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With microfluidic devices gaining prominence for many applications in chemistry and biology, the hunt is on to find ways of accurately controlling the motion of liquid droplets. In Angewandte Chemie, Antoine Diguet et al. describe a method for using light to trap and move oil droplets floating on an aqueous solution (
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.
By combining ultraviolet and visible light, Diguet et al. made a chromocapillary trap that captured oil droplets cast onto the surface of the surfactant solution. The authors could then drag the droplets across the surface of the solution, at speeds of about 300 micrometres per second, simply by moving the trap around. The image above is a montage of superimposed frames from a movie, and shows a droplet (gold colour) being directed by a trap (cyan halo) along a heart-shaped path; the Petri dish is 5.1 centimetres in diameter.
Chromocapillary traps should work for various combinations of immiscible liquids, and could thus be useful for controlling droplets in micro- or millifluidic devices. The authors' system could also be used to safely handle dangerous liquids, or in light-responsive materials.
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