Interfacial dynamics of a confined liquid-vapour bilayer undergoing evaporation

The dynamics of an interface between a thin liquid-vapour bilayer undergoing evaporation is studied. Both phases are considered to be hydrodynamically and thermally active, with momentum and thermal inertia taken into account. Areduced-order model based on the weighted-residual integral boundary layer method is used to investigate the dynamical behaviour for two cases, viz., phase change in the absence of gravity and then phase change in the presence of gravity. In the first case, it is shown that evaporative instability may cause rupture of either liquid or vapour layer depending on system parameters. Close to interfacial rupture, the disjoining pressure due to intermolecular forces results in the formation of drops (bubbles) separated by a thin film for low liquid (vapour) hold-up. Momentum inertia is shown to have a stabilizing effect, while thermal inertia has a destabilizing effect. In the second case, evaporative suppression of Rayleigh-Taylor (R-T) instability shows emergence of up to two neutral wavenumbers. Weak nonlinear analysis of these neutral wavenumbers suggests that the instability may be either supercritical or subcritical depending on the rate of evaporation. At high rates of evaporation, both neutral wavenumbers are supercritical and computations on the interface evolution lead to nonlinear saturated steady states. Momentum inertia slows down the rate of interface deformation and results in an oscillatory approach to saturation. Thermal inertia results in larger interface deformation and the saturated steady state is shifted closer to the wall. At very low evaporation rates, only one neutral wavenumber of subcritical nature exists. The nonlinear evolution of the interface in this case is then similar to pure R-T instability, exhibiting spontaneous lateral sliding as it approaches the wall.