Patterning Electro-osmotic Flow with Patterned Surface Charge

This Letter reports the measurement of electro-osmotic flows (EOF) in microchannels with surface charge patterned on the $200\ensuremath{\mu}\mathrm{m}$ scale. We have investigated two classes of patterns: (1) Those in which the surface charge varies along a direction perpendicular to the electric field used to drive the EOF; this type of pattern generates multidirectional flow along the direction of the field. (2) Those in which the surface charge pattern varies parallel to the field; this pattern generates recirculating cellular flow, and thus causes motion both parallel and perpendicular to the external field. Measurements of both of these flows agree well with theory in the limit of thin double layers and low surface potential.