for confinement by identical walls, it is of crucial importance for opposing surface fields. Analysis of a Landau functional and other mean-field treatments show that if h1 is such that critical wetting occurs at a single wall (L = infinity) at a transition temperature T(w) then phase coexistence, for finite wall separation L, is restricted to temperatures T < T(c,L), where the critical temperature T(c,L) lies below T(w). In the temperature range T(c,b) > T > T(w) there is a single soft mode phase that is characterized, for zero bulk field and large L, by a + - interface located at the centre of the slit, a transverse correlation length xi(parallel-to) approximately e(L) and a solvation force that is repulsive. For large h1, T(w) can lie arbitrarily far below the bulk critical temperature T(c,b). Scaling arguments, whose validity we have confirmed in two dimensions by comparison with exact solutions for interfacial Hamiltonians. predict that such behaviour persists beyond mean-field for systems with short-ranged forces. They also predict similar phase behaviour for long-ranged forces, but with xi(parallel-to) increasing algebraically with L in the soft mode phase. The solvation force f(s) changes from repulsive to attractive (at large L) as the temperature is reduced below T(w) i.e. the sign of f(s) reflects wetting characteristics.