Process model for latex film formation: Optical clarity fronts

Horizontal drying fronts are often observed in latex films. Current underst anding is that thin films Horizontal drying fronts are often observed in la tex spread on a dry substrate at an edge first, due to the reduced height. Capillary pressure keeps the solvent at the film surface and evaporation co ntinues unhindered for some time. This capillary pressure, along with the p article-solvent interfacial tension, leads to deformation of particles and an increase in local volume fraction. At a volume fraction close to unity, the film becomes optically clear and, due to the lateral inhomogeneity, a f ront of optical clarity passes across the film. Recently there has been con siderable interest in drying fronts. They have been noticed by many includi ng Sheetz(1) and Croll.(2) Films drying in a petri dish are also reported t o have drying fronts moving from the center outwards, due to the reduced he ight in the center caused by the fluid meniscus. Winnik and Feng(3) propose d a drying process, shown in Figure 1, where propagation of the front is du e to evaporation from the consolidated region. We formulated this concept m athematically to examine horizontal compaction fronts for non-deforming par ticles(4) and found the front propagation to depend on the maximum capillar y pressure and initial film profile. Once particles come into contact within the film, they can deform at a rate that increases with temperature relative to the glass transition temperatu re (T-g) of the polymer.