Highly asymmetric bidisperse and polydisperse colloidal mixtures consisting of particles (or macromolecules) with very different sizes are common in both natural and industrial systems. Examples include groundwater (contains particles of various types and humic acids), industrial dispersions (e.g., titanium dioxide particles with anionic stabilizers), and numerous food products (e.g., milk products contain proteins, casein micelles, and emulsion droplets).

The prevalence of asymmetric colloidal mixtures continues to motivate numerous studies of the effective equilibrium interaction forces between particles in such systems. The most widely known example is the depletion attraction that arises between two large particles in a solution of much smaller particles. Far less understood, however, are the nonequilibrium properties of highly asymmetric binary dispersions. The dynamic behavior of the larger species in bidisperse mixtures is important for many applications, as the depletion interactions may be used to produce deposition, flocculation, gelation, or a phase change in a colloidal suspension.

Our goal is to develop a fundamental understanding of the dynamics of particle interactions in highly asymmetric binary colloidal mixtures.

Our approach involves derivation of a closed-form evolution equation for the large particles in the mixture and theoretical evaluation of the effective mobility tensor for near-contact motion of the particles. Our results will be compared with measurements performed in the laboratory of Prof. John Walz in the Department of Chemical Engineering. A calculation of the effective mobility tensor for the large particles in near contact requires an analysis of the motion of small Brownian particles in the gap between the surfaces of the large particles (or between a particle and a wall); see hydrodynamic confinement effects.

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