Light Walks Oil Drops Across Water

Chemists have developed a new method for driving liquid droplets across a liquid surface with the power of light: controlling the droplets’ direction by changing the wavelength of the light (Angew. Chem. Int. Ed., DOI: 10.1002/anie.200904868). Dubbed the “chromocapillary effect” by Damien Baigl of École Normale Supérieure, in Paris, and colleagues, the method has numerous potential uses, including in “smart” photoactive materials and for the safe handling of dangerous liquid samples. Photomanipulation of liquid droplets is an active area of research, but this new work is the first to demonstrate reversible, wavelength-dependent motion of a liquid droplet, the researchers say. Baigl’s group placed oleic acid droplets on the surface of an aqueous solution of a cationic photosensitive azobenzene trimethylammonium bromide surfactant in a petri dish. When a droplet is illuminated with light, a certain surface-tension gradient is established at the oil-water interface. When the wavelength is changed, the surfactant flips between its cis and trans isomers, changing the surface-tension gradient and forcing the droplets to respond. “This new strategy makes the controlled manipulation of millimeter-sized liquid solutions by light possible,” Baigl and coworkers write.