| Summary: | 碩士 === 國立成功大學 === 化學工程學系碩博士班 === 96 === In this thesis, I experimentally investigate effects of asymmetric polarization on the motions of particles and fluids under high-frequency ac electric fields. Two strategies are invoked in this work, and constitute the main body of this thesis.
In the first part of the thesis, I employ a series of asymmetric coplanar electrode pairs to design an interdigitated microelectrode array and observe the motion of submicron latex particles in a deionized water subjected to ac electric fields. At high frequencies (≧1MHz), I find particle aggregation of particles due to negative dielectrophoresis and the aggregation occurs near the inner edges between an electrode pair. At low frequencies such as 10kHz~100kHz, the prevailing asymmetric ac electro-osmotic flow leads to rapid particle trapping on the smaller electrodes while the aggregation on the wider ones becomes sluggish.
In the second part of the thesis, I adopt an oblique electrode array to see how it affects the motion of silica particles in ac electric fields. At high frequencies (≧1MHz), I again find negative dielectrophoretic aggregation of the particles. And yet, I find that the larger tilt angle the more aggregated particles, suggesting that the induced dipoles of the particles and the applied field must be disaligned by the oblique electrode arrangement. I also find that the aggregation is rapidly re-suspended by an ac electro-osmotic flow, due possibly to Faradaic charging, at large voltage like 50 Vpp. At moderately high frequencies (1kHz~100kHz), the phenomena are dominated by electrokinetic flow due to Ohmic charging, and exhibit rapid spiral fluid motion toward the sidewalls or pumping toward the end of the channel with a shorter distance to the electrode array. At low frequencies (~100Hz), instead of being re-suspended, the particles are trapped by a flow due to Faradaic charging.
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