Diffusiophoresis of Colloidal Particles in an Electrolyte Solution

• Main Authors: Shih-Han Lou, 羅仕瀚
• Other Authors: Eric Lee
• Format: Others
• Language: zh-TW
• Published: 2009
• Online Access: http://ndltd.ncl.edu.tw/handle/09882994975722611124

博士 === 國立臺灣大學 === 化學工程學研究所 === 97 === Diffusiophoretic behavior of colloidal particles subject to a uniform electrolyte concentration gradient is investigated theoretically for arbitrary double layer thickness and surface potential. The governing general electrokinetic equations are put in terms of spherical coordinates and bipolar spherical coordinates, and solved numerically with a pseudo-spectral method based on Chebyshev polynomial. Without any assumption about particle surface potential or double layer thickness, the effects of key factors are examined such as the effect of double layer polarization, double layer overlapping, and boundary effect. It is found, among other things, that the diffusiophoretic mobility exhibits a local maximum as well as a local minimum with varying particle surface potential. In contrast to the case of identical diffusivity of cations and anions, a local electric field is induced in the present case due to an unbalanced charge distribution between higher and lower concentration regions. Depending upon the direction of this induced electric field, the diffusiophoretic mobility can be larger or smaller than that for the case of identical diffusivity. The effect of volume fraction ratio of colloids is also examined. The higher the ratio is, the lower the mobility. Furthermore, if the surface pf a particle contains dissociable functional groups, the dissociation of which yields a variation of particle surface potential. The diffusiophoretic mobility of the particle will change accordingly. Besides, it is found that the internal flow of a liquid drop, characterized by their viscosity, has the most significant impact on the diffusiophoretic motion of a liquid drop when the viscosity ratio is approach 1.0. In the study of a spherical colloidal particle normal to a planar boundary, it is found, among other things, that the presence of a planar boundary results in a local concentration gradient, provided that the double layer does not touch the planar boundary. If it does, however, the diffusiophoretic mobility of the particle will exhibit a maximum as the double layer just touches the planar boundary, thanks to the competitive force of hydrodynamic drag. Distinctive features pertinent to a planar metal surface, a non-conducting plane, and an air-water interface are also examined in particular. It is concluded that the planar boundary poses not only as a conventional hydrodynamic retarding force, but may lead to different electrostatic interaction hence alter the polarization situation, which has profound electrostatic impact on the motion of the particle when it is close to the plane. Keywords: diffusiophoresis, Electrokinetics, colloids, liquid drop, boundary effect.