PHASE COEXISTENCE IN BINARY-MIXTURES IN THIN-FILMS WITH SYMMETRICAL WALLS - MODEL-CALCULATIONS FOR 2-DIMENSIONAL AND 3-DIMENSIONAL ISING LATTICES

Binary mixtures (A, B) that undergo phase separation in the bulk are c onsidered in thin film geometry, assuming that one of the components i s preferentially attracted to one of the walls. We discuss the average profile of the order parameter {volume fraction phi(z) of one of the components} in the z-direction (perpendicular to the surfaces), paying attention to the lateral inhomogeneity of the thin film when the aver age volume fraction corresponds to a state inside the coexistence curv e. We consider the situation where due to (short range) surface forces a second-order wetting transition would occur in semi-infinite geomet ry: in the thin film geometry, this transition is rounded off and its only remnant is a smooth increase of the adsorbed mass in the surface- enriched layer in the transition region. Monte Carlo calculations for nearest neighbor Ising square and simple cubic lattices are used to de rive typical concentration profiles. In the two-dimensional case, also the kinetics of domain formation after the quench from a disordered s tate is considered, and it is shown that the typical concentration osc illations perpendicular to the wall (''surface directed spinodal decom position'') do not occur, due to strong lateral fluctuations of the lo cal position of the interface between the enrichment layer at the surf ace and the neighboring depleted region. Finally, also phase-separated states in thin films with competing walls (where one surface prefers A and the other prefers B) are briefly treated, and experimental appli cations are discussed.