Supercooled water is basically two liquids in one. that is the conclusion reached by a research team at the U.S. Department of Energy’s Pacific Northwest National Laboratory after making the 1st ever measurements of liquid water at temperatures much colder than its typical freezing-point .
The finding, published on 17sep, 2020 in journal Science, provides long-sought experimental data to elucidate a number of the bizarre behavior water exhibits at extremely cold temperatures found in space and at the far reaches of Earth’s own atmosphere. Until now, liquid water at the foremost extreme possible temperatures has been the topic of competing theories and conjecture. Some scientists have asked whether it’s even possible for water to really exist as a liquid at temperatures as low as -117.7 F (190 K) or whether the odd behavior is water rearranging on its inevitable path to a solid.
The argument matters because understanding water, which covers 71% of the surface , is critical to understanding how it regulates our surroundings , our bodies and life itself.
“We showed that liquid water at extremely cold temperatures isn’t only relatively stable, it exists in two structural motifs,” said Greg Kimmel, a chemical physicist at PNNL. “The findings explain a long-standing controversy over whether or not deeply supercooled water always crystallizes before it can equilibrate. The solution is: no.”
Supercooled water: A tale of two liquids
You’d think we understand water by now. It’s one among the foremost abundant and most studied substances on the earth . But despite its seeming simplicity—2 atoms of hydrogen and 1 atom of oxygen per molecule—H2O is deceptively complicated.
It is surprisingly difficult for water to freeze slightly below its melting point: water resists freezing unless it’s something to urge it started, like dust or another solid to hold close . In pure water, it takes an energetic active nudge to jostle the molecules into the special arrangement needed to freeze. And it expands when it freezes, which is weird behavior compared with other liquids. But that weirdness is what sustains life on Earth. If ice cubes sank or water vapour in atmosphere didn’t retain warmth, life on Earth as we all know it wouldn’t exist.
Water’s weird behavior has kept chemical physicists Bruce Kay & Greg Kimmel occupied for quite 25 years. Now, they and postdoctoral scientists Loni Kringle & Wyatt Thornley have accomplished a milestone that they hope will expand our understanding of the contortions liquid water molecules can make.
Various models are proposed to elucidate water’s unusual properties. The new data obtained employing a kind of stop-motion “snapshot” of supercooled water shows that it can condense into a high-density, liquid-like structure. This higher density form co-exists with a lower-density structure that’s more in line with the standard bonding expected for water. The proportion of high-density liquid decreases rapidly because the temperature goes from -18.7 F (245 K) to -117.7 F (190 K), supporting predictions of “mixture” models for supercooled water.
Kringle and Thornley used infrared spectroscopy to spy on the water molecules trapped in a type of stop motion when a skinny film of ice got zapped with a laser, creating a supercooled liquid water for a couple of fleeting nanoseconds.
“A key observation is that each one of the structural changes were reversible and reproducible,” said Kringle, who performed many of the experiments.
Graupel: it’s supercooled water!
This research may help explain graupel, the fluffy pellets that sometimes fall during cool- weather storms. Graupel forms when a snowflake interacts with supercooled liquid water in upper atmosphere.
“Liquid water within the upper atmosphere is deeply cooled,” says Kay, a PNNL lab fellow and expert within the physics of water. “When it encounters a snowflake it rapidly freezes then in right conditions, falls to Earth. It’s really the only single time most of the people will experience the consequences or effects of supercooled water.”
These studies can also help understand how liquid water can exist on very cold planets—Jupiter, Saturn, Uranus and Neptune—in our system , and beyond. Supercooled water vapour also creates the gorgeous tails that trail behind comets.
Water molecule gymnastics
Here on Earth, a far better understanding of the contortions water can perform when placed during a tight situation, like one water molecule wedged into a protein, could help scientists design new medicines.
“There isn’t much of space for the water molecules that surround individual proteins,” said Kringle. “This research could shed light on how liquid water behaves in closely packed environments.”
Thornley noted that “in future studies, we will use this new technique to follow the molecular rearrangements underlying a broad range of chemical reactions.”
There is still much to be learned, and these measurements will help lead the way to a far better understanding of the foremost abundant life-giving liquid on Earth.
