AS LONG AS IT’S NOT ICE 9: Direct evidence emerges for the existence of two forms of liquid water.

Thirty years ago, a team working at Boston University made the dramatic suggestion that there are not one but two forms of liquid water, which can interconvert at high pressure well below water’s normal freezing point.1 Researchers have been searching for this putative liquid–liquid phase transition ever since, and evidence has slowly accumulated that it really exists. New experiments now supply what seems to be a direct observation of such a transformation between liquid states of different density – not in pure water but in solutions of the sugar trehalose.2 Understanding how such supercold solutions behave could have implications for biology and cryopreservation – where damage to biological tissues by ice crystals must be avoided – as well as for the water-rich states that might exist in the atmospheres of gas giants.

Liquids are structurally disordered, so it’s not immediately obvious how they can support two distinct structures with different densities. But that does seem to be possible for liquids in which some degree of directional bonding, such as hydrogen bonds between adjacent water molecules, makes distinct local structures possible.

Fascinating.