Diffusiophoresis of a rigid sphere through a viscous electrolyte solution

Numerical predictions of the migration velocity are presented for slightly non-uniform aqueous solutions of KCl or NaCl over a wide range of zeta potential (ζ) and particle radius divided by Debye length (κa). The characteristic speed of migration is comparable in magnitude to the ensemble mean diffusion velocity of ions forming the concentration gradient. In the absence of a macroscopic electric field induced by the gradient, the particle usually migrates toward higher salt concentration (the chemiphoretic contribution). However, for moderate values of κa, the particle reverses direction when |ζ| becomes sufficiently large. With an induced electric field, competition between electrophoresis and chemiphoresis can result in as many as four reversals in direction over the range of ζ. In some cases, the direction of migration was found to depend on the magnitude of the ionic drag coefficients, which seems to preclude predicting the direction on the basis of equilibrium thermodynamics.