微粒子尺寸與電極間距對介電泳性質影響之研究

Effects of Particle Size and Distance between Electrodes on Dielectrophoretic Phenomena of Micro Particles.

翁平侑(Ping-You Weng)

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(2014年)

https://doi.org/10.6342/NTU.2014.02390

摘要

在微粒子操控、微流道系統與實驗室晶片的研究中，介電泳係一重要之應用性質，透過改變外加交流電場之頻率與電極幾何的搭配可將流體中微米粒子分流、捕捉與操控。然而，目前多數研究使用之理論模型假定的理想假設在對粒子所受之介電泳力的計算上會在粒子尺寸較大時產生大誤差，與實驗結果不符合。因此，本研究利用馬克斯威應力張量探討粒子的介電泳性質以避免理想假設所帶來的誤差。
本研究利用有限元素法建構二維軸對稱模型，以電場頻域分析的方法模擬計算交流電場中粒子與介質內的電場分布，並利用有限元素軟體以馬克斯威應力張量積分計算粒子所受介電泳力。以此結果得到所受介電泳力交越頻率並與傳統理論及文獻實驗結果比較，並對於傳統理論型在粒子尺寸方面提出使用上的建議，闡述傳統理論在不同粒子尺寸下與實驗結果貼近及有誤差之原因。此外，為了解交越頻率與粒子尺寸關係曲線變動之原因，本研究利用固定相對幾何尺寸的模型，避免幾何尺寸變動造成的影響，了解在不同粒子尺寸下粒子內外電場分布的情況，並發現曲線趨勢之變動與粒子和介質複數電容率比值相關。另外，本研究利用建立的有限元素模型，改變介質溶液的導電率與粒子導電率修正項中的Ks以探討介質溶液性質改變後對於粒子介電泳性質的影響。
最後，本研究建立橢圓球形粒子之模型，初步探討橢圓球形粒子情況下傳統理論的適用性並提出適用的粒子尺寸臨界值，並對於橢圓球形粒子的傳統理論計算適用範圍較球形粒子大之現象由電場分布情況方面切入探討提出解釋。

關鍵字

介電泳 ； 馬克斯威應力張量 ； 電雙層 ； 表面電導 ； 微流道 ； 微粒子操控

並列摘要

Dielectrophoresis (DEP) is an important application for micro particle manipulation, microfluidic system and lab on-chip which can be used for trapping, separating and manipulating of micro particles suspended in a liquid medium. However, the most widely used analytic mode, dipole model, has several assumptions that will cause errors when calculating crossover frequency in the case of large particles. Thus, we propose another analytic model, Maxwell stress tensor (MST), combined with finite-element method to study the DEP phenomenon without the assumptions on particle sizes and shapes.
We conduct two-dimensional (2D) axial symmetric frequency domain electric field finite-element analysis to study the electric field distribution of the system in an alternating current (AC) electric field and use MST to calculate the DEP force exerted on the micro particle. We compare the relationship between crossover frequency and particle diameter obtained by the dipole mode, MST model and published experimental data from the literature. We find that the crossover frequency calculated by MST shows a similar trend as that measured experimentally, in particular the existence of dual slopes and a transitional diameter. On the contrary, dipole model fails to predict the crossover frequency for larger particles. As a result MST model is more applicable in general cases than the conventional dipole model, and may provide valuable insights into some puzzling experimental findings reported in the literature.
To understand the physics and mechanism that determine the crossover frequency curve as a function of particle diameter, we conduct two-dimensional (2D) axial symmetric simulation which has the same relative geometry and avoid the influence of the relative geometric size change. Studying the electric field distribution of various particle sizes, we find that the crossover frequency curve is highly correlated to the ratio of complex permittivity of the particle and medium. Moreover, we use the finite-element model to study of the sensitivity of DEP properties to medium material properties by adjusting the medium conductivity and Ks, included to account for the electric double layer in the modified equation, for particle conductivity.
Lastly, we conduct simulations to analyze ellipsoid particle, and give guidelines on the applicability of conventional analytic dipole model about the particle size. We find that the dipole model is more applicable to an ellipsoid than a spheroid, i.e. the transitional diameter for an ellipsoid is larger than that of a spheroid, which is then explained by the distribution of electric field.

並列關鍵字

Dielectrophoresis ； Maxwell stress tensor ； electric double layer ； surface conductance ； microfluidics ； micro-particle manipulation

參考文獻

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