Electroosmotic flow (EOF) in microchannels induced by the electric field could purvey attractive implements to control the fluid motion in microfluidic devices such as LOCs. Most of electroosmotic flows in microchannels is laminar and the mixing occurs very slowly. In this paper, a new method is suggested to enhance the mixing in electroosmotic microchannels. In our preliminary experiments on an electroosmotic microchannel made of PDMS/glass and having the hydraulic diameter of about 150μm, interesting phenomena happened to be observed when a periodical DC voltage was applied to the secondary anode located at the middle of an electroosmotic microchannel. More specifically, a inverse flow was observed in the center area of the channels especially between the secondary anode and the existing cathode electrode when the secondary DC voltage was applied; and the flow recovered the initial direction when the secondary voltage was stopped. The reverse flow in the center area seemed to occur due to the combination of the increased flow velocity near the wall and the repulsive force which prevents the cations between the primary and secondary anode from flowing to the same direction as the primary osmotic flow. It is thought that these unsteady flow phenomena could be used to improve the mixing which is often required in microfludic devices. The effect of important parameters, such as the frequency of the secondary voltage and the location of the secondary anode, will be further investigated and discussed in detail.

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