How to Make Ice with Lasers

Supercooled water

Ice is the result of water passing from its liquid state to a solid one and occurs when liquid water is cooled below 0 °C (273.15 K, 32 °F) at standard atmospheric pressure. Right? Well, sort of. Scientists have now succeeded in creating ice at room temperature.

As a naturally occurring crystalline solid, ice is considered a mineral. But physicists at the University of Goettingen, Germany, have managed for the first time to make supercooled water freeze without crystallization occurring.

This unusual phenomenon was achieved using pulses of laser light pointed to a sample of water, that was chilled down through its normal freezing point (0 C) without passing through the normal crystallization process. This can happen in a small sample and if no "nucleation" site presents itself, around which solid ice (a crystal structure) can form.

Nucleation is the creation of a new phase from a metastable (either supersaturated or supercooled) state occurring via germs, or embryos. These are small, transient clusters of the new phase, or something approximating it, which can exist in various sizes, dispersed within the parent phase.

An incoming laser pulse produces an optical breakdown in the structure of the small sample, as some of the water molecules are ionized, creating a momentary plasma. This plasma is very hot and expands to form a bubble of vapor that collapses very rapidly.

Plasma is characterized by high pressure, that produces pressure waves, which collapse the bubble to trigger the rapid crystallization. Previously, an acoustic equivalent of this process - sonocrystallization - had been seen, but this is absolutely the first time a laser has been proven to be able to initiate crystallization.

Robert Mettin, one of the researchers, believes these applications could be extended to help scientists understand the process of solidification in other materials, as well.

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