Run-and-Halt Behavior of Motile Droplets in Response to Light

We report the run-and-halt behavior of motile droplets immersed in an aqueous solution of amphiphilic molecular switch. These oil droplets move autonomously as the switch solubilizes the oil into the water. Droplet movement stops in response to UV light, and picks up again in response to visible light. This motile behavior is a consequence of the reversible trans-to-cis photo-conversion of the switch in water, because the trans photo-isomer stabilizes the oil droplets better than the cis photo-isomer, and therefore it also solubilizes the droplet more effectively. Notably, the droplets also evolve positive photokinesis under illumination with visible light, and, in patchy light environments, their complex motility pattern directs the droplets at the periphery of the illuminated areas.