Phase separation patterns in irradiated thin liquid films due to optical interference effects

The pattern formation in irradiated thin liquid films on solid substrates is investigated within the framework of the long-wave approximation. The focus is placed on a transparent film/absorbable substrate system irradiated by a monochromatic wave with laterally uniform intensity distribution. The evolution of the film surface profile is described by an equation of Cahn-Hilliard type, and the free energy density that is a function of the film thickness has local minima due to optical interference between waves reflected from the gas-liquid and liquid-solid interfaces. Therefore, a small perturbation develops into a phase separation pattern if the unperturbed uniform state is unstable.