Researchers at the University of Birmingham have found that ordinary water becomes ice when it gets really cold (which we all already knew) but also can become a second, denser liquid form (which is a bit of a surprise):
A new kind of ‘phase transition’ in water was first proposed 30 years ago in a study by researchers from Boston University. Because the transition has been predicted to occur at supercooled conditions, however, confirming its existence has been a challenge. That’s because at these low temperatures, water really does not want to be a liquid, instead it wants to rapidly become ice.
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This new evidence, published in Nature Physics, represents a significant step forward in confirming the idea of a liquid-liquid phase transition first proposed in 1992. Francesco Sciortino, now a professor at Sapienza Università di Roma, was a member of the original research team at Boston University and is also a co-author of this paper.
The team has used computer simulations to help explain what features distinguish the two liquids at the microscopic level. They found that the water molecules in the high-density liquid form arrangements that are considered to be “topologically complex”, such as a trefoil knot (think of the molecules arranged in such a way that they resemble a pretzel) or a Hopf link (think of two links in a steel chain). The molecules in the high-density liquid are thus said to be entangled.
In contrast, the molecules in the low-density liquid mostly form simple rings, and hence the molecules in the low-density liquid are unentangled.
Andreas Neophytou, a PhD student at the University of Birmingham with Dr Dwaipayan Chakrabarti, is lead author on the paper. He says: “This insight has provided us with a completely fresh take on what is now a 30-year old research problem, and will hopefully be just the beginning.”
The researchers used a colloidal model of water in their simulation, and then two widely used molecular models of water. Colloids are particles that can be a thousand times larger than a single water molecule. By virtue of their relatively bigger size, and hence slower movements, colloids are used to observe and understand physical phenomena that also occur at the much smaller atomic and molecular length scales.
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Sciortino adds: “Water, one after the other, reveals its secrets! Dream how beautiful it would be if we could look inside the liquid and observe the dancing of the water molecules, the way they flicker, and the way they exchange partners, restructuring the hydrogen bond network. The realisation of the colloidal model for water we propose can make this dream come true.”
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You can read more of the water research here, in Nature Physics.