We have observed flow-induced migration of a 10 mm sphere away from the wall during linear shear flow of pure glycerol (10 poise) along the wall at a shear rate of 6 sec-1. This apparent hydrodynamic lift is observed to weaken either with the addition of water to the glycerol (which decreases viscosity and increases conductivity) or with the addition of salt to the glycerol (which increases conductivity). Estimates of hydrodynamic lift caused by fluid inertia suggest inertial lift is negligible in these experiments.

In this talk, we will explore an electrokinetic mechanism for lift. In the absence of fluid motion, the sphere is electrostatically levitated above the wall at an elevation equal to a few percent of its radius. Sliding motion between the sphere and wall during linear shear flow causes convection of the diffuse counterion cloud adjacent to either surface. Conservation of charge requires an tangential electric field to arise (streaming potential) which in turn generates normal electrical stresses in the fluid. A lubrication analysis of these stresses reveal that the sphere is always pushed away from the wall by the streaming potential Ð even if the sphere and wall are oppositely charged. This electrokinetic lift is qualitatively consistent with all of the trends observed but quantitative estimates are a factor of 100 too small to produce the effects observed.