Analysis of convection flow of a self-propelled alcohol droplet in an exoskeleton frame

Abstract

This study aims to analyze the convection flow of a self-propelled 1-pentanol droplet. The droplets move spontaneously when 1-pentanol droplets are dropped into an aqueous 1-pentanol solution. This self-propulsion is due to the interfacial tension gradient caused by the concentration differences. The shape of the droplet is closely related to its behavior because the shape of the droplet changes the interfacial tension gradient. In this study, an exoskeleton is used to fix the droplet shape. In our preliminary experiments, we observed Marangoni convection in droplets dropped in exoskeleton frames with boomerang and round holes. The results showed that a large difference in surface tension was necessary to control the self-propulsion of the 1-pentanol droplets. Herein, we prepared two exoskeletons with different holes, an elongated symmetrical elliptical shape, and an asymmetrical shape to fix the shape of the droplet. The droplets were then dropped into each exoskeleton, and the droplet behavior, Marangoni convection inside the droplet, and convection in the aqueous phase were analyzed. We found that the direction of the self-propulsion of the droplet was determined by these exoskeletons, particularly in the case of the asymmetrical exoskeleton, and the direction of self-propulsion was fixed in one direction. Marangoni convection was observed in the droplet from the direction of lower surface tension to that of higher surface tension. In the aqueous phase, two convections were generated from the aqueous phase to the droplet because of the diffusion of 1-pentanol. In particular, when an asymmetrical exoskeleton was used, two convections of different sizes and velocities were observed in the aqueous phase. Based on these experimental results, the relationship between droplet behavior and convection is discussed.