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

Abstract

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 number of induced electrons 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. We reached to the following conclusions: in a sliding electrolyte-droplet, (i) the proportion of ions in the bulk solution that are adsorbed is almost 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.