垂直於氣液交界面及固體平面之液滴電泳現象

Abstract

本論文探討液滴垂直於氣液交界面或一固體平面的電泳運動現象，並檢視極化效應、邊界效應對粒子泳動行為造成的影響。本研究發現當粒子距邊界近時，邊界的存在對粒子運動造成阻力，並且削弱極化效應對粒子泳動度的影響。同時觀察出當電雙層越厚以及粒子離邊界越近時，邊界給予粒子的阻力越大，使得粒子泳動度下降的幅度越明顯。同時亦觀察出，當粒子表面電位高(ζa*≧3)時，邊界的存在會削弱極化效應的影響。因此粒子距邊界近時，雖然邊界給予粒子的阻力變大，但極化效應的影響減弱，使得粒子距邊界近時，泳動度下降幅度會較不顯著。本研究以擾動離子分布圖、流場圖、以及平衡電位場圖觀察邊界效應造成粒子電雙層變形的現象。 另一方面，當液滴內外黏度比愈小，液滴所受的流體阻力越小，其電泳速度隨之增加，反之當內外黏度比越大的時候，所受的流體阻力越大，其電泳速度越接近硬球粒子的結果。此外，本研究提供了修正因子表格解釋邊界效應以及表面電位對粒子泳動度造成的影響，以提供學者預測液滴電泳受邊界效應的影響。

Electrophoretic motion of a charged liquid droplet or a bubble normal to an air-water interface is investigated theoretically in this study, motivated by the rapid development of various practical applications involving micro/nano emulsions, as well as the fascinating potentials of using the air-water interface as a platform for the two-dimensional colloidal crystallization, for instance.

The presence of an air-water interface is found to reduce the droplet/bubble mobility in general, especially when the double layer is thick or the particle is close to the interface. Special attention is given to the boundary effect upon the convection-induced motion-deterring double layer polarization effect pertinent to highly charged colloids. As demonstrated by various contour plots of both electric and flow fields, the presence of an air-water interface affects the particle motion mainly through the deformation of the double layer when it touches the interface. A counterclockwise vortex flow around the droplet/bubble is observed which pumps up the counterions originally in the wake to the front region. This significantly alleviates the motion-deterring polarization effect when the thickness of double layer is comparable to the droplet/bubble radius. The release of this brake (polarization effect) refuels the electric driving force of the droplet/bubble hence affects its motion near the interface as observed. Convenient charts of correction factors for the boundary effect, expressed as a function of separation distance, are provided to facilitate the possible usage by interested researchers and engineers.