Phenomenological theory for the formation of interfaces via the interdiffusion of layers

Cahn-Hilliard theory is applied to the situation where a film rich in species A is brought on top of a film rich in species B, assuming that the binary phase diagram of the two species exhibits a miscibility gap. If the two films have concentrations exactly according to the coexistence curve, the initially sharp interface broadens with time (t) according to a t1/4 law and then exponentially saturates out to its equilibrium value. If the initial concentrations of the films exceed the coexistence-curve concentrations, the t1/4 law holds only initially and the late stages of the broadening of the interface follow a t1/2 law. Recent experiments are discussed in terms of these predictions.