Nature Mater. http://doi.org/hrm (2012)

Credit: © ISTOCKPHOTO/THINKSTOCK

When it comes to its physical properties, water is weird. The most obvious demonstration of this is the fact that ice floats. Less widely known but even more unusual is the fact that it exhibits at least two distinct phases in its amorphous ice state — a low-density phase and a high-density phase. How it undergoes the transition between states has been a subject of debate.

It has been suggested that it could be mirrored by an equivalent transition between different liquid states. But simulations of this liquid–liquid transition predict its critical point to be in the middle of water's so-called no man's land — an experimentally inaccessible region of water's phase diagram where it spontaneously crystallizes. And so, its very occurrence has remained speculative.

To try to gain some insight into the structural dynamics of liquid water in this region, Ken-ichiro Murata and Hajime Tanaka have explored the phase diagram of mixtures of water and glycerol. It is well known that glycerol hinders crystallization of water. This allowed the mixtures to reach supercooled liquid states, enabling the authors to identify a distinct transition between low- and high-density liquid states, providing the strongest suggestion so far that such a transition should indeed be possible in pure water.