In this paper we present experimental observations of the migration away from a wall of 9.87 - and (5.2-\mu \mathrm{m})-diameter polystyrene latex spheres in solutions of varying viscosity and conductivity. During slow linear shear flow, this lateral migration is diminished by an increase in the fluid conductivity caused either by adding salt to increase the number of charge carriers or by adding water to decrease the viscosity. At a shear rate of (6 \mathrm{~s}^{-1}), for example, the largest conductivity for which lift of (5.2-\mu \mathrm{m}) spheres is observed is (0.12 \mu \mathrm{S} \mathrm{cm}^{-1}). In addition, the increase in the particle-wall separation distance upon the initiation of flow generally increases with particle zeta potential for a given fluid conductivity. These observations support the hypothesis 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 refine the electrokinetic lift theory in the sequel to this paper. O 1995 Academic Press, Inc.
Key Words: electrokinetic lift; sphere-wall interactions; linear shear flow; zeta potential; particle migration.