本論文的主要目的是探討高頻交流電場下之非平衡電荷動力現象，及其在操控微流體與次微米膠體粒子之應用。
本論文包含三個部份。第一部份探討一個由高頻交流電場下之非線性電滲流所形成之拉伸流動。此現象是由於在非對稱電場作用之下相鄰電滲流漩渦交互作用之下產生之結果。我們發現在100Hz時流場因電化學充電作用(Faradaic charging)呈現拉伸流結構且流速可達300m/s。然而在1kHz時，充電機制轉為歐姆充電作用(Ohmic charging)，此時所形成之拉伸流動方向與前者相反且流速減慢。我們發現拉伸速率與電場強度之平方成正比符合非線性Smoluchowski尺度。
第二部份，我們使用與第一部份相同的電極設計，來觀察在高頻交流電場下，不同DNA溶液所產生的非平衡電荷動力現象。藉此，我們發現溶液性質與作用頻率會影響DNA溶液的流動行為。
第三部份，我們提出一個運用電荷動力現象來收集並濃縮DNA分子的新方法。此法是運用高頻交流電場下，非平衡電荷動力現象產生的非線性交流電滲流來作用。其遷移率(mobility)與外加電場成正比，這比傳統直流電場更為快速。我們使用不對稱電極設計，由於ACEO漩渦流動交互作用使流體產生匯流，進而形成一遠距且超快速的收集結構。藉此，局部區域內的DNA分子濃度可以在數秒內瞬間提升，且收集DNA分子的作用範圍可長達1mm。同時，本方法不需要連續提供樣品，對於限量之樣品就可以達到明顯的濃度提升。除此之外，我們可以藉由電場的開關來重複收集與釋放DNA分子。由於本方法具有遠距收集的特性，所以對於極稀薄的DNA溶液(10-2 pM)也有明顯的提濃效果。故我們揭示了一個可應用在連續微程序中輸送與濃縮生物分子的新方法。

This thesis focuses on non-equilibrium, induced-charge electrokinetic flow (ICEO) under high-frequency AC fields and its applications to micromanipulation of fluids and colloids.
There are three parts in this thesis. In Part I, we demonstrate a microelongational streaming generated by nonlinear electro-osmosis due to AC polarization. The phenomenon is attributed to the unique rectification mechanism that coordinates three-dimensional flow interactions between adjacent microvortices set by an asymmetric quadrupole electric field. This streaming exhibits a stagnation-point structure with velocity ~ 300 um/s at 100Hz due to Faradaic polarization, but is reversed with slower velocity at 1kHz by Ohmic charging. The measured extensional rate shows a quadratic dependence on the field in line with nonlinear Smoluchowski scale.
In Part II, we employ the same electrode system in Part I and examine the ICEO behavior using different DNA solutions. We find that there exhibit a variety of flow structures, depending on the properties of solutions and applied frequencies.
In Part III, we report a new electrokinetic scheme capable of trapping and concentrating a trace amount of DNA molecules both efficiently and effectively. It invokes non-equilibrium charge polarization under high-frequency AC fields, creating a nonlinear electro-osmotic flow with the mobility growing linearly with the field and hence rendering response much faster than that under conventional DC fields. With an asymmetric quadruple electrode design, rectified intense converging and focusing streams transform into a robust electrokinetic funnel with a long-range and superfast trapping capability. We demonstrate that DNAs not only are rapidly concentrated into a compact cone within just few seconds, but also are trapped remotely in the form of focused threads that can extend as far as 1mm. More importantly, the concentration can be enhanced by several decades without any continuous DNA feeding. In addition, this funnel is shown to possess a reversible concentration/release switch when successively turning on/off the field. While this long–range funnel is capable of concentrating dilute DNA solutions as low as 10-2 pM, it further offers a potential means for transporting and concentrating biomolecules in a continuous fashion using a microdevices.

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