Near-wall effects on suspended particle electrokinetics

Colloidal polystyrene particles in a very dilute (< 0.5 vol%) suspension become attracted towards, and accumulate in, the high shear regions near the wall, when the particles are subject to pressure and voltage gradients along the same direction.  This behavior is qualitatively similar to inertial migration, although the particles lead the flow in this case due to electrophoresis. Estimates based on tracking tracer particles that are about 1% of the radius a = 250 nm particles imply that near-wall particle concentrations grow exponentially by a factor of 100-200 as the particles are attracted to the wall, increasing the near-wall concentration to at least 25 vol%.  The time scales for accumulation suggest that the attractive force is comparable to that predicted by recent models of phoretic or weakly inertial lift.  Measurements of streamwise near-wall particle velocities, even when the average interparticle spacing exceeds 20a, differ significantly from the sum of the flow and particle electrophoretic velocities.  If the flow velocity is equal to what would be predicted by theory, these results suggest that wall effects suppress particle electrophoresis by a factor of 2-3, in disagreement with models that predict that wall effects enhance electrophoresis.