(749g) Stratification Dynamics in Drying Colloidal Mixtures

Drying colloidal mixtures can undergo a spontaneous, nonequilibrium vertical separation (stratification) into layers enriched in particles of a specific size, an effect which can be exploited to create functional coatings. It was recently observed in experiments and simulations that for large particle size ratios and high drying rates, smaller colloids migrate to the top of the film, while big colloids are pushed to the bottom, creating a peculiar and counterintuitive “inverted” stratification. To investigate this behavior, we performed implicit-solvent molecular dynamics simulations of binary colloidal mixtures for a wide range of particle size ratios and drying rates, and quantified the stratification dynamics in detail. We found that the stratified layer of small colloids grew faster and to larger thicknesses for larger size ratios. Interestingly, inverted stratification was observed even at moderate drying rates, but the thickness of the stratified layer decreased. We propose a model based on dynamical density functional theory which captures the salient features of our simulations and can help to explain the observed phenomena.