| Summary: | 碩士 === 國立臺灣大學 === 應用力學研究所 === 99 === Mixing is an important topic in micro-fluidic systems. We studied here an electro-thermal micro mixer via both theoretical simulation and experiment. The mixer is essentially a Y-type micro channel with designed electrodes built on the bottom wall of the straight main channel. When two striped electrodes (with width 100μm here) are placed with their ends closed to each other (30μm spacing in the present study), we observed four vortices in the electrolyte above the four corners of the electrodes when they are powered by ac voltages with 180 degree phase difference. The vortices are flat but three dimensional and their senses are counter rotating for any two neighboring vortices. Many pairs of such electrodes are built as arrays and arranged in a saw-tooth manner on the bottom wall of the channel. Those vortices are electro-thermal in origin according to a theoretical study, and were employed to perform active mixing of two different fluid streams entering the Y-type channel. The flow structure was studied quantitatively via numerical calculation with the aid of the COMSOL software, and via experiment using PIV technique. The general feature of the flow according to the image of the motion of the suspended particles agrees qualitatively and fairly quantitatively with the calculation. In particular, we compared the fluid velocity on the vertical mid plane of the electrode gap region obtained from both calculation and PIV measurement. Effects of applied voltages (from 8 to 12 volts peak-to-peak) and electrolyte conductivity (from 0.1 to 1 S/m) were also studied. The strength of the vortices increases rapidly with the applied voltage and electrolyte conductivity. Different electrode shapes were also studied numerically, and it was found that the rectangular shape is more favorable for generating stronger vortices.
Mixing experiment were carried out using solutions with fluorescent and dark blue dye feeding separately into the two inlets (with width 300 μm, height 30 μm, flow rate 0.25 μl/min) of the Y-channel. Mixing occurs above the electrode region of the main channel with 400μm in width and 30μm in height at a total flow rate 0.5μl/min (averaged speed is 694μm/s, Reynolds number is 0.039). The mixing indexes increase with the applied voltage and the electrolyte conductivity, and the voltage effect is more significant. Mixing index reaches 81% under 12V for an electrolyte with conductivity 1 S/m.
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