Dewetting Patterns in a Drying Liquid Film.

Abstract

Failure of a liquid coating to remain continuous on a substrate that exhibits a significant equilibrium contact angle is a common occurrence in industrial applications. The term "reticulation" is sometimes used to describe the resulting formation of a pattern of defects. The failure may take the form of coating perforations and dewetting, and it may ultimately lead to a set of isolated drops. We present mathematical and experimental results for reticulation. The theoretical and numerical results use a disjoining-conjoining pressure model to represent the substrate energetics. The theory uses the small-slope or "lubrication" approximation and also includes the effects of evaporation and drying of the coating. The model employs a two-component liquid where the viscosity depends on local values of the nonvolatile mixture fraction. A linear analysis for a slightly perturbed uniform layer predicts a most-unstable wavelength and an associated growth rate. These are in approximate agreement with the modeling results. Computations employing the full nonlinear model show the wide variety of patterns that can arise in the drying liquid. These patterns are both qualitatively and quantitatively similar to actual patterns that we observe experimentally. Small defects that are visible in the experiment are used to initiate reticulation in the numerical simulation. Copyright 2001 Academic Press.