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

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.