PATTERN-FORMATION IN DRYING WATER FILMS

A film of a volatile medium in contact with unsaturated vapor is bound to a plane substrate by both van der Waals and polar forces. We prese nt a thermodynamic description of this system, including exchange of m aterial between the fluid and the vapor under nonequilibrium condition s. For a range of values of the vapor pressure, a two-phase system dev elops, involving the coexistence of molecularly thin and macroscopical ly thick layers whose dynamics are controlled by the vapor pressure. T he theory is used to explain the origin of experimentally observed spa tial patterns in water films evaporating from clean mica substrates, w here scale and other features are a function of the vapor pressure. An analogy is developed between the initial stages of the pattern format ion and the diffusion-controlled solidification problem in two dimensi ons and typical features such as ''doublons'' or parity-broken dendrit es are observed. At later stages in the pattern formation hydrodynamic s becomes important.