Indian Institute of Science in Bangalore scientists, along with researchers in Italy, have found two types of liquid water that have long been suspected to exist below water's normal freezing point.
Unlike most liquids, water becomes less rather than more dense when it freezes, and it is densest not when it is coldest (at 0 degrees Celsius, just before it freezes) but at 4 degrees C.
These are just two of water's host of anomalous properties, some of which are crucial to its behavior in the natural environment.
In 1992, Gene Stanley of Boston University, Massachusetts, and his co-workers carried out computer simulations of water, which suggested that hydrogen bonds in water might produce two different types of liquid if water was made very cold and squeezed to high pressures.
In one form, the hydrogen bonds create a rather open, sparse network of water molecules, called low-density liquid (LDL) water. In the other, water molecules press closer at the cost of breaking some hydrogen bonds, forming a high-density liquid (HDL).
Stanley and his colleagues found that the two types of liquid water changed from one to the other in an abrupt 'phase transition', like the freezing/melting transition that separates ice and ordinary liquid water.
In this view, anomalies such as the density maximum at 4 degrees C are a reflection of the same competition between dense and less-dense states that creates the phase transition at much lower temperatures.
Now, Dino Leporini of the University of Pisa in Italy and his co-workers at the Indian Institute of Science in Bangalore say they have seen the two phases that Stanley's team proposed in 1992.
The team used a technique called electron spin resonance to study the mobility of water molecules within tiny pockets of liquid trapped between crystallites of ice at temperatures down to around -183 degrees C.
They report that between about -140 and 0 degrees C, they can see two types of 'liquid-like' motion of the TEMPOL probes, presumably reflecting the presence of two types of water in the ice pockets.
One is slower than the other, and they interpret this as evidence for the presence of two distinct types of water: the more viscous LDL form, and the more fluid HDL.
According to Debenedetti, the results seem to reveal two different types of water, whose relative amounts change as the temperature changes.