Rayleigh-Marangoni oscillatory instability in a horizontal liquid layer heated from above: coupling and mode mixing of internal and surface dilational waves

An oscillatory instability mechanism is identified for a horizontal liquid layer with undeformable open surface heated from the air side. Although buo yancy and surface tension gradients are expected to play a stabilizing role in this situation, we show that, acting together, they may lead to the ins tability of the motionless state of the system. The instability is a conseq uence of the coupling between internal and surface waves, whose resonant in teraction and resulting mode mixing are discussed. Predictions amenable to experimental test are given together with a thorough analytical and numeric al study of the problem.