Negative phototaxis behavior of organic droplets in channels
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Phototaxis describes the locomotory movement that occurs when an organism moves in response to a light stimulus. Various organisms like green algae, cyanobacteria and ciliates have dramatically phototactic responses, underlying the complicated biological, physical and photochemical mechanisms involved. Inspired by them we developed an inanimate/chemical system in which an organic droplet is self-propelled in response to a photo-stimulus. The centimetre-scale directional movement of the organic droplet on the aqueous solution is powered by the combination of photo-induced pH change and surface tension effects. Upon irradiation with white light the pH of the aqueous solution containing a spiropyran derivative is lowered from 5 to 3.5, which causes the local release of surfactant from the organic droplet to the aqueous solution. The release of surfactant dramatically reduces the local surface tension of the aqueous solution which in turn is sufficient to power the movement of a microliter droplet of dichloromethane away from the illuminated area with speeds up to 4000 μm s-1.