流體在平行板間受導電梯度與水平電場作用之動力穩定特性分析

Abstract

在微流體裝置(microfluidic device)中，以電力取代壓力來驅動流體是最有效的方式。電流體力學(electrohydrodynamics, EHD)整合了電學與流體力學，產生了相當豐富有趣的新現象。特別是當流體內部具有導電度梯度(conductivity gradient)存在時，通電的結果導致自由電荷的堆積，進而引發不穩定對流現象的發生。這種不穩定現象最常被應用至微流體的混合器上，因此是目前微機電技術中相當重要的研究課題之一。本論文主要是在利用線性穩定性分析方法探討具有導電梯度分佈的電滲流場(electroosmotic flow, EOF)之穩定性特性。理論模型則假設在填滿稀薄二元電解液(dilute binary electrolyte solution)的兩無限平板間，通入一水平方向的電場，電雙層(electric double layer, EDL)內的流體受電場作用而產生邊界滑移速度，進而推動流體運動。由於流場內部存在導電梯度，使得自由電荷與電力呈現不均勻的分佈。當外加電場持續增加超過某一臨界值時，流場便會開始變得不穩定。為了瞭解此滑移速度在此系統中所扮演的角色，我們分別計算有EDL及無EDL兩個情況。研究結果發現，在低導電度梯度時，EDL所產生邊界滑移速度的確增強了此流體系統的不穩定作用。然而在高導電梯度時，在EDL中的則塔電位(zeta potential)增強時，似乎對系統有更穩定的影響。本研究藉由大量數值結果的呈現，嘗試解釋其中重要的物理機制，成果將有助於電流體穩定現象的進一步了解。

In the microfluidic device, it is the most effective way that the fluid is driven by the electric force is better than pressure. The electrohydrodynamics (EHD) is integrated electricity and the fluid mechanics have the interesting phenomenon. Specially the DC causes the free charge to accumulate when the fluid has conductivity gradient, then occurs unstable convection phenomenon. This kind of stabilization is most often applied to the micromixer. In the present the Micro Electro Mechanical Systems (MEMS) technology is quite important research subjects. This paper mainly considers electroosmotic flow (EOF) of the conductivity gradient distribution to use the linear stability analysis method. The model use dilute binary electrolyte solution between two infinite plates passes over a horizontal electric field. The electric field makes the electric double layer (EDL) in fluid to produce the boundary slip velocity then impetus fluid motion. As a result of the flow field has electric conduction gradient inside to cause the free charge and the electric force are the non-uniform distribution. When the applied electric field continues to increase the critical value then the flow will become unstable. In order to understand that this slip velocity which acts in this system, we calculate separately to have EDL and no two situations. we discover when low conductivity gradient , EDL had the boundary slip velocity indeed to strengthen system unstable. However, at high conductivity gradient when EDL of zeta potential is stronger that system is more stable. This paper use massive value result to explain the important physical mechanism, the achievement will be helpful to the electrohydrodynamic stability a little understanding.