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

Biology demonstrates many examples of response and adaptation to external stimuli, and here we focus on self-propulsion (motion) while presenting several self-propelling droplet systems responsive to pH gradients. We use light as the gating source to gain reversibility, avoid the formation of chemical wastes, and control the self-propulsion remotely. To achieve light-stimuli ultra-fast response, we use photoacids and photobases, capable of donating or capturing a proton, respectively, in their excited-state. We control the movement and directionality of the droplet’s self-propulsion by introducing the photo-acid/base either in bulk solution, on the surface of the droplet, or inside the droplet. We show that proton transfer between the photo-acid/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 our systems together with a record-breaking ultra?fast response to light makes them highly attractive for the design of various controlled cargo?carrier systems.