Self‐Propulsion of Droplets via Light‐Stimuli Rapid Control of Their Surface Tension

Abstract Nature demonstrates many examples of response and adaptation to external stimuli. Here, this study focuses on self‐propulsion (motion) while presenting several self‐propelling droplet systems responsive to pH gradients. Light is used as the gating source to gain reversibility, avoid the formation of chemical wastes, and control the self‐propulsion remotely. To achieve light‐stimuli ultrafast response, photoacids and photobases are used, capable of donating or capturing a proton, respectively, in their excited state. The movement and directionality of the droplet's self‐propulsion are controlled by introducing the photoacid/base either in bulk solution, on the surface of the droplet, or inside the droplet. It is shown that proton transfer between the photoacid/base and the droplet results in a rapid change in the droplet's surface tension, which induces the self‐propulsion movement. The high versatility of the systems together with a record‐breaking ultrafast response to light makes them highly attractive for the design of various controlled cargo‐carrier systems.