GRAVITATIONAL INSTABILITIES OF THIN LIQUID LAYERS - DYNAMICS OF PATTERN SELECTION

We present recent results obtained in the study of the instabilities induced by gravity on liquid layers hanging below solid surfaces, in situation of perfect wetting (without contact lines). Two cases are discussed: two-dimensional films of constant volume, and films continuously supplied with ''fresh'' liquid by an external source. In the first case, we show experimental evidences suggesting that the formation of a two-dimensional structure within the film arises by means of front propagation mechanisms. In the simplest situation, propagation of a one-dimensional structure (''roll'' formation), the measured front speed is close to that deduced from the linear marginal stability theory. In the second case, the liquid film is hanging below an horizontal overflowing half-cylinder. Depending on the rate of supply, different regimes are observed (dripping, arrays of parallel jets, triangular sheets). The arrays of drops and jets exhibit interesting spatiotemporal phase dynamics: oscillations, pairing or nucleation of cells, forced tilt waves.