Basic Study for Micro Pump Using Electroosmotic Flow

The characteristic features of electroosmotic flow have been studied to obtain important information for applying the flow to a micro pump as a driving device. Here, an electroosmotic flow of water was generated in a donut channel constructed by a gap between two parallel donut-shaped glass plates. The flow rate was measured in relation to the applied voltage, the gap width of the channel, the pressure gradient and the properties of the fluid. The experimental results were compared with a theoretical equation of electroosmosis, the Helmholtz-Smoluchowski equation, which predicts that the electroosmotic flow rate is proportional to the applied voltage as well as to the gap width. The Electroosmotic flow rate increased linearly with the applied voltage. however, there appeared some particular voltage ranges of nonlinear relation unlike the Helmholtz-Smoluchowski equation. The water properties (the conductivity and the kinds of impurities included) had a great influence on the electroosmotic flow rate characteristics including the degree of non-linearity and even the flow direction. The cause of these phenomena has not been clarified. It is conjectured that the zeta potential of the glass-water interface was altered by the applied electric field. It is confirmed that the electroosmotic flow rate is proportional to the gap size of the channel. When a pressure gradient existed in the direction of the electroosmotic flow, the total flow rate was given as a sum of the flow rates of the electroosmotic and Poiseuille flows. These findings provide important information for micro pumps. For measuring the electroosmotic flow rate, we developed a micro flow meter consisting of a capillary and two fine wires. This flow meter makes it possible to measure a flow rate the order of 1×10⁻³ mm³/sec (= 1 nl/sec) and has a potential ability to measure even much smaller flow rates.