Diffusiophoresis of particles under the influence of pH

We study experimentally and theoretically diffusiophoresis of colloidal particles with varying surface potential. When the system pH is varied, the ionization processes associated with acid-base equilibria change the fraction of charged groups on the surfaces of polymeric microspheres. Such change in the surface chemistry affects the diffusiophoretic motion of particles under a concentration gradient of ions. We demonstrate with microfluidic experiments how the solute and pH gradients, combined with the bulk flow, are manifested in the transient distribution of polystyrene particles. The ideas are tested with both positively and negatively charged particles, and compared with corresponding theoretical predictions.