A stop-motion experiment reveals supercooled water’s dual nature

Liquid water at temperatures well below freezing has two different arrangements of molecules

water molecule illustration

Liquid water cooled to very low temperatures has high-density and low-density arrangements of molecules, a new study suggests. The high-density structure is illustrated here with molecules containing oxygen atoms (red spheres) and hydrogen atoms (silver spheres).

Timothy Holland/Pacific Northwest National Laboratory

Supercooled water may be a two-for-one deal.

A long-standing theory holds that liquid water at temperatures well below freezing is composed of two different arrangements of molecules, one with high density and one with low density. Now, an experiment provides new evidence for that theory, researchers report in the Sept. 18 Science.

Typically, water freezes below 0° Celsius thanks to impurities, such as dust in the water, on which ice crystals can nucleate. But pure water, which lacks those crystallization kick starters, can remain liquid to much lower temperatures, a phenomenon called supercooling.

In the 1990s, a group of physicists proposed that, at high pressures and very low temperatures, supercooled water splits into two distinct liquids of different densities. At atmospheric pressure, under which the new experiment took place, supercooled water would retain some traces of that behavior, resulting in small-scale, transient arrangements of molecules in high-density and low-density formations. Normal liquids have only one such arrangement, rather than two.

Although experiments have hinted at this effect, scientists haven’t been able to fully pin it down (SN: 6/18/14). “There’s a temperature region where [supercooled water is] just experimentally very difficult to look at,” says Loni Kringle of Pacific Northwest National Laboratory in Richland, Wash.

Between about –113° C and –38° C, the liquid crystallizes extremely rapidly, even if it’s entirely pure. That makes teasing out its properties difficult, as measurements must be made in the fraction of a second before the water freezes.

Now, Kringle and colleagues have glimpsed that murky temperature regime with an experiment that works a bit like a stop-motion movie. They heated a thin film of water using a laser and then rapidly cooled the liquid. Hitting the film with infrared light revealed how the water molecules jostled around, hinting at the water’s structure. The team then repeated this process to take snapshots of how that structure evolved over time as the film was heated and cooled. That let the scientists measure the properties of the liquid at temperatures at which it would quickly crystallize if held there for longer periods of time.

The researchers conclude that the water’s behavior as it was heated and cooled could be explained by the coexistence of two different molecular arrangements, as previously predicted. However, the team hasn’t directly measured the density of those structures, so more work is still needed to confirm whether the theory is correct.

“The combination of techniques is quite new and original,” says chemical engineer Pablo Debenedetti of Princeton University, who was not involved with the study.

Better understanding the strange properties of supercooled water might help scientists understand water’s quirks. For example, unlike most substances, water expands when it freezes, making it less dense than its liquid form. That’s the reason why ice floats in your cup and why it sits atop a lake, leaving a liquid layer underneath that can shelter aquatic life over the winter.

“Water is a very strange liquid,” says physicist Greg Kimmel, a coauthor of the study, also at the Pacific Northwest National Laboratory.  “But everybody’s familiar with it, so we don’t really realize how weird it is.”

Physics writer Emily Conover has a Ph.D. in physics from the University of Chicago. She is a two-time winner of the D.C. Science Writers’ Association Newsbrief award.

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