Instability and dewetting of evaporating thin water films on partially and completely wettable substrates

Abstract

Stability, dynamics and dewetting of thin (<100 nm) evaporating water films on partially and completely wettable substrates are studied based on numerical solutions of the nonlinear thin film equation, as well as by simplified semianalytical approaches. The instability and rupture of aqueous films are engendered by the hydrophobic attraction, whereas the net van der Waals force is repulsive for aqueous films on most substrates. An evaporating aqueous film on a partially wettable surface thins uniformly to a critical thickness, and then spontaneously dewets the substrate by the formation of growing holes. Complete nonlinear simulations as well as the linear analyses are used to predict the most important, experimentally accessible characteristics of the instability such as the length scale and time scale of the instability and the mean film thickness at the instant of rupture. Curiously, in contrast to nonthinning films, the number density of holes decreases slightly with increased strength and range of hydrophobic attraction, and also with decreased strength of LW repulsion, even though both of these factors promote the macroscopic nonwettability. The rate of evaporation has the most significant influence on the length scale, λ˜E^q, where the exponent, q lies in a narrow range from −0.17 to about −0.26, depending on the rate of evaporation and the critical thickness. Thin aqueous films on completely wettable (free energy per unit area is positive) surfaces are also unstable when the free energy does not decrease monotonically with the film thickness. Simulations show that instability in such cases leads to the formation of quasiequilibrium microscopic "islands" or "pancakes" made up of largely flat thin and thick films.