Applications of AC and DC Electroosmotic Flows in Microchannels
| Autor: | Jia-Kun Chen, 陳佳堃 |
|---|---|
| Rok vydání: | 2008 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 96 This thesis presents the applications of electroosmotic flow (EOF) toward achieving easy fabrication and high performance operation of lab-on-chip (LOC). The primary parts of the thesis concern the DC EOF in a curved microchannel, in a zigzag microchannel, and with a sharp corner in a microchannel, as well as the AC EOF in a microchannel. Classic electrokinetic phenomena are discussed in Chapter 1, and the experiments are introduced in Chapter 2. The numerical investigation of electroosmotic flows driven by externally applied DC and AC electric fields in curved microchannels are given in Chapter 3. For the DC electric driving field, the velocity distribution and secondary flow patterns are investigated in microchannels with various curvature ratios. We use the Dean number to describe the curvature effect of the flow field in the DC electric field. The results show that the effect of the curvatures and the strengths of the secondary flows become stronger when the curvature ratio of C/A is smaller (where C is the radius of curvature of the microchannel and A is the half-height of the rectangular curved tube). For the AC electric field, the velocity distribution and secondary flow patterns are investigated for driving frequencies in the range of 2.0 kHz (Wo = 0.71) to 11 kHz (Wo = 1.66). The numerical results reveal that the velocity at the center of the microchannel becomes lower at higher AC electric field frequencies and that the strength of the secondary flow decreases. When the applied frequency exceeds 3.0 kHz (Wo = 0.87), vortices are no longer observed at the corners of the microchannel. Therefore, it can be concluded that the secondary flow induced at higher AC electric field frequencies has virtually no effect on the axial flow field in the microchannel. Chapter 4 presents numerical and experimental investigations concerning the mixing of electroosmotic flows 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 simulated 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 cleaning process. Accordingly, a zigzag microchannel with a 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. A scaling analysis shows that this corner geometry leads to an effective increase in the mixing length and the experimental results also show 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 superior to that of the conventional sharp-corner microchannel. Finally, the results of the 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. The aim of chapter 5 is the study of vortices occurred when sharp wedges are set in a micochannel where electroosmotic flow occurred: vortices are induced near the wedges when a DC electric field is 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. The formation of vortices is due to the concentration depletion in the microchannel, and they can be used as a micromixer. The experimental results show 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. The purpose of chapter 6 is to perform an experimental investigation into the micro-mixing capabilities of three different types of AC electroosmotic flow (ac EOF), namely capacitive charging (CC), Faradaic charging (FC), and asymmetric polarization (AP). The results show that vortex structures generated by the FC phenomenon are stronger than those induced by the CC mechanism and, therefore, provide an improved mixing effect. However, in the FC system, the frequency of the external AC voltage must be carefully controlled to avoid electrode damage as a result of Faradaic reactions. The experimental results indicate that the AP polarization effect induces more powerful vortex structures than either the CC method or the FC method, and therefore yields a better mixing efficiency. Two AP-based micromixers were 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. Thus, the results show that the device with an asymmetric electrode configuration represents a more suitable solution for micro-mixing applications in microfluidic devices. |
| Databáze: | Networked Digital Library of Theses & Dissertations |
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