Dynamical effects and phase separation in cooled binary fluid films

Abstract

We study phase separation in thin films using a model based on the Navier-Stokes Cahn-Hilliard equations in the lubrication approximation, with a van der Waals potential to account for substrate-film interactions. We solve the resulting thin-film equations numerically and compare to experimental data. The model captures the qualitative features of real phase-separating fluids, in particular, how concentration gradients produce film thinning and surface roughening. The ultimate outcome of the phase separation depends strongly on the dynamical back reaction of concentration gradients on the flow, an effect we demonstrate by applying a shear stress at the film's surface. When the back reaction is small, the phase domain boundaries align with the direction of the imposed stress, while for larger back-reaction strengths, the domains align in the perpendicular direction.