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

Abstract

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 µm 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 (∼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.

High-performance, low-voltage electroosmotic pumps with molecularly thin silicon nanomembranes

“…In Table 1 we show that this figure of merit exceeds low-voltage EOPs fabricated from porous silicon (10), alumina (11), and track-etched polymer (14) by three orders of magnitude. Normalized flow rates for ultrathin pnc-Si EOPs are also 20 times higher than carbon nanotube (CNT) EOPs, which are considered to have unique properties due to a slippery surface (15).…”

“…Note that only a small portion of the applied voltage V app falls across the membrane. One method to determine the transmembrane voltage V TM is through the electrical resistance in the system (11,16),…”

“…For these reasons, the development of a low-voltage EOP is a current focus in the literature. Several recent low-voltage EOPs have been fabricated from porous silicon (10), alumina (11)(12)(13), track-etched polymer (14), and carbon nanotube membranes (15). These low-voltage EOPs are much thinner than their highvoltage predecessors (60-350 μm compared with >10 mm).…”

Recent advances in microscale pumping technologies: a review and evaluation

“…The packed beads increase the interfacial area available for EO flow, thereby increasing maximum pressure; pressures above 2 MPa have been reported (Zeng et al 2001). As an alternative to the use of packed beads, an innovative EO pump method using porous anodic alumina (PAA) was reported (Vajandar et al 2007). This pump had the highest normalized flow rate as compared to any previous EO pump at lower voltages.…”

Electroosmotic flow in nanoporous membranes in the region of electric double layer overlap

“…The authors also found that the flow rate decreased with decreasing porosity, as expected. The effect of surface charge on EOF was observed with a two-orders-of-magnitude increase in the maximum flow rate for SiO 2 -coated AAMs compared to uncoated AAMs using a borate buffer (Vajandar et al 2007). Another study looked at the effect of three different AAM pore diameters of 25, 46 and 240 nm) on ion transport and found that ion diffusion coefficients decreased with decreasing pore diameter for ions with the same charge as the pore wall (Romero et al 2012).…”