Stratification in Drying Colloidal and Polymer Films*

Drying of colloidal and polymer films is a ubiquitous nonequilibrium process where evaporation, fluid dynamics, diffusion, phoretic motion, and capillarity are coupled. Recently, various auto-stratification phenomena have been discovered when polydisperse colloidal suspensions are dried rapidly. We employ molecular dynamics simulations to model drying solutions of colloids, polymers, and their mixtures. Our purpose is twofold: first, to elucidate the physical mechanism underlying the stratification phenomena; and secondly, to explore various means of controlling the resulting stratified structures. For suspensions of bidisperse particles, we observe the counterintuitive “small-on-top” stratification when the suspensions are dried quickly. Our results generally support the diffusiophoretic mechanism of stratification but the drying process can be complicated by the emergence of temperature gradients during rapid solvent evaporation. The corresponding thermophoretic response of colloidal particles is size-dependent and can be utilized to control the structure of the resultant stratified film by imposing externally controlled temperature gradients. Stratification can also be induced and controlled using a binary solvent with the two components having different volatilities. During fast evaporation, the two components develop opposite concentration gradients with the less volatile one enriched at the evaporating front. These concentration gradients can be used to stratify colloidal particles that have contrasting preference to the solvent components, even when the particles are identical otherwise. The stratification behavior also occurs in drying suspensions of polymer-particle mixtures, which leads to useful strategies of fabricating polymer nanocomposites with controlled dispersion of filler particles.