SiO2-coated porous anodic alumina membranes for high flow rate electroosmotic pumping

Electroosmotic pumping has been extensively used in lab-on-a-chip devices and micropumps for microelectronic cooling. High flow rate per unit area with a low applied voltage is a key performance requirement to achieve compact design and efficient operation. In this paper, we report work on using SiO2-coated porous anodic alumina membranes for high flow rate electroosmotic pumping under low applied voltages. High quality porous alumina membranes of controllable pore diameters in the range of 30-100 nm and pore lengths of 60-100 mu m were fabricated by electrochemical anodization. The pores are straight, uniform and hexagonally close-packed with a high porosity of up to 50% of the total area. The inner surface of the pore was coated conformally with a thin layer (similar to 5 nm) of SiO2 to achieve a high zeta potential. The electroosmotic pumping performance of the fabricated anodic alumina membranes, coated and uncoated, was investigated using standard relevant aqueous electrolyte buffer solutions. The high zeta potential of the SiO2 coating increases the pumping flow rate even though the coating reduces the porosity of the membrane. Results show that nanostructured SiO2-coated porous anodic alumina membranes can provide a normalized flow rate of 0.125 ml min(-1) V-1 cm(-2) under a low effective applied voltage of 3 V. This compares favourably with other microporous materials such as glass frits.