Melting of a single ice microparticle on exposure to focused near-IR laser beam to yield a supercooled water droplet

We observed for the first time that a single ice microparticle supported on a substrate melted photothermally to form a supercooled water droplet on exposure to tightly focused illumination with a 1064-nm laser beam that generated a point heat source. In situ Raman micro-spectroscopy clearly showed the formation of liquid water at the expense of ice. The observation of this melting is only possible when the experiment is performed with micrometer-sized ice particles. A previous attempt to melt millimeter-sized ice through photothermal heating of gold nanoaggregates fell short of expectations because only vapor formation, rather than liquid water formation, has been postulated. Our observation is significant because thermal confinement in a microscale compartment using a water-air interface as a heat-insulated wall can achieve particle temperatures above the melting point of water, whereas, in an unlimited space of ice, heat transfer from the heating center to the surroundings causes steep temperature decays, resulting in limited temperature increase. Micro-Raman spectroscopy revealed the laser-heating-induced melting of ice microparticles resulting in supercooled liquid droplet. Confinement using a water-air interface as a heat-insulated wall enabled particle temperatures above the melting point.