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Using X-ray lasers, Stockholm University researchers were able to track the transformation between two distinct different liquid states of water, both of which consist of H2O molecules. At about -63 degrees centigrade the two liquids exist at different pressure regimes with a density difference of 20%. By rapidly varying the pressure before the sample could freeze, it was possible to observe one liquid transforming into another in real time. Their findings are published in the journal Science.
Water, common and necessary for life on Earth, behaves in a very strange way compared to other substances. The way water properties such as density, specific heat, viscosity and compressibility respond to changes in pressure and temperature is completely opposite to other liquids we know of. Consequently, water is often called “anomalous”. If water had behaved like a “normal liquid” we would not exist, as marine life could not have developed. However, it is still an open question: what causes these anomalies?
There have been numerous explanations for the strange properties of water, and one of them proposes that water has the ability to exist as two different liquids at different pressures and at low temperatures. If we were able to keep the two liquids in a glass, they would separate with a clear interface in the middle, like water and oil. Ordinary water in our environmental conditions is just a liquid and no interface would be seen in a glass – but at the molecular level, it fluctuates creating small local regions of similar density to the two liquids, causing the water to behave strangely. The challenge was that no experiments were possible at temperatures where the two liquids would coexist as ice formed almost instantaneously. So far it has only been possible to study water under these conditions using different types of computer simulations, which has led to many contradictory results depending on the model used.
“The special thing was that we were able to take X-rays unimaginably fast, before the water froze, and we could observe how one liquid turns into another,” says Anders Nilsson, professor of chemical physics at Stockholm University. “For decades there has been speculation and different theories to explain these anomalous properties and why they get stronger when the water gets colder. Now we have discovered that the two liquid states are real and can explain the strangeness of water. “
“I have been studying different forms of disordered ice for a long time with the aim of determining whether they can be considered a glassy state that represents a frozen liquid,” says Katrin Amann-Winkel, senior researcher in chemical physics at Stockholm University. “It’s a dream come true to see that they actually represent real liquids and we see the transformation between them.”
“We have worked so hard for several years to conduct water measurements at such low temperatures without freezing and it is so gratifying to see the result,” says Harshad Pathak, a researcher in chemical physics at Stockholm University. “Many attempts around the world have been made to look for the two liquids by putting water in tiny compartments or mixing it with other compounds, but here we could follow it as just plain water.”
“I wonder if the two liquid states as fluctuations could be an important ingredient for biological processes in living cells,” says Fivos Perakis, assistant professor of chemical physics at Stockholm University. “The new result may open up many new directions for water research in the life sciences as well.”
“Perhaps one of the liquid forms is most prominent for water in the small pores within membranes used to desalinate water,” says Marjorie Ladd Parada, Postdoc at Stockholm University. “I think that access to clean water will be one of the main challenges with climate change.”
“There has been an intense debate on the origin of the strange properties of water for over a century since Wolfgang Röntgen’s early work,” explains Anders Nilsson. “Researchers studying the physics of water can now settle on the model that water can exist as two liquids in the super-cooled regime. The next step is to find out if there is a critical point where the two liquids cross to become a single liquid, as pressure and temperature vary. A great challenge for the next few years “.
The study was conducted in collaboration with POSTECH University in Korea, PAL-XFEL in Korea, SLAC National Accelerator Laboratory in California, Brooklyn College of the City University of New York in the USA and St. Francis Xavier University in Canada. Other people who contributed to the study include former members of Stockholm University’s chemical physics group: Kyung Hwan Kim, Alexander Späh, Daniel Mariedahl, Tobias Eklund, and Matthew Weston.
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