Boundary Effects on Diffusiophoresis Motion of Charged Porous Spheres
Date Issued
2012
Date
2012
Author(s)
Lai, Tung-Ting
Abstract
Diffusiophoretic behavior of porous colloidal particles subject to a electrolyte concentration gradient is investigated theoretically for arbitrary double layer thickness and surface potential. The governing general electrokinetic equations are put in terms of 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.
We find that the fixed charge density of the porous particle will increase the chemiphoresis and diffusiophoresis. 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. When the fixed charge density is positive, the direction of electrophoresis will have opposite pattern with chemiphoresis. The competition between chemiphoresis and electrophoresis will result in diffusiophoresis.
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 porous particle will exhibit significance differences between planar metal surface and air-water interface. In the condition of planar metal surface, there will be an increase in chemiphoresis and demonstrates an decrease in the state of air-water interface
Subjects
diffusiophoresis
electrokinetucs
porous particle
bipolar coordinate
polarization
boundary effect
electric double layer
Type
thesis
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