Fig. 1. Working principle of OTTs and trapping of a PFP droplet.
a A temperature gradient is created by the photothermal conversion of light at a plasmonic substrate, and a water-dispersed oil droplet migrates to a hot region. Water molecules and negative charges are arranged at the water/oil interface, having a radial orientation of water dipoles (indicated by arrows). The temperature gradient (∇T = dT/dr, r: distance from the laser beam) induces a permittivity gradient (∇εint) along the interfacial water layer by thermal perturbation of interfacial water ordering, leading to a thermophoretic migration of the droplet toward the hot region. b Optical image of PFP droplets dispersed in water (scale bar: 10 μm). Inset is a photograph of PFP-in-water emulsions. c Optical images of a PFP droplet under an optical input in the course of time (scale bars: 10 μm). Crossbars indicate the position of the laser beam. The droplet is trapped once it is close to the laser beam.