ELECTROKINETIC LIFT OF A SPHERE MOVING IN SLOW SHEAR-FLOW PARALLEL TOA WALL .1. EXPERIMENT

In this paper we present experimental observations of the migration aw ay from a wall of 9.87- and 5.2-mu m-diameter polystyrene latex sphere s in solutions of varying viscosity and conductivity. During slow line ar shear flow, this lateral migration is diminished by an increase in the fluid conductivity caused either by adding salt to increase the nu mber of charge carriers or by adding water to decrease the viscosity. At a shear rate of 6 s(-1), for example, the largest conductivity for which lift of 5.2-mu m spheres is observed is 0.12 mu S cm(-1). In add ition, the increase in the particle-wall separation distance upon the initiation of flow generally increases with particle zeta potential fo r a given fluid conductivity. These observations support the hypothesi s that the migration is electrokinetically induced, but predictions of lift based on the current lubrication theory are more than one order of magnitude weaker than gravity. This disagreement has led us to refi ne the electrokinetic lift theory in the sequel to this paper. (C) 199 5 Academic Press, Inc.