Photoinduced Collective Motion of Oil Droplets and Concurrent Pattern Formation in Surfactant Solution

Collective motion is ubiquitous in living systems. Although various biomimetic artificial systems have been constructed, there have been few studies reported on collective motion induced by the coupling of chemical reactions, diffusion and convection in a far-from-equilibrium state. In this study, we report an artificial system of oil droplets in a surfactant solution wherein the collective motion of multiple droplets and pattern formation occurred concurrently. Using photo-responsive surfactants with an azobenzene moiety, the assembly of droplets and the formation of circular patterns around the formed droplet clusters occurred under UV illumination, whereas the disassembly of droplets and disappearance of the patterns occurred under subsequent visible light illumination. The observed dynamics were induced by Marangoni flows based on the reversible photoisomerisation of azobenzene-containing surfactants. The phenomena were considered analogous to the bioconvection of microorganisms. These findings could be useful for understanding the mechanism of motion of life in terms of physicochemical aspects.