Nonuniformities in miscible two-layer two-component thin liquid films

Depositing and obtaining a liquid film of uniform thickness is a problem integral to numerous applications and requires an understanding of capillary leveling, Marangoni flows, evaporation, and various other phenomena. These applications often demand multilayer films where each layer has distinct properties, and this gives rise to additional challenges. It has been experimentally demonstrated that two-layer films in which the layers are miscible can undergo dewetting, but theoretical understanding of this phenomenon is lacking. Through a lubrication-theory-based model, this work studies the mechanisms that may initiate dewetting in miscible two-layer two-component films. The model film consists of nonvolatile solvent and solute with constant density and viscosity. Two coupled fourth-order nonlinear partial differential equations describing the time evolution of the film height and solute concentration are derived and then solved with a pseudospectral method. It is found that a disparity in initial solute concentration between the film layers drives flows that lead to significant film-height nonuniformities. A parametric study is conducted to examine the influence of system parameters on this behavior and to develop several scaling relations that shed light on the underlying physical mechanisms.