Formation of a Hemispherical Droplet in a Temperature Responsive Ionic Liquid by Optical Tweezers

Phase separation is related to a variety of chemical and biological processes. It proceeds stochastically in solutions, such that controlling it is a challenging issue. Here, we demonstrate local liquid-liquid phase separation of a temperature-responsive ionic liquid (IL) solution by utilizing a near-infrared laser beam. Upon focusing the laser beam onto the glass/solution interface, a single liquid droplet is formed due to optical trapping and a photothermal effect. The resultant droplet deposits on to the glass substrate, exhibiting a hemispherical shape. Under further laser irradiation, the droplet grows to a large size and increases in ionic liquid concentration, which is confirmed by Raman and fluorescence spectroscopic analysis. We observe that the droplet formation strongly depends on the surface properties of a glass substrate. We discuss the formation dynamics by considering optical and thermal potentials in addition to the interaction between the Ionic liquid and the substrate.