Exploring Supercooled Water Structures

Using femtosecond X-ray pulses on a stream of water droplets, researchers have for the first time been able to experimentally study the structure of liquid water supercooled to below −38 °C under vacuum (Nature 2014, DOI: 10.1038/nature13266). Scientists are interested in the temperature region because theoretical work suggests that in this regime water has a critical point, in which multiple phases coexist. Understanding water’s behavior in this temperature range could also help explain other unique properties of the substance. A research team led by Jonas A. Sellberg and Anders Nilsson of SLAC National Accelerator Laboratory used X-ray pulses to get scattering data from individual micrometer-sized water droplets. The scientists then used the data to determine whether droplets were pure liquid or contained some ice at varying temperatures. They found that liquid water could exist at temperatures as low as −46 °C. As the temperature dropped, the water became more ordered, increasingly forming tetrahedral hydrogen-bonded structures. But the supercooled water did not have the long-range order of ice and was not a glass. Droplets showed an accelerated structural change around −45 °C, possibly indicating that a critical point might exist there at high pressure.