Electro-osmotic pumping on application of phase-shifted signals to interdigitated electrodes

Alternating-current (ac) electro-osmotic flow can be implemented in microfluidic pumping systems with the advantage over conventional electro-osmosis that it requires low applied potentials that can be supplied by normal batteries so that it is highly suitable for use in portable lab-on-a-chip systems. We report how the combination of applying phase shifted signals to the interdigitated electrodes and the use of a ground layer causes ac electro-osmotic flow of aqueous electrolytes. The direction of pumping may be changed by varying the phase difference. Velocities up to 100 μ m/s were observed for applied potentials of less than 1 Vrms. The maximum velocity was observed at phase differences of around ±90°. How surface conductivity affects ac electro-osmosis can be inferred from the effect of varying electrolyte concentration on the frequency of peak velocity.