报告人：Dr. Zuli Xu （The Hong Kong University of Science and Technology）
This talk involves the theoretical and experimental study of electroosmotic pump (EOP) and its reverse effect, the generation of streaming potential through a pressure gradient across a porous sample. We designed and fabricated samples with various fluid channel diameters. Electroosmotic effect in a porous medium arises from the electrically charged double layer at the fluid-solid interface, whereby an external voltage can give rise to fluid flow. In particular, we use the Onsager relation to check the consistency of the two effects and to obtain a value for the surface conductivity of our samples. The results show that the efficiency of the EOP decreases with the increasing channel diameter, from 2.5 μm to 4.5 μm. This is opposite to the trend observed for samples with much smaller channel diameters (20~200 nm) fabricated on anodized aluminum oxide films. Thus there is an optimal diameter for achieving maximum efficiency in EOP, in agreement with the theoretical prediction that the best efficiency is at a Debye length that is on the order of 1/5 the diameter of the microchannel. We have also developed a digital approach to flow rate control in EOP, using pulsed voltage with varying duty cycles. This approach has shown much more stability and tunability at both high and low flow rates, thus making EOP better suited for various applications in micro/nano total analysis systems (μTAS/nTAS).