Self-Propelled Motion of a Nitrobenzene Droplet on Au Electrode during Sn Electrodeposition: Factors Creating Imbalance of Interfacial Tension and Marangoni Effect

Abstract

A nitrobenzene droplet moves in a self-propelled manner on Au electrode surface during the Sn electrodeposition in H2SO4 solution because the interfacial tension of solid-water interface acting on the front side of the droplet is larger than that on the rear side. The electrodeposition, which increases the interfacial tension, is suppressed by a side reaction such as hydrogen evolution reaction and by adsorbed nitrobenzene molecules. If the electrode surface is non-uniform, the electrodeposition as well as the side reaction takes place non-uniformly on the electrode surface, leading to an initiation of the droplet motion. If the electrode surface is uniform, the motion is initiated when nitrobenzene molecules are adsorbed non-uniformly on the surface. Once the motion is initiated, the imbalance is created spontaneously: the electrodeposition occurs less efficiently at the rear side because it is suppressed by nitrobenzene molecules remaining on the surface. When the electrodeposition is performed in HNO3 solution, the formation of aniline by the reduction of the non-uniformly adsorbed nitrobenzene induces the Marangoni effect and consequently the droplet continually changes in shape and moves like an amoeba. This amoeba-like motion is also self-propelled because nitrobenzene molecules remaining on the rear side are reduced to form aniline molecules.