The first experimental evidence of the hypothesized liquid-liquid critical point in supercooled water
Liquid water fluctuates between two different states when it is cooled and these fluctuations reach a maximum at -44 oC. An international team of researchers headed by the Stockholm University reported the first experimental evidence of the existence of the Widom line which is supposed to emanate from the liquid-liquid critical point. The experiments were performed at PAL-XFEL in South Korea and at SACLA in Japan and are now reported in the journal Science.
Water is the most important liquid for life on Earth and plays an essential role in physics, chemistry, biology and geoscience. What makes water unique is not only its importance but also the anomalous behavior of many of its macroscopic properties. For example, density, specific heat, viscosity and compressibility of water behave opposite to other liquids that we know. If we look at a glass of ice water, everything is upside down. Strangely enough for the liquid state, water is the densest at 4 oC, and therefore it stays on the bottom. This is why life can exist at the bottom of a lake and an ocean during winter even when the surface is frozen.
There has been an intense debate about the origin of the strange properties of water for over a century since the early work of Wolfgang Röntgen. One major hypothesis, that has strong indirect support from theoretical work, is that there could exist two different liquid states, high density liquid (HDL) and low density liquid (LDL). This liquid-liquid transition (LLT) line is proposed to end with decreasing pressure and increasing temperature in a liquid-liquid critical point (LLCP) and its extension into the one-phase region corresponds to the Widom line. At the Widom line, the density fluctuations would reach a maximum.
The scientific team led by Anders Nilsson performed X-ray measurements on deeply supercooled liquid water with the help of ultra-short x-ray pulses at x-ray lasers in Japan and South Korea. They successfully found that water reaches the peak of its strange behavior such as compressibility and correlation length at -44 oC. This is the first experimental evidence of the existence of the Widom line and LLCP which can explain the origin of water’s anomalous properties.
“What was special was that we were able to X-ray unimaginably fast before the ice froze and could observe how it fluctuated between the two states”, says Anders Nilsson, Professor of Chemical Physics at Stockholm University. “For decades there has been speculations and different theories to explain these remarkable properties and why they got stronger when water becomes colder. Now we have found such a maximum, which means that there should also be a critical point at higher pressures”.
Another remarkable finding of the study is that the unusual properties are different between normal and heavy water and more enhanced for the lighter one. “The differences between the two isotopes, H2O and D2O, given here shows the importance of nuclear quantum effects”, says Kyung Hwan Kim, postdoc in Chemical Physics at Stockholm University.
“The possibility to make new discoveries in a much studied topic such as water is totally fascinating and a great inspiration for my further studies”, says Alexander Späh, PhD student in Chemical Physics at Stockholm University.
“It was a dream come true to be able to measure water under such low temperature condition without freezing” says Harshad Pathak, postdoc in Chemical Physics at Stockholm University. “Many attempts over the world have been made to look for this maximum”.
“Researchers studying the physics of water can now settle on the model that water has a critical point in the supercooled regime.”, further explains Anders Nilsson. “The next stage is to find the location of the critical in terms of pressure and temperature. A big challenge in the next few years.”
These studies were led by Stockholm University and involve a collaboration including the KTH Royal Institute of Technology, PAL-XFEL in South Korea, SACLA in Japan.
Maxima in the Thermodynamic Response and Correlation Functions of Deeply Supercooled Water;
H. Kim, A. Späh, H. Pathak, F. Perakis, D. Mariedahl, K. Amann-Winkel, J. A. Sellberg, J. H. Lee, S. Kim, J. Park, K. H. Nam, T. Katayama, and A. Nilsson; Science, 358, 1589-1593 (2017)