Dewetting patterns in a drying liquid film

Failure of a liquid coating to remain continuous on a substrate that exhibi ts a significant equilibrium contact angle is a common occurrence in indust rial applications. The term "reticulation" is sometimes used to describe th e resulting formation of a pattern of defects. The failure may take the for m of coating perforations and dewetting, and it may ultimately lead to a se t of isolated drops. We present mathematical and experimental results for r eticulation. The theoretical and numerical results use a disjoining-conjoin ing pressure model to represent the substrate energetics. The theory uses t he small-slope or "lubrication" approximation and also includes the effects of evaporation and drying of the coating. The model employs a two-componen t liquid where the viscosity depends on local values of the nonvolatile mix ture fraction. A linear analysis for a slightly perturbed uniform layer pre dicts a most-unstable wavelength and an associated growth rate. These are i n approximate agreement with the modeling results. Computations employing t he full nonlinear model show the wide variety of patterns that can arise in the drying liquid. These patterns are both qualitatively and quantitativel y 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. (C) 2001 Academic Press.