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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Extended data figures and tables
Extended Data Tables
- Supplementary Information (8.4 MB)
This file contains Supplementary Text, Supplementary Figures 1-22, Supplementary Tables 1-6 and Supplementary References.