This isn't the first time that light has been used to push droplets around. But Diguet and colleagues take a new approach based on the chromocapillary effect, in which light generates a tension gradient at a liquid–liquid interface. This gradient can induce an interfacial flow between droplets and bulk liquids, which propels the droplet in the opposite direction to the gradient.
The authors' technique depends on the compound dissolved in the bulk liquid. Diguet et al. used a surfactant that isomerizes in response to different wavelengths of light — it adopts a polar isomeric form when illuminated with ultraviolet light, and a less polar form when lit with visible light. The light-induced changes in polarity modulate the surface tension between the surfactant solution and oil droplets floating on its surface. So, when the authors partially illuminated such a droplet with ultraviolet light, the tension gradient caused the droplet to move away from the lit area. If they then partially irradiated the droplet with visible light, the droplet moved towards the lit area.
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