Soft Matter and Complex Systems Seminar

Two oppositely charged surfaces separated by a dielectric medium attract each other. In contrast, using the Surface Force Apparatus, we observe a strong repulsion between two plates of a capacitor that is filled with an aqueous electrolyte upon application of an alternating potential difference between the plates. This long-range force is observed on the distances two orders of magnitude larger than Debye length, the typical length scale for the extension of electrostatic effects. It increases with the ratio of diffusion coefficients of the ions in the medium and reaches steady state after a few minutes, which is much larger than the millisecond time scale of diffusion across the narrow gap. The repulsive force is an order of magnitude stronger than the electrostatic attraction observed in the same setup in air. We find that it results from the increase in osmotic pressure as a consequence of the field-induced excess of cations and anions due to lateral transport from adjacent reservoirs. Oppositely to what is observed with DC fields the ion concentration is elevated across the whole narrow gap instead of in the double layer region only. The unexpected distribution of ions in the thin electrolyte film under AC voltage points to the new design principles for fine control of local ionic concentration.