Sedimentation of Charged Colloids In Strong Gravitational Fields

The sedimentation of an electrically charged micron-sized particle is a fundamental problem in colloid science. The vast majority of previous theoretical work has focused on the “linear-response” limit, wherein the gravitational field is sufficiently weak (in an appropriate dimensionless sense) to only slightly perturb the equilibrium ionic screening cloud surrounding the colloidal particle. In this talk, we examine the motion of a charged spherical colloid under a “strong” gravitational field, which is relevant to ultra-centriguation and “streaming flows” in microfluidics, for instance.  Here, importantly, the screening cloud can be highly distorted.  We compute the sedimentation velocity of the particle over a range of field strengths and screening cloud thicknesses, via numerical solution of the linearized (i.e. small zeta potential) Poisson-Nernst-Planck equations. Time permitting, we will discuss the related, but significantly more challenging, problem of electrophoresis of colloids in strong electric fields.