Phototaxis Motion Behavior of a Self‐propelled Submarine‐like Water Droplet Robot in Oil Solvent

Abstract Controllable self‐propelled droplet robots show great potential to be used as carriers, sensors and actuators in biological medicine and oil exploration. Current research mainly focuses on studying oil droplet robots floating in water phase which has limited application. Different from the previous robots, a light‐driven self‐propelled water‐phase droplet micro robot, which is doped with Fe₂O₃ nanoparticles, moving in oil solvent is proposed. Unlike light‐driven oil droplets, the water drop robot is constantly diving towards the light source like a submarine moving in oil environment under blue laser irradiation. Numerical simulations are performed to investigate the water/oil interface behavior and the motion mechanism of micro robots. It is found that a photocatalytic Fenton reaction occurs on the irradiated Fe₂O₃ nanoparticles of water droplet robot, which causes uneven ion concentration to change the interfacial tension distribution of the water drop generating Marangoni flow. In addition to phototaxis behavior, further experiments confirmed that multiple water droplet micro robots show collective behavior under the control of surface light source. The proposed water droplet micro robot provides new possibilities for the development of targeted drug delivery, oil‐water separation as well as oil pollution treatment.