Horizontal drying fronts during solvent evaporation from latex films



Latex films cast on a substrate often dry nonuniformly, with a drying front separating fluid domains from solidified regions passing across the film. For initial film thicknesses that are smaller than the characteristic horizontal distance, the analysis predicts surface-tension-driven horizontal flow. In a limit that ensures vertical homogeneity it is shown how a front of close-packed particles forms and propagates. Imposing a maximum for the capillary pressure causes a solvent front to recede into the film. This recession is minimal, but can markedly affect the propagation of the particle front. An overall mass balance offers a solution for infinite capillary pressure, thereby illustrating the mechanism for propagation of the front. The positions of the fronts are predicted for both infinite and finite domains as a function of the maximum capillary pressure. Selective or nonuniform evaporation produces final film profiles, while the evaporating regions are still visible. After predictions over different size areas are made, the smallest area is compared with experiment.