Diffusiophoresis and Auto Stratification in Rapidly Drying Colloidal Suspensions

Abstract

Because of the coupled effects of diffusion, advection, and phoretic transport under non-equilibrium conditions, polydisperse colloidal suspensions exhibit interesting auto-stratification phenomena during rapid drying, where particles of different sizes segregate into layered structures. The physical mechanism responsible for these phenomena is believed to be diffusiophoresis, which refers to the phoretic motion of a particle driven by the concentration gradients of other solutes in the suspension. Although molecular diffusiophoresis, where salts with a molecular size much smaller than the phoretic particles serve as the solutes, is well documented, colloidal diffusiophoresis in polydisperse suspensions displays new features that are not well understood. In this dissertation, a model is developed to enable a systematic study of colloidal diffusiophoresis with molecular dynamics simulation, where the solutes are also colloidal particles with sizes comparable to or even larger than the phoretic particles. The results reveal the effects of the strength of the concentration gradient and the size of the phoretic particles on their diffusiophoretic mobility, which provides a foundation to understand the auto-stratification phenomena in rapidly drying colloidal suspensions featuring size dispersity. Further simulations show that mixtures of particles with similar mass but different shapes also stratify upon rapid drying, alluding to the potential effect of particle shape on diffusiophoresis. Also, analytical interaction potentials are developed for disks in two dimensions (2D) based on the Lennard-Jones 12-6 potential. These potentials are used to study the behavior of 2D suspensions of disks, including their equilibrium and drying properties. Auto-stratification is found to also occur in 2D suspensions of bidisperse disks that are rapidly dried. This points to new features of colloidal diffusiophoresis in 2D suspensions. Finally, a facile strategy is proposed to use a binary mixture of solvents to induce and control the stratification of a binary mixture of colloidal particles suspended in the solvent mixture upon solvent evaporation. Overall, this dissertation provides a molecular understanding of colloidal diffusiophoresis and its relation to auto-stratification phenomena in rapid drying by revealing the effects of particle size and shape, system dimensionality, and solvent composition on diffusiophoresis.