We report a novel versatile droplet system where droplets of organic solvent, 3-phenylpropionaldehyde (3PPA), spontaneously propel themselves both on the surface of surfactant solution, that is, at the air-solution interface, and on the bottom surface of the Petri dish, that is, at the glass-solution interface. Moreover, we found that 3PPA can be polymerized with the help of basic catalysts, resulting in a compound with enhanced surface activity. This polymerized compound then alters the mode of droplets’ self-propulsion. We propose that this 3PPA droplets system can be used as a primitive cell model for its ability of propulsion and chemical reaction both induced by its environment.

Active matter sometimes exhibits life-like complex spatiotemporal patterns. Here we report on complex oscillatory behaviors of droplets floating on an aqueous surfactant solution. Even if the droplets consist of only simple chemicals, the behaviors they exhibit are unexpectedly complicated. They are likely induced by the interaction among droplets, which is mediated through the surface tension field as well as Marangoni flow field created by the droplets themselves.