Gravitational settling in the electrophoresis of spheroids with small potentials in quiescent fluid

Abstract

Electrophoretic motion is analyzed for non-neutrally buoyant, spheroidal particles settling in an unbounded fluid otherwise at rest. The double-layer thickness is arbitrary, and the surface potential of the spheroid is assumed to be small so that the deformation of the double-layer is neglected. Examples are presented for spheroids when the applied electric field is perpendicular to the gravity. The electrophoretic motions are determined for spheroids of arbitrary aspect ratios at arbitrary orientations. Depending on their orientations the gravitational drift can contribute up to 10% of the observed particle velocity in the direction of the electric field. The gravitational contribution is maximum when the angle between the axisymmetric axis of spheroids and the gravity is pi/4. However, for Brownian spheroids with a uniform orientation distribution the sedimentation effects are averaged out, thus there is no net gravitational drift in the direction of the applied electric field. (C) 1997 Academic Press.