Physics Department -  Stratification and Diffusiophoresis in Drying Suspensions of Colloidal Mixtures

Abstract
The complex interplay among several factors, including solvent flow, solute advection, solute diffusion, and different gradients, in the drying process of soft matter solutions often leads to interesting outcomes. The coffee-ring effect is a famous example. Recently, counterintuitive stratification phenomena, where one type of particle is enriched while the other type is depleted near the evaporation front, were discovered in drying suspensions of bidisperse mixtures of colloidal spheres. Through large-scale molecular dynamics (MD) simulations, we demonstrated that similar stratification also occurs in drying mixtures possessing sufficient shape dispersity. A unified framework, based on the competition between diffusion and diffusiophoresis where a colloidal particle migrates in response to the concentration of other solute particles, has emerged for understanding these stratification phenomena. Using MD simulations, we have found direct evidence for colloidal diffusiophoresis. Our simulations of the drying behavior of colloidal mixtures containing aspherical particles and two-dimensional suspensions of colloidal disks further reveal that diffusiophoretic mobility may depend on particle shape and a system’s dimensionality.