Suppression of Electrokinetic Flows by Surface Roughness

In microfluidic systems, electro-osmotic flows are a promising alternative to mechanical pressure-driven flows, since electrokinetic flow rates are independent of channel dimensions and may enable the design of portable (e.g., battery-operated) devices. We show that nanoscale surface roughness, which commonly occurs on microfabricated metal electrodes, can dramatically suppress electro-osmotic flows when excess surface conductivity is appreciable. We identify the different regimes under which electro-osmotic flow suppression occurs, demonstrate the physical mechanisms of flow suppression due to surface curvature, and compute the effects of varying surface conductivity and roughness amplitudes on the slip velocities of a model system. We suggest that roughness may be one factor that accounts for large discrepancies between classical electrokinetic theory and modern microfluidic experiments.