A new study shows that supercooled water is one of the only ones.
Scientists have long suspected that water at freezing temperatures comes in two distinct varieties: a high-density liquid that appears at very high pressures and a low-density liquid at lower pressures. Now, ultrafast measurements have caused water to transform from one type of liquid to another, confirming that heartbeat. The discovery, reported in the journal Science on Nov. 20, could help explain some peculiarities of water.
The experiment "adds more and more evidence to the idea that water is really two-component … and that's the reason why water is so rare," says physicist Greg Kimmel of the Pacific Northwest National Laboratory in Richland, Washington. did not participate in the study.
When free of impurities, water can remain liquid below its typical freezing point of zero degrees Celsius, forming what is called a supercooled liquid. But the dual nature of supercooled water was expected to appear in a temperature realm so difficult to study that it was nicknamed "no man's land." Below –40 ° C, the water remains liquid for moments before it crystallizes into ice. Making the task even more daunting, the high-density phase only appears at very high pressures. Still, “people dreamed of how to do an experiment,” says Anders Nilsson of Stockholm University.
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Thanks to rapid experimental maneuvers, Nilsson and his colleagues infiltrated no man’s land by monitoring the properties of water on a nanosecond scale. “This is one of the main achievements of this work,” says computational chemist Gül Zerze of Princeton University. "I'm impressed with his work."
Scientists began by creating a type of high-density ice. Then a pulse from an infrared laser heated the ice, forming high-pressure liquid water. Then that water expanded and the pressure dropped quickly. Meanwhile, the researchers used an X-ray laser to investigate how the structure of the water changed, depending on how the X-rays dispersed. As the pressure decreased, the water went from a high-density fluid to a low-density fluid before crystallize on ice.
Previous studies have used ultrafast techniques to find clues to the behavior of two faces of water, but these have been done mainly at atmospheric pressure (SN: 28/09/20). In the new work, water was observed at about 3,000 times atmospheric pressure and -68 ° C. "It's the first time we've had real experimental data at these pressures and temperatures," says physicist Loni Kringle of the Pacific Northwest National Laboratory. was not involved in the experiment.
The result could indicate that supercooled water has a “critical point”: a certain pressure and temperature at which two distinct phases merge into one. In the future, Nilsson hopes to identify that point.
Such a critical point could explain why water is a strange liquid. For most liquids, cooling makes them denser and harder to compress. Water becomes denser as it cools to 4 ° C, but becomes less dense as it cools even more. Similarly, its compressibility increases as it cools.
If supercooled water has a critical point, this could indicate that the water experienced in daily life is strange because, under typical pressures and temperatures, it is a supercritical liquid, a rare state that occurs beyond a critical point. Such a liquid would not be in the form of high or low density, but would consist in some regions with a high density arrangement of water molecules and other low density bags. The relative amounts of these two structures, which result from different arrangements of hydrogen bonds between molecules, would change as the temperature changes, explaining why water behaves strangely as it cools.
Thus, even though the experiment involved extreme pressures and temperatures, Nilsson says, "it influences water in our ordinary lives."