Spontaneous dewetting and ordered patterns in evaporating thin liquid films on homogeneous and heterogeneous substrates

The growth of instabilities and the initial stages of dewetting of volatile thin aqueous films on partially wettable solid substrates are investigated based on 2D nonlinear simulations. Dewetting by the formation of holes occurs by a spinodal mechanism due to the hydrophobic attraction on chemically homogeneous surfaces. The number density of holes and, consequently, the rate of dewetting can be enhanced by as much as an order of magnitude by evaporation on a homogeneous surface. At moderate to high rates of evaporation, all the holes do not form at the same time uniformly over the surface but form gradually in a rather ordered way around the earliest holes which act as "seeds". On a chemically heterogeneous substrate, spatial gradients of the interaction potential and the rate of evaporation engender the surface instability. A chemical heterogeneity can induce faster rupture at a higher mean thickness and, thus, control the hole size distribution and the pattern of drying very significantly. A locally ordered, complex pattern often forms that consists of a central giant "nucleated" hole surrounded by a few concentric rings of smaller spinodally created satellite holes. An increase in the rate of evaporation encourages the formation of a larger number of ringlike structures containing the satellite holes but reduces the size difference between the spinodal satellite holes and the heterogeneously nucleated holes. The results obtained are in accord with recent experimental observations.