Interdisciplinary Applied Mathematics

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AC Electroosmosis

AC electroosmosis is an important electrokinetic effect that can be utilized for particle manipulation and separation, as well as for flow pumping, and mixing enhancement applications. An advantage of AC electroosmosis is observed in designing electroosmotic pumps. Although DC electroosmotic pumps utilize large electric fields (see Table 7.1), AC electroosmotic pumps utilize low voltages (< 10 V) due to the close proximity of the electrodes, and they can maintain flowrates on the order of 1 to 10 mm3/s (see Chapter 8 in Morgan and Green, 2003). For particulate flows, AC electroosmosis is often accompanied by dielectrophoresis (see Section 7.6).

AC electroosmosis is observed when an AC electric field is applied on two electrodes that are placed next to each other, as shown in Figure 7.9. Electric field lines near the electrode surfaces display tangential components, especially in the vicinity of the two electrodes. Interactions of the tangential electric field with the induced charges on each electrode (schematically shown on the figure with + and — signs) result in AC electroosmotic force and fluid flow. Since the tangential electric field is larger near the two neighboring edges of the electrodes, the electroosmotic velocity is also large at these locations, and the velocity decays away from these neighboring edges. Here we must note that changing the electrode polarity in Figure 7.9 does not change the direction of the AC electroosmotic force. Therefore, AC electroosmosis results in unidirectional flow. In a series of papers, Green and coworkers presented experimental measurements, theoretical analysis, and numerical simulation of AC electroosmotic flows (Green et al., 2000; Gonzalez et al., 2000; Green et al., 2002). The experimental results have shown that AC electroosmosis is effective in the 10 Hz to 100 kHz frequency range, after which electrothermal/Joule heating effects (Section 7.4.6) become dominant. The electric field frequency greatly affects the electroosmotic velocity. For example, using a field of 1.25 V at electrolyte conductivity of 8.6 mS/m, velocity magnitudes of 75, 300, and 50 p,m/s

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