In this study we performed numerical and experimental investigations into the mixing of EOFs in zigzag microchannels with two different corner geometries, namely sharp corners and flat corners. In the zigzag microchannel with sharp corners, the flow travels more rapidly near the inner wall of the corner than near the outer wall as a result of the higher electric potential drop. The resulting velocity gradient induces a racetrack effect, which enhances diffusion within the fluid and hence improves the mixing performance. The simulation results reveal that the mixing index is approximately 88.83%. However, the sharp-corner geometry causes residual liquid or bubbles to become trapped in the channel at the point where the flow is almost stationary, when the channel is in the process of cleaning. Accordingly, a zigzag microchannel with flat-corner geometry is developed. The flat-corner geometry forms a convergent-divergent type nozzle which not only enhances the mixing performance in the channel, but also prevents the accumulation of residual liquid or bubbles. Scaling analysis reveals that this corner geometry leads to an effective increase in the mixing length. The experimental results reveal that the mixing index is increased to 94.30% in the flat-corner zigzag channel. Hence, the results demonstrate that the mixing index of the flat-corner zigzag channel is better than that of the conventional sharp-corner microchannel. Finally, the results of Taguchi analysis indicate that the attainable mixing index is determined primarily by the number of corners in the microchannel and by the flow passing height at each corner.
A special phenomenon was found when sharp wedges were set in a microchannel where electroosmotic flow occurred, vortices were induced near the wedges when a DC electric field was imposed. The strength of the induced vortices depends on the concentration of electrolytes and the intensity of the electric field. Latex particles are used to aid the flow visualization. Formation of vortices is due to concentration depletion in the microchannel. Furthermore, the vortices are used to enhance mixing in a micromixer. Experimental results showed that the vortex structures created within the mixing section increase the mixing index from a value of 3% in the upstream region of the microchannel to 78% at the outlet of the mixing section.
This study performs an experimental investigation into the micromixer capabilities of three different protocols of AC electroosmotic flow (AC EOF), namely capacitive charging (CC), Faradaic charging (FC) and asymmetric polarization (AP). The results reveal that the vortices generated by the FC protocol (the frequency is around 50-350 Hz) are stronger than those induced by the CC protocol (the frequency is higher than 350 Hz), and therefore provide an improved mixing effect. However, in the FC protocol, the frequency of the external AC voltage must be carefully controlled to avoid damaging electrodes as a result of Faradaic reactions. The experimental results indicate that the AP polarization effect (the applied voltage and frequency are V(1) = 1 V(pp) and V(2) = 20 V(pp)/5 kHz) induces more powerful vortices than either the CC protocol or the FC protocol, and therefore yields a better mixing performance. Two AP-based micromixers are fabricated with symmetric and asymmetric electrode configurations, respectively. The mixing indices achieved by the two devices after an elapsed time of 60 seconds are found to be 56.49 % and 71.77 %, respectively. This result shows that of the two devices, an asymmetric electrode configuration represents a more suitable choice for micromixer in microfluidic devices.
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