Diffusiophoresis of a spherical particle normal to an air–water interface

Abstract

The diffusiophoresis of a spherical colloidal particle normal to an air–water interface subject to a uniform electrolyte concentration gradient is investigated theoretically. The governing electrokinetic equations are solved numerically with a pseudo-spectral method based on Chebyshev polynomials. Key parameters such as the distance between the particle and the interface, the double-layer thickness, and the surface potential of the particle are examined to analyze their respective effect on the diffusiophoretic velocity of the particle. Distinctive features pertinent to an air–water interface are investigated in particular as compared with a solid metal surface. It is found in this study, among other things, that the diffusiophoretic velocity of a particle moving toward an air–water interface is always less than that toward a planar metal surface when the diffusivities of cations and anions are identical. This can be explained nicely by the classic image-charge analogue in electrostatics, where the former (air–water interface) stands for an image-charge of the same sign, while the latter (planar metal surface) stands for an image-charge of the opposite sign. In the case of distinct diffusivities of cations and anions, the situation is much more complicated. No such simple and convenient analogue is observed.