Boundary effect on electrophoresis:non-integrated/ eccentrically positioned colloidal particle
|關鍵字:||電泳;邊界效應;超環形粒子;軟粒子;軸偏移;電荷可調整;Electrophoresis;Mobility;Boundary effect;Toroid;Soft particle;Eccentricity;Charge regulation||公開日期:||2008||摘要:||本論文係以有限元素法，研究膠體粒子在邊界存在下受一外加電場之電泳行為。吾人延伸前人的研究方向朝具孔洞的超環形粒子，及粒子位置相對於邊界為軸偏移。探討的主題包括超環形粒子沿著無窮圓孔軸心之電泳、有限圓柱體在無窮圓孔中進行軸偏移電泳、超環形粒子正向於無窮大圓盤面之電泳，及超環形軟粒子沿著無窮圓孔軸心的電泳。另外，在粒子或邊界的帶電模式部分，分別採用了固定表面電位、固定體積電荷及表面電荷可調整模式等。在計算技巧上，由於低粒子表面電位及弱外加電場的假設條件，將相互偶合的電場及流場方程式線性化，因此可用疊加原理求解。 研究結果發現下列幾項因素對膠體粒子的電泳速度有顯著的影響：粒子與邊界的帶電情形、粒子與邊界的幾何形狀、粒子孔洞之大小、粒子與邊界的距離、粒子在邊界內軸偏移的程度、電雙層的厚度、軟粒子外層膜的厚度及膜內流體阻力等。研究結果顯示平行邊界電泳的膠體粒子，邊界雖有延滯膠體粒子泳動的效果，但對於具孔洞的超環形膠體粒子，有時會隨邊界效應加大而加速粒子的泳動速度，產生一局部極小值，特別是對於硬粒子；對於正向於邊界電泳的超環形粒子，電泳速度隨著電雙層厚度變化會有局部極大值發生。另外，圓柱形粒子在圓孔中的電泳，一般情形下，粒子速度隨著偏離圓孔軸心的程度增加而增加，但對於短圓柱粒子在某些條件下則呈現一局部極小值。
The electrophoretic mobility of a colloidal particle near the boundary is investigated in this study for various types of problems by using finite element method. For the present study, we extend the integrated particle to non-integrated one like the toroid, and also consider the case a particle move along the axis of a long cylindrical pore eccentrically. The analysis include a toroid along the axis of a cylindrical pore, a finite cylinder eccentrically along the axis of a long cylindrical pore, a charge-regulated toroid normal to a large disk, and a soft toroid along the axis of a cylindrical pore. Another, about the charge model of both the particle and the boundary, we adapt the constant surface electric potential, constant volume charge density, and charge-regulated respectively. Referring to the skill of calculation, the coupled flow field and electric field equations or so-called electrokintic equations can be linearized assuming the applied electric field is weak and the surface potential is low, and therefore, a superposition is used to solve problem. We found that the particle mobility is affected by several factors, the charge conditions on the particle and the boundary, the geometric of the particle and the boundary, the size of the hole of the particle, the separation distance between the particle and the boundary, the eccentricity of the particle within the boundary, the double layer thickness, and the relative magnitude of the thickness of the charged membrane layer and its resistance. Results reveal that although the presence of the boundary has the effect of retarding the movement of a particle, it becomes advantageous if a toroid is sufficiently close to the boundary and cause a local minimum, especially for the case of hard toroid. For the case of the electrophoresis of a toroid normal to a large disk, the mobility may have a local maximum as the thickness of double layer varies. In addition, when a finite cylinder moves along the axis of a long cylindrical pore, generally speaking, the mobility of the particle increases with the increasing eccentricity, but for a short particle the mobility may have a local minimum as the eccentricity varies.
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