PATTERN-FORMATION IN DRYING WATER FILMS

A thick water film is deposited on a mica substrate, which it wets, fr om pure water vapour at a low temperature. The vapour pressure is redu ced so that the water evaporates. Despite the fact that under equilibr ium conditions the film wets the substrate, when it reaches a thicknes s of about 500 Angstrom, dry patches nucleate in the film and the wate r recedes to the wet parts. During the recession, a thick rim of water is created, which becomes hydrodynamically unstable and produces a be autiful reproducible pattern of water drops. Experiments have been don e showing both homogeneous and heterogeneous nucleation. The basic phy sics of the de-wetting the nucleation and the origin of the pattern fo rmation are discussed. The effect is not restricted to water on mica, but has been observed with other fluids on wetted substrates.