Diffusiophoretic and diffusioosmotic dynamics under a variable zeta potential model*

Diffusiophoresis and diffusioosmosis refer to particle and fluid motions induced by fluid flow over a surface exposed to a local solute concentration gradient. Electrochemical properties of the surface, characterized by the zeta potential (the electrical potential at the shear plane) determine the diffusiophoretic/diffusioosmotic mobilities. Although the zeta potential is known to be a function of local solute concentration, it is frequently treated as constant in order to simplify calculations and facilitate theoretical progress. Here, we revisit recent studies that have assumed constant zeta potential and numerically re-solve the system dynamics using a proposed variable zeta potential model. First, we reexamine the particle banding in an NaCl gradient studied by Staffeld et al. and the particle injection in a dead-end pore by Ault et al. Next, we consider the particle spreading in a Gaussian NaCl distribution studied by Chu et al. and finally calculate the diffusioosmotic pressure drops in the microchannels described by Ault et al. Although the assumption of constant zeta potential is reasonable for certain systems, it is not always obvious a priori whether it is reasonable. In general, the variable zeta potential model is required to predict quantitatively accurate dynamics.