Electrokinetic Phenomena of Charged Porous Colloidal Spheres
Date Issued
2010
Date
2010
Author(s)
He, Yan-Ying
Abstract
The electrokinetic behavior including electrophoresis and diffusiophoresis in either dilute or concentrated suspensions of charged porous particles is investigated. Brinkman model is adopted to simulate the porous structure. A pseudo-spectral method based on Chebyshev polynomials is used to solve the resulted general electrokinetic equations. Instead of the classic hard sphere model, porous particle model may be a better choice in describing bio-particles and polyelectrolytes, which are usually permeable to ions and fluid.
We found, among other things, that the polarization effect due to the convection flow within the porous sphere is a crucial factor in determining its electrophoretic behavior. An induced electric field opposite to the applied electric field is generated, which deters the particle motion significantly when the particle is highly permeable. Approximate analytical prediction for dilute suspensions neglecting convection flow can overestimate the mobility severely in this situation. The approximate analytical prediction is satisfactory when the permeability of particle is low, though. Counterion condensation happens at high fixed charge density which decreases the mobility drastically and the mobility approaches a constant value asymptotically. The mobility profile of the particles with increasing volume fraction can exhibit local minimum if the corresponding dimensionless parameter Qfix/(λa)2 is high, where Qfix and λa are respectively the fixed charge density and the friction coefficient of the porous particles in dimensionless form. This is due to the overlapping of counterion clouds surrounding particles, which offsets the polarization effect, becomes significant as the suspension gets concentrated. No such phenomenon for low Qfix/(λa)2, where the mobility profile decreases monotonously with increasing volume fraction. Comparison with experimental data available in the literature for polyelectrolyte suspensions is excellent, indicating the reliability of this analysis, as well as the success of using charged porous sphere to model a polyelectrolyte system.
Subjects
porous particle
electrophroesis
diffusiophoresis
suspension
electrokinetic phenomena
Type
thesis
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