探討電雙層及電液動效應對介電泳現象之影響

The Effects of Electrical Double Layer and Electrohydrodynamic Flow on Dielectrophoretic Phenomenon of Micro-particles

孫捷(Chieh Sun)

指導教授 ： 莊嘉揚

國立臺灣大學/工學院/機械工程學研究所/碩士(2018年)

https://doi.org/10.6342/NTU201802458

摘要

近年來介電泳現象被廣泛應用在實驗室晶片技術中，藉由改變外加電場頻率、溶液種類及電極形狀，即可操控流體中不同大小、性質之微粒子。
在探討介電泳現象時，不可以忽略微粒子表面所形成之電雙層，其存在使微粒子之導電度顯著提高。在相關理論研究中，目前最常見的計算介電泳力之方法為電偶模型及馬克斯威應力張量，這兩種方法都需要以表面電導(surface conductance)對微粒子進行導電度修正，然而此一修正模型通常假設電雙層為均質層，故與現有之模型相違背，此外表面電導通常需要藉由擬合實驗數據才能得到，失去使用有限元素法預測介電泳現象的優勢。
本研究針對電雙層理論探討其物理結構及電化學機制使用更嚴謹之模型，並結合電動力學理論推導出不同於MST及電偶之介電泳力乃至交越頻率之計算方法，稱為體積積分法，最後以不同導電度及粒徑來比較這三種方法之適用範圍。
另外也將流體效應納入考慮，因電場所造成之電液動現象在電極處會十分明顯，故微粒子會同時受到介電泳及電液動之影響。本研究使用光鉗系統進行不同實驗條件之量測，再結合有限元素法模擬將介電泳力及流體黏滯力量化後進行分析與探討電液動及介電泳對微粒子之影響。

關鍵字

介電泳 ； 電雙層 ； 交越頻率 ； 電液動 ； 光鉗

並列摘要

In recent years, the phenomenon of dielectrophoresis(DEP) has been widely implemented to Lab-on-chip technology. By simply changing the frequency of the external AC electric field, the type of medium or the geometry of the micro-electrode on a chip, we are able to manipulate micro-particles with various sizes and dielectric
properties.
When considering the DEP, one cannot ignore the effect of electrical double layer (EDL) that forms around the micro-particle, the presence of EDL will make the conductivity of the particles rise drastically. In the field of DEP theory, the dipole model and the Maxwell stress tensor(MST) theory are two common approaches to calculate the DEP force. Both of them use surface conductance (KS ) to incorporate the conductivity of EDL with the particle, assuming the EDL being a homogeneous conductive layer. However, this assumption deviates from the existing EDL models in this project. Moreover, in our experimental setting the value of KS can only be obtained by numerical analysis through curve fitting the collected experimental data, the finite element (FEM) method can no longer be used to predict the DEP phenomenon accurately.
In this research, we have been focusing on the physical structure and electro-chemical mechanism of EDL and implemented a more rigorous model, along with the electrokinetic theory, we applied a method that is different from the MST and dipole for computation of the DEP force and associated crossover frequency. This method is called the volumetric integration approach(VI) in this thesis. The applicability and performance of the three methods(dipole, MST and VI) are also evaluated and compared under various experimental conditions in this work.
In addition to DEP, we also studied the fluidic effect. Due to the fact that the electric field induced electrohydrodynamic(EHD) flow is significant near the electrodes, the particles may be affected by the effect of DEP and EHD flow simultaneously. We used optical tweezer to conduct the measurement under different experimental conditions and then used the FEM method to quantify the DEP force and drag force. The experimental and simulation results are analyzed and compared to discuss and conclude the effect of DEP and EHD on the particles in this thesis.

並列關鍵字

dielectrophoresis ； electrical double layer ； crossover frequency ； electrohydrodynamics ； optical tweezer

參考文獻

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