(460c) Colloidal Dynamics Under Confinement: Effects of Particle Softness

The presence of a solid wall imposes constraints on the flow field and affects the hydrodynamic mobility of a particle. As a result, Brownian diffusion of colloids is suppressed significantly near walls. For hard, spherical colloids, confinement effects have previously been studied experimentally and theoretically. However, for soft colloidal systems they are relatively unexplored, in spite of their abundance and relevance for various important applications: droplet flow in microfluidics, cells and nanoparticles in microvascular networks, and multiphase flow in porous media, such as in enhanced oil recovery.

We therefore present an experimental study of the hindered Brownian diffusion of various types colloidal particles under confinement between parallel plates: hard spheres, droplets and microgels. These experiments provide new insight into the fundamental hydrodynamic interactions that exist in colloidal systems. In particular, the aim was to investigate the effects of interfacial mobility and internal flow inside emulsion droplets, and the porosity of microgel particles on their respective mobilities near flat solid substrates.

Hindrance effects on colloidal mobility were quantified by measuring diffusion coefficients of confined suspensions using particle tracking video microscopy in a quasi-2D confinement cell. For polystyrene hard spheres, hindrance effects were found to be in excellent agreement with analytical predictions from literature across a much wider range of confinement than was previously explored. As incompressible, deformable droplets we used monodisperse, toluene-swollen polystyrene spheres: due to their deformability and potential internal flow, these soft spheres were found to exhibit significantly greater mobility under confinement than hard spheres. By varying the swelling ratio, diameter of swollen sphere normalized to original polystyrene particle, the mobility of the swollen sphere can be tuned between hard sphere and fully swollen droplet. Mobility enhancements of similar magnitude were observed in core-shell microgel particles with a porous outer layer, which enables liquid flow through these particles. We varied the crosslink density in the hydrogels and the ratio between core diameter and hydrogel layer thickness to determine the key contributing factors towards mobility enhancement.