Phase separation of binary mixtures in thin films: Effects of an initial concentration gradient across the film

We study the kinetics of phase separation of a binary (A,B) mixture confined in a thin film of thickness D by numerical simulations of the corresponding Cahn-Hilliard-Cook (CHC) model. The initial state consisted of 50% A:50% B with a concentration gradient across the film, i.e., the average order parameter profile is Psi(av)(z, t = 0) = (2z/D - 1)Psi(g), 0 <= z <= D, for various choices of Psi(g) and D. The equilibrium state (for time t -> infinity) consists of coexisting A-rich and B-rich domains separated by interfaces oriented perpendicular to the surfaces. However, for sufficiently large Psi(g), a (metastable) layered state is formed with a single interface parallel to the surfaces. This phenomenon is explained in terms of a competition between domain growth in the bulk and surface-directed spinodal decomposition (SDSD) that is caused by the gradient. Thus, gradients in the initial state can stabilize thin-film morphologies which are not stable in full equilibrium. This offers interesting possibilities as a method for preparing novel materials.