INDUCED CHARGE ELECTRO-OSMOSIS : THEORY AND MICROFLUIDIC APPLICATIONS

Electrokinetic techniques are one of the most popular of non-mechanical fluid manipulation strategies for microfluidic flows (e.g. [1]). This popularity stems in part from the fact that many microfluidic applications naturally integrate electronic circuits for electrophoretic separations. Electro-osmotic flows are further favored in that they allow dispersionless plug flows (as opposed to parabolic pressure-driven flow) and because the large surface/volume ratios of microchannels suggest surface-driven flows. However, electro-osmosis has drawbacks: they depend sensitively on solution pH, salt concentration, and surface charge densities, and electrochemical (Faradaic) reactions are required to maintain a constant electric field, which introduces bubbles, metallic ions, and concentration gradients. Furthermore, high voltages are required to apply appreciable fields over significant distances. In traditional electro-osmosis, an applied field acts upon the diffuse cloud of counter-ions surrounding a charged surface. The resulting body force drives the fluid into motion, resulting in Smoluchowski’s ‘slip velocity’