Physicists at the University of Goettingen have for the first time gotten supercooled water to freeze using pulses of laser light. Supercooling occurs when a sample of water is chilled down through its normal freezing point (0 C) without crystallization occurring.
This can happen in a small sample and if no “nucleation” site presents itself around which solid ice (a crystal structure) can form. The incoming laser pulse brings about an optical breakdown: some of the water molecules are ionized, creating a momentary plasma. The hot plasma expands and forms a vapor bubble that collapses very rapidly.
It is the pressure waves emitted by the tiny plasma and the bubble collapse which, the Goettingen scientists believe, trigger the rapid crystallization. Previously an acoustic equivalent of this process---sonocrystallization-had been seen, but this is the first time crystallization has been initiated by a laser pulse.
One of the
researchers, Robert Mettin (R.Mettin@physik3.gwdg.de, +49-551-39-2285), suggests that laser icemaking can be extended to studying solidification of other materials. (Lindinger et al., Physical Review Letters, 27 July 2007