New details have been discovered about the ability of water to split into two different liquids. 

Researchers in Europe have examined the behaviour of molecules in pressurised liquid water, placed under conditions that would usually cause it to crystallise into snowflakes.

Back in 1992, researchers theorised that water could undergo a liquid-liquid phase transition under supercooled conditions. 

Unlike everyday examples of phase transitions in water between a solid or vapour phase and a liquid phase, a liquid-liquid phase has been extremely difficult to spot. 

But new evidence, published in Nature Physics, represents a significant step forward in confirming the idea, based on research by a member of the original 1992 research team. 

The new team at the University of Birmingham and Sapienza Università di Roma used computer simulations to help explain what features distinguish the two liquids at the microscopic level. 

They found that water molecules in high-density liquid form arrangements that are considered to be “topologically complex”, such as a trefoil knot (molecules arranged in such a way that they resemble a pretzel) or a Hopf link (resembling 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.

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.

It enabled the team to, the first time, present a view of the liquid-liquid phase transition based on network entanglement ideas.

“Water, one after the other, reveals its secrets!” exclaims Dr Francesco Sciortino, a member of the 1992 research team. 

“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.”

The team says their modelling can pave the way for new experiments to validate the theory and extend the concept of ‘entangled’ liquids to other liquids such as silicon.

Dr Pablo Debenedetti - a world-leading expert in this area of research - says it is an important step. 

“This beautiful computational work uncovers the topological basis underlying the existence of different liquid phases in the same network-forming substance,” he says. 

“In so doing, it substantially enriches and deepens our understanding of a phenomenon that abundant experimental and computational evidence increasingly suggests is central to the physics of that most important of liquids: water.”