Effects of zeta potential and electrolyte on particle interactions on an electrode under ac polarization

The relative motion between two colloidal particles loosely deposited on an electrode passing alternating current was investigated. Parameters such as zeta potential, electrolyte composition, electrolyte concentration, and frequency were varied. At a low frequency (100 Hz), the particles aggregated in both sodium bicarbonate and sodium chloride solutions but separated in sodium hydroxide solutions. At 1000 Hz, the particles separated in both bicarbonate and hydroxide solutions, and the rate of separation was slower than at 100 Hz for the hydroxide solutions. The effect of zeta potential was negligible, indicating a convective mechanism causing the relative motion between the particles. Electrolyte concentration had no appreciable effect on the motion. These results are qualitatively consistent with predictions of a theory based on electrohydrodynamic flow induced by the interaction between a space charge in the liquid adjacent to the electrode's surface, generated by concentration gradients of the ions, and an electric field tangent to the electrode which is caused by deflection of current around each particle. The interparticle separation velocity in hydroxide solutions predicted from the theory without adjustable parameters is comparable to the experimental values.