Thin films dewetting with phase separation: Dependence of surface tension and Hamaker constant on concentration

Abstract

We study the instability of a thin film composed of two miscible fluids (binary fluid) placed on a solid planar surface. We include the fact that both the free surface and wetting energies depend on the mixture concentration. By assuming a linear relationship between these energies and both the bulk and surface concentrations, we analyze their effect on the phase separation of the constituent fluids. The problem is formulated within the gradient dynamics formulation applied to the thin film limit of the Cahn-Hilliard Navier-Stokes equations. The dependence of the free surface energy on concentration leads to a Marangoni type of effect, while the wetting energy resulting from fluid--solid interaction between the film and the substrate is described by a concentration dependent Hamaker constant. The linear stability analysis uncovers that both monotonous and oscillatory evolution is possible. While our problem formulation applies to any binary mixture that can be consistently modeled via the presented approach, a particular interpretation of the results is provided for the case of liquid metal alloy films on nanoscale. In this context, we find that rich dynamics is possible, including the evolution that may lead to formation of drops of the Janus--type as well as of core-shell configurations.