A surfactant-coated oil-droplet can be regarded as a supramolecular dynamic system comprising a surfactant monolayer as an ordered surface phase and a fluid as a disordered phase inside. Owing to such a characteristic structure, an oil-droplet is able to show a unique macroscopic dynamics in a hierarchal manner, initiated from a chemical reaction in the molecular scale1).
We prepared oil-droplets with a diameter around 100 μm composed of a caged oleic acid, 2-nitrobenzyl oleate (NBO), a surface of which was covered with oleic acid/oleate as surfactants in a basic water solution (pH = 9.2)2). When UV light irradiated a fresh NBO droplet, the droplet started to move toward the light source, and a velocity of its one-dimensional movement was accelerated in a non-linear manner.
Accompanied by this macroscopic movement, the inner dynamics of the photo-irradiated droplet exhibited a distinct convection flow as visualized by the real-time optical microscopy. It means that the macroscopic movement is tightly linked with the inner dynamics triggered by UV irradiation, and the linkage induces a non -linear droplet's motion. This characteristic movement of photo-irradiated NBO droplet can be regarded as a phototaxis, which would give a hint to reveal the mechanism of the taxis of a living system.
1) K. Suzuki et al., Chem. Lett. 2009, 38,1010.
2) K. Suzuki, T. Sugawara, Chem. Phys. Chem. 2016, 17, 2300.
Kentaro Suzuki, Department of Chemistry, Faculty of Science, Kanagawa University