(98i) Forced Spreading and Evaporation of Films and Droplets of Colloidal Suspensions

When a thin film of a colloidal suspension flows over a substrate, uneven distribution of the suspended particles can lead to an uneven coating.  To gain insight into this phenomenon, we analyze the flow of films and droplets of colloidal suspensions down an inclined plane.  Lubrication theory and the rapid-vertical-diffusion approximation are used to derive a coupled pair of one-dimensional partial differential equations which describe the evolution of the liquid height and particle concentration.  A precursor film is used at moving contact lines, the colloidal particles are assumed to be hard spheres, and particle and liquid dynamics are coupled through a concentration-dependent viscosity and diffusivity.  We find that for sufficiently high Peclet numbers, even small initial concentration inhomogeneities produce viscosity gradients that cause the film front to evolve continuously in time instead of traveling without changing shape as happens in the absence of colloidal particles.  At high enough particle concentrations, particle diffusion can lead to the formation of long-lived secondary flow fronts. The influence of evaporation is also examined, and it is found that evaporation can strongly suppress the effects of viscosity gradients.  Our observations suggest a possible mechanism for the onset of patterns that are observed experimentally in flowing films of colloidal suspensions.