Liquid–Liquid Phase Separation of Single Optically Levitated Water–Ionic Liquid Droplets in Air

In this study, to investigate the equilibrium morphology of liquid–liquid phase-separated droplets in air, a temperature-responsive ionic liquid (IL) showing lower critical solution temperature behavior was employed. ILs have negligible vapor pressure and do not evaporate from aerosol droplets during dehumidifying processes. We demonstrated that the liquid–liquid phase separation of single optically levitated aqueous droplets containing the temperature-responsive IL can be induced by controlling the air relative humidity. The formation of liquid–liquid phase-separated droplets of partially engulfed morphology was successfully observed under an optical microscope, and their configurations were compared with those calculated by a thermodynamic model based on interfacial tensions and relative volume ratios of two immiscible phases.