When water is cooled very quickly below its freezing point, it exhibits some unusual physical properties that are still not fully understood. Subray Bhat at the Indian Institute of Science in Bangalore and co-workers1 have now shown that this ‘supercooled’ water can exist in two liquid phases of different densities at much lower temperatures than were previously thought possible.

Fig. 1: Water does not always turn to ice when it is cooled below freezing point; it can enter a number of ‘supercooled’ liquid states.© 2009 Subray Bhat

Water can be supercooled from room temperature down to around 235 K (–38 °C), where it usually crystallizes into ice. Water has also been observed in a viscous liquid form at far lower temperatures, between 136 and 150 K. However, the region between 150 and 235 K has often been called a ‘no-man’s land’ where liquid water cannot exist.

Bhat and co-workers added magnetic ‘spin probes’ to ultrapure water samples to allow them to monitor the water dynamics by electron spin resonance (ESR) spectroscopy. They applied a burst of liquid helium to rapidly cool the water to 4.2 K, and then recorded the ESR signal as the samples warmed up.

Their experiments revealed that at low temperatures, supercooled water is dominated by a thick, low-density liquid phase that has some structural order due to the formation of hydrogen bonds. At higher temperatures, the signal indicated the formation of a more normal, disordered, high-density water state.

Most importantly, both liquid phases were observed at ‘no-man’s land’ temperatures, mainly in spaces between ice grains. This could explain some anomalies observed in the properties of water, and has several possible applications as Bhat explains: “The findings have implications in diverse areas such as cryopreservation, the dynamics of glaciers where water can act as a lubricant, the tails of comets containing amorphous ice, and the possibility of finding life in pockets of water trapped in ice in polar regions or even on other planets.”

Bhat and his team now hope to conduct similar experiments using nuclear magnetic resonance, which needs no external spin probes. “The final word on the water phase diagram is yet to be spoken,” he claims, “and the sheer complexity of this very simple and common material will continue to inspire theoretical and experimental investigations in the years to come.”