Laser-induced fabrication of a supercooled liquid droplet embedded in an ice microcrystal†

Optical heating of an ice microparticle supported on a hydrophobic substrate enabled local melting, yielding a liquid microdroplet. To demonstrate this effect, a liquid droplet entrapped in an ice microcrystal was fabricated by illuminating a focused continuous-wave laser beam in the near-IR at temperatures below −10 °C. Droplets formed in this way are either persistent as a supercooled liquid or short-lived, resulting in recrystallization, depending on the presence of salt additives. Salts were added to reduce the vapor pressure of water because vaporization from ice due to laser heating competed with melting. Without salts, melting occurred only during illumination and there was marked vaporization. In situ Raman micro-spectroscopy assisted by optical microscopy imaging provided clear evidence of liquid water formation at the expense of ice. During illumination, the initial Raman signal of ice was gradually replaced by that of liquid water suggesting that melting proceeded with ice and liquid coexisting. Supercooled droplets embedded in ice microparticles eventually transformed into isolated liquid droplets because the surrounding ice either vaporized or merged into the original droplets. Fundamental features of fabrication based on optical-heating-induced melting were elucidated in the experiments described here.