Solitary waves on superconfined falling liquid films

Solitary traveling waves are prominent features covering the surface of a falling liquid film and are known to promote heat and mass transfer. We focus on the little studied case where they are subject to an extremely confined countercurrent gas flow, and we identify two secondary instabilities. At high gas velocities, a catastrophic instability develops, leading to flooding through wave reversal and liquid arrest. At lower gas velocities, an oscillatory instability occurs, producing a high-frequency periodic modulation of the wave height. Conjunction of this self-sustained oscillatory state and vortices forming in the liquid is shown to enhance mixing. We also show that the gas flow can cause extreme local film thinning, leading to almost dry patches where the liquid thickness is very small.