Supercooled Water Is Really Two Liquids In One

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Water is key to life on Earth and one of the most studied substances in the universe. Most of its properties are simple and familiar to us, but some are complex and somewhat baffling.

For example, while you can make ice in your freezer by cooling water below 0°C (32°F), water is also perfectly happy to stay liquid well below its freezing point. Water actually resists freezing until a speck of dust or another solid gives it something to cling on to.

If it can’t find that something, then water remains liquid – and this supercooled water is supremely weird. Many popular videos show how it can immediately become ice with a jolt, which is really (pardon the pun) cool! As reported in Science, researchers tested supercooled water to temperatures of -83.15°C (-117.7°F) and discovered that supercooled water is actually two liquids in one.

To uncover this, the team zapped ice with a laser and created a supercooled water film for a few nanoseconds. They were then able to monitor the evolution as the water froze again and saw that a high-density form of water was co-existing with a low-density version.  

“We showed that liquid water at extremely cold temperatures is not only relatively stable, it exists in two structural motifs,” senior author Dr Greg Kimmel, from the Pacific Northwest National Laboratory, said in a statement. “The findings explain a long-standing controversy over whether or not deeply supercooled water always crystallizes before it can equilibrate. The answer is: no.”

The team tested this duality of supercooled water across a range of temperatures from -28.15°C (-18.7°F) to -83.15°C (-117.7°F). In agreement with theoretical models, the fraction of high-density supercooled water decreases with the temperature.  

“A key observation is that all of the structural changes were reversible and reproducible,” added lead author Loni Kringle.

There is a lot of interest in supercooled water’s properties and behaviors. It is believed that this phase of water can form in the upper atmosphere and form peculiar precipitations such as graupel, which is when a snowflake is encrusted in a thin shell of ice. Supercooled water is also expected to exist in planets and the tail of comets.

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