Water has a number of anomalous physical properties, and some of these become drastically enhanced on supercooling below the freezing point. Particular interest has focused on thermodynamic response functions that can be described using a normal component and an anomalous component that seems to diverge at about 228 kelvin (refs 1,2,3 ). This has prompted debate about conflicting theories4,5,6,7,8,9,10,11,12 that aim to explain many of the anomalous thermodynamic properties of water. One popular theory attributes the divergence to a phase transition between two forms of liquid water occurring in the ‘no man’s land’ that lies below the homogeneous ice nucleation temperature (TH) at approximately 232 kelvin13 and above about 160 kelvin14, and where rapid ice crystallization has prevented any measurements of the bulk liquid phase. In fact, the reliable determination of the structure of liquid water typically requires temperatures above about 250 kelvin2,15. Water crystallization has been inhibited by using nanoconfinement16, nanodroplets17 and association with biomolecules16 to give liquid samples at temperatures below TH, but such measurements rely on nanoscopic volumes of water where the interaction with the confining surfaces makes the relevance to bulk water unclear18. Here we demonstrate that femtosecond X-ray laser pulses can be used to probe the structure of liquid water in micrometre-sized droplets that have been evaporatively cooled19,20,21 below TH. We find experimental evidence for the existence of metastable bulk liquid water down to temperatures of kelvin in the previously largely unexplored no man’s land. We observe a continuous and accelerating increase in structural ordering on supercooling to approximately 229 kelvin, where the number of droplets containing ice crystals increases rapidly. But a few droplets remain liquid for about a millisecond even at this temperature. The hope now is that these observations and our detailed structural data will help identify those theories that best describe and explain the behaviour of water.
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We acknowledge the US Department of Energy (DOE) through the SLAC Laboratory Directed Research and Development Program, Office of Basic Energy Sciences through SSRL and LCLS; the AMOS programme within the Chemical Sciences, Geosciences, and Biosciences Division of the Office of Basic Energy Sciences; and the Swedish Research Council for financial support. The molecular dynamics simulations were performed on resources provided by the Swedish National Infrastructure for Computing at the NSC and HPC2N centres. Parts of this research were carried out at LCLS at the SLAC National Accelerator Laboratory. LCLS is an Office of Science User Facility operated for the DOE Office of Science by Stanford University. We also acknowledge the support of the SSRL Structural Molecular Biology group funded by the National Institutes of Health, National Center for Research Resources, Biomedical Technology Grant and the US Department of Energy, Office of Biological and Environmental Research. We wish to thank D. Schafer and M. Hayes for mechanical support; W. Ghonsalves and F. Hoeflich for software support; the SLAC detector group for assistance with the Cornell-SLAC pixel array detector; H. Nakatsutsumi, K. Beyerlein and C. Gati for nozzle support; D. Bowron for providing the data files from ref. 22; J. Spence and C. Stan for discussions; and H. E. Stanley, V. Molinero, C. A. Angell and D. Chandler for critical reading of the manuscript.
Extended data tables
This file contains Supplementary Text, Supplementary Figures 1-22, Supplementary Tables 1-6 and Supplementary References.
About this article
Nature Communications (2018)