電場下液滴在介電層上運動的機制

Mechanism of droplet motion on the dielectric layer under electric field

沈明慧(Ming-Huei Shen)

指導教授 ： 江宏仁

國立臺灣大學/工學院/應用力學研究所/碩士(2021年)

https://doi.org/10.6342/NTU202100545

摘要

近年來，實驗室晶片為生物晶片主要發展的目標，而介電質電濕潤之技術為不需要微流道也可以控制流體的方式，但是目前為止，尚未完全明瞭介電質電濕潤開放式系統下，影響液滴移動行為的各種因素，因此本研究希望透過總結觀察結果與見解，能夠對此系統有更加深入的理解。
本篇論文使用的開放式介電質電濕潤裝置，採用石蠟膜作為介電層，並添加矽油作為潤滑層。發現該系統下介電質電濕潤的效應與電極的電位極性無關。透過定電壓探針碰觸液滴的方式，使液滴獲得淨電荷。當液滴帶正電，其會往高電位的方向移動，而當液滴帶負電，則會往低電位的方向移動。因此，進一步改變施加在電極上的電壓與液滴的淨電荷量，發現液滴移動的平均速度與施加電壓與淨電荷量呈正相關，但是若低壓驅動電極或是液滴淨電荷不足時，發現液滴移動後會停在對稱電極中間。當施加高頻交流電壓，靜電作用力會消失，液滴會移動到高電場強度的位置，推論存在介電泳現象，並且發現施加的頻率與電壓大小，皆會影響液滴移動的速度。當施加頻率越高，液滴移動速度會變慢，而當施加電壓越大，會增加液滴受到的介電泳力，使液滴移動速度變快，提出介電泳力會使液滴停在對稱電極中間。在本篇論文中亦檢視液滴在移動過程中的接觸角，發現液滴只有跨電極時才有明顯的接觸角變化，推論液滴的接觸角變化所產生的壓差並非主要的驅動力。除了驅動力的分析，基於研究結果提出驅動單電極的操作方式，液滴可以不受介電泳力的吸引，透過靜電力到達下一電極上。
最後提出此研究在開放式系統下，液滴整體主要受到靜電力與介電泳力的作用，而接觸角變化產生的壓差，只有在電極與電極之間的小範圍區域。由於上述機制的了解，本篇論文提出一種新穎的操作方式，即先驅動雙電極再驅動單電極。此方式可以使裝置不需要嚴苛的條件，例如高介電常數與夠薄的介電質材料，就能有低壓操作的可能性。

關鍵字

介電質電濕潤 ； 開放式系統 ； 介電泳 ； 低電壓操作

並列摘要

In recent years, lab-on-a-chip is target of the biochip. The electrowetting-on-dielectric technology is a way to control fluid without requiring micro-channel. However, the various factors affecting the motion of droplet in a electrowetting-on-dielectric(EWOD) open system has not been fully understood so far. Therefore, the effects of electrostatic force, dielectrophoretic force and contact angle under EWOD are major in this studied.
The open EWOD device used in this study is composed of parafilm as the dielectric layer and silicone oil as the lubricating layer. With a constant voltage probe touching a droplet, a droplet obtains a net charge. When a droplet is positively charged, it moves toward a higher potential, and when a droplet is negatively charged, it moves a lower potential. Therefore, by changing the voltage of the electrodes and the net charge of a droplet, it is found that the average velocity of droplet motion is positively related to the applied voltage and net charge. But in the case of low voltage or insufficient net charge of a droplet, it is found that a droplet stop between two electrodes. When a high-frequency AC voltage is applied, a droplet moves to a position with high electric field strength. It is inferred that there is a dielectrophoresis phenomenon, and it is found that the applied frequency and voltage affects the velocity of the droplet motion. The higher the applied frequency, the slower a droplet motion velocity, and the higher the applied voltage, faster the motion velocity of droplet. It is proposed that the dielectrophoretic force causes the droplet to stop between two electrodes. In this study, we also examined the contact angle of a droplet in the process of moving, and found that the contact angle changes when a droplet crosses the electrode. It is inferred that the pressure difference caused by the change of the contact angle of a droplet is not the main driving force. In addition to the analysis of the driving force, based on the research results, the operation method of driving a single electrode is proposed. A droplet can reach the next electrode through electrostatic force without being attracted by the dielectrophoretic force.
Finally, it is proposed that under a open system, a droplet is mainly subjected to acting forces of static electricity and dielectrophoresis. The pressure difference caused by the change of the contact angle is only in a small area between the electrodes. Basing on the results, this study proposes a novel operation method, which is to drive a droplet by double electrodes first and then by a single electrode. This method can drive a droplet at low voltage, and more applications can be realized in the future.

並列關鍵字

electrowetting-on-dielectric ； open system ； dielectrophoresis ； low voltage operation

參考文獻

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