The diffusiophoresis of a concentrated spherical dispersion of colloidal particles subject to a small electrolyte gradient is analyzed theoretically for an arbitrary zeta potential and double layer thickness. In particular, the influence of the difference in the diffusivities of cations and anions is discussed. A unit cell model is used to simulate a spherical dispersion, and a pseudospectral method is adopted to solve the equations governing the phenomenon under consideration. We show that, as in the case of an infinitely dilute dispersion, when the diffusivities of cations and anions are different, the diffusiophoretic mobility is no longer an even function of the zeta potential or double layer thickness. 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 diffusiophoretic mobility is influenced mainly by the induced electric field arising from the difference in the ionic diffusivities, the concentration gradient, and the effect of double layer polarization.

Diffusiophoresis of a spherical colloidal particle normal to a plane 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 bipolar spherical coordinates and solved numerically with a pseudospectral method based on Chebyshev polynomial. The effects of key parameters are examined such as the double layer thickness, surface potential, and the distance between the particle and the plane. It is found, among other things, that the presence of the boundary has a retardation effect on the motion of the particle, provided that the double layer does not touch the planar boundary. If it does, however, the velocity of the particle will exhibit a maximum as the double layer just loses touch of the plane, thanks to the competitive force of the polarization effect. The planar boundary poses not only as a conventional hydrodynamic retarding force, but also may distort the shape of the double layer greatly, hence altering its polarization situation, which has a profound electrostatic impact on the motion of the particle when it is close to the plane.

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The diffusiophoresis of concentrated suspensions of liquid drops subject to a small electrolyte gradient is analyzed theoretically at arbitrary levels of surface potential and double-layer thickness. The effect of doublelayer polarization and double-layer overlapping is taken into account. Kuwabara's unit cell model is employed in modeling the suspension system, and a pseudospectral method based on Chebyshev polynomial is adopted to solve the resulting general electrokinetic equations. Key factors are examined such as the thickness of the electric double layer, the magnitude of the surface potential, the volume fraction of liquid drops, and the viscosity of the internal fluid. The results presented here add another dimension to the previous corresponding study of rigid particles by considering the internal flow of liquid drops, characterized by their viscosity. It is found, among other things, that the lower the viscosity of the internal fluid, the greater the diffusiophoretic velocity of liquid drops. In particular, the diffusiophoretic velocity of an invicid drop is about 3 times higher than that of a rigid one, whereas it is about 2 times higher for a liquid drop with a viscosity similar to that of the suspending medium. The surfactant that might be absorbed on the droplet surface is crucial to the overall observations made above, though.

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