Macroscopic movement of electrolyte-droplet reveals the characteristics of microscopic ion dynamics.

This paper aims to elucidate the microscopic ion dynamics at the solid-liquid interface of a moving droplet, which is difficult to characterize experimentally. We investigated the ion dynamics at the solid-liquid interface by measuring the induced current at the semiconductor surface due to ion-electron interaction at the semiconductor-electrolyte interface. The electric signal exhibited two non-monotonic behaviors corresponding to the ionic concentration and velocity of the electrolyte droplet, neither of which could be explained by the previous model. Revising the characteristic equation of the device to include microscopic ion dynamics led us to the following conclusions: in a sliding electrolyte-droplet, (i) the proportion of ions in the bulk solution that are adsorbed is almost the same regardless of the velocity when ion adsorption is not saturated, (ii) ion-electron interactions decrease rapidly beyond a certain distance, and (iii) the time scale on which the ion-electron interaction becomes a steady state is on the order of seconds. We further extended the charge neutrality condition incorporating ion-electron interaction at the semiconductor-electrolyte interface, combined with the modified Poisson-Boltzmann model. Through the macroscopic motion of the droplet, we are able to estimate the microscopic dynamics inside the droplet.