Is free surface free in micro-scale electrokinetic flows?

Abstract

When a rigid boundary is replaced by a free surface, or by a liquid-liquid interface, the shear stress is reduced near the boundary because the no-slip on the rigid boundary is relaxed. A nonzero fluid velocity exists on the free surface, and the flow rate would increase by the replacement. For electrokinetic flows, however, the replacement from rigid to free boundary can lead to an increase (decrease) in shear stress (flow rate). This counterintuitive effect of free surface is presented in this note for a generic electroosmotic flow in a microchannel. (C) 2010 Elsevier Inc. All rights reserved.