在電熱耦合場下之微流體與粒子操控

Manipulation of Fluids and Particles under an Electrothermal Field

陳昱良(Yu-Liang Chen)

指導教授 ： 江宏仁

國立臺灣大學/工學院/應用力學研究所/博士(2017年)

https://doi.org/10.6342/NTU201703643

摘要

近年來，流體與粒子之電、熱操控技術已廣泛運用在實驗室晶片的應用上。過去的許多研究中指出可以利用熱梯度在流體與粒子週圍產生局部介電性質之非均質性並且藉由此非均值性進一步地利用外加電場驅動流體與粒子。相較於在單一物理場下的流體與粒子操控技術，在電熱耦合場下的流體與粒子操控展現了較廣大的流場與較快的運動速度。基於這些優點與特性，我們計劃利用電熱耦合場來操控流體、粒子甚至是驅動單一非對稱粒子。在本篇論文中，我們提出一種新的旋轉電熱流體操控技術，透過粒子不同的介電泳反應以及向加熱中心流動的電熱流體，可以快速地分離以及搜集不同的粒子。由於不同粒子之介電泳反應的差異，可能會被吸附在電極邊緣(正介電泳)或是被排斥遠離電極電極邊緣(負介電泳)，而負介電泳反應的粒子會進一步地被電熱流所搜集，達到粒子分類與搜集的目的。此熱電耦合技術可以在鹽水溶液以及在高頻率域範圍操作，因此可克服在低頻時所發生的電滲流現象，改善在微流體晶片中操控的精確性。此技術的應用範圍廣泛，例如生醫檢測之訊號加強、光晶體的製造以及生化微感測器的應用等。

為了要進一步地利用電熱耦合場來驅動單一非對稱粒子，我們必須了解非對稱粒子在電場下的極化特性。在本篇論文中，我們利用電旋轉技術來量測金屬/二氧化矽之非對稱粒子的極化特性。根據不同的電場頻率與金屬濺鍍層的厚度，非對稱粒子的電旋轉方向會有所不同。在非對稱粒子、全金屬球殼粒子與均值介電粒子的比較中，半邊鍍金屬之非對稱粒子的幾何特性造成屏蔽效應的下降進而增加粒子的極化，因此提高非對稱粒子的特徵頻率。我們提出在非對稱粒子中存在一與濺鍍厚度相關之長度尺度，其會影響非對稱粒子在電場下的極化進而造成特徵頻率的改變。

接下來，電熱耦合場進一步地被用來驅動單一非對稱粒子。對一個在自熱泳情況下的非對稱粒子施加一交流電場，並量測其運動速度的改變。發現在同時施加電場與熱梯度的情況下，粒子的運動速度遠大於單一物理場所驅動的速度。基於此實驗結果，我們提出運動速度的增加來自於外加電場所產生的感應介達電位加強了粒子的自熱泳效應。

根據實驗結果，相較於在單一場下的操控技術，在電熱耦合場下之流體與粒子操控技術更加有效率。基於此概念，若是使用一個物理場改變流體或是粒子之物理特性，並且利用另一個物理場來驅動可能有助於操控或增強流體或粒子的運動。此種熱電耦合的操控技術在微流體晶片應用中提供了一個可以動態加強或是控制流體與粒子運動的方法。

關鍵字

非對稱粒子 ； 電動力學 ； 電旋轉 ； 介電泳 ； 粒子操控 ； 電熱流 ； 熱泳

並列摘要

Electrically and thermally based manipulations of fluids or particles has been a technological trend used in lab-on-a-chip applications in last decades. In particular, several studies report that it could generate local inhomogeneities of electric propertiy on fluids and particles by a thermal gradient and further drive the fluids and particles by an electric field acting these inhomogeneities, such as electrothermal flow. Compared with the manipualtion of fluids and particles under a single physical field, the manipulation under an electrothermal field reveals a wide range flow pattern and fast velocity. Based on these features, we plan to apply an electrothermal coupled field to manipulate fluids, particles and even a single asymmetric particle. In this thesis, we demonstrate a functional rotating electrothermal technique for rapidly concentrating and sorting a large number of particles on the microchip by the combination of particle dielectrophoresis (DEP) and inward rotating electrothermal (RET) flows. Different kinds of particles can be attracted (positive DEP) to or repelled (negative DEP) from the electrode edges, and then the n-DEP responsive particles are further concentrated at the heated region by RET flows. This multi-field technique can be operated in salt solutions and at higher frequency without external flow pressure. It can avoid the electrokinetic phenomena at low frequency to improve manipulation accuracy for lab-on-chip applications.

To further apply the electrothermal field to drive a single asysmmetric particle (Janus particle), the characters of polarization of particles under an electric field must be understood in advance. In this thesis, the polarization of metal-coated Janus particles is characterized by electrorotation (EROT) measurements. The rotational direction of Janus particles following or countering the direction of the rotating electric field are observed depending on the field frequency and thickness of metallic coating. The comparison of Janus, metallic, and dielectric particles reveals that the hemispherical coating reduces the screening effect and promotes polarization, thereby exhibiting a higher characteristic frequency. We propose that there is a special length scale introduced by metallic coating in the polarization of Janus particle.

Subsequently, the electrothermal field is used to drive a singlemetal-coated Janus particle. By applying an AC electric field on self-thermophoretic Janus particles in a defocused laser beam, the velocity becomes faster than that of usual self-thermophoretic Janus particles. We propose that the enhancement of self-thermophoresis could be explained by the induced zeta potential resulting from applying an AC electric field.

Based on the experimental results, the manipulation technique under an electrothermal field is more effective than the technique under a single physical field. It would be useful if the properties of fluids and particles can be dynamically tuned by one field and driven by the other field. The functional manipulation technique under an electrothermal field may give a new way to power and control the motion of particles and fluids in a microchip.

並列關鍵字

Janus particle ； electrokinetics ； electrorotation ； dielectrophoresis ； particle manipulation ； electrothermal flow ； thermophoresis

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