Institute of Fundamental Technological Research

Two oppositely charged surfaces separated by a dielectric medium attract each other. In contrast 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 increases with the ratio of diffusion coefficients of the ions in the medium and reaches a steady state after a few minutes, which is much larger than the millisecond timescale of diffusion across the narrow gap. The repulsive force, an order of magnitude stronger than the electrostatic attraction observed in the same setup in air, 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.

Electrostatic forces are the strongest interactions in soft matter physics, yet they are usually screened by ions present in the solutions. Here we demonstrate that the extent of ionic polarization of liquid-liquid interfaces can be dynamically controlled via appropriate choice of the strength and frequency of the external electric field. Results of microfluidic experiments on electro-coalescence of droplets of aqueous solutions of salt provide guidelines for optimization of the process.

electro-coalescence, droplets, microfluidics