A new kind of “phase transition” in water was first proposed 30 years ago. However, as this transition has been predicted to occur at deeply supercooled conditions, confirmation of its existence has proven challenging. Now, researchers at the University of Birmingham, UK, and Sapienza Università di Roma, Italy, have made a significant step forward to confirming the idea of a liquid–liquid phase transition. They reported their work in a recent issue of Nature Physics (https://doi.org/10.1038/s41567-022-01698-6).

figure a

Representative snapshots of the (a) low-density liquid (LDL) and (b) high-density liquid (HDL) networks of colloidal water at reduced temperature T* = 0.105, a temperature below the critical temperature Tc, and pressures either side of the critical pressure Pc (reduced pressure P* = 0.005 and P* = 0.016). Here, vertices correspond to the centers of tetrahedral clusters, while edges indicate existing (weaker) B–B bonds (where B represents the energetics and geometry of the patchy particles). Tetrahedral clusters in the HDL network forming a trefoil knot (right top) and a Hopf link (right bottom) are highlighted in (b), visualized in a reduced representation as tetrahedral patchy particles. Credit: Nature Physics.

The research team used computer simulations to help identify features that distinguish the two liquids at the microscopic level. They found that the water molecules in the high-density liquid are organized in “topologically complex” arrangements such as a trefoil knot (resembling a pretzel) or a Hopf link (like two links in a steel chain). The molecules in the high-density liquid are therefore 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 simulations, and then two widely used molecular models of water. “This colloidal model of water provides a magnifying glass into molecular water, and enables us to unravel the secrets of water concerning the tale of two liquids,” says Dwaipayan Chakrabarti of the University of Birmingham.

The researchers expect that the model they have devised will pave the way for new experiments that will validate the theory and extend the concept of “entangled” liquids to other liquids such as silicon.

Cristian Micheletti, a professor at the International School for Advanced Studies in Trieste, Italy, who was not involved in this study, expects the results will have impact across diverse scientific areas. “First, their elegant and experimentally amenable colloidal model for water opens entirely new perspectives for large-scale studies of liquids,” says Micheletti.

“Beyond this, they give very strong evidence that phase transitions that may be elusive to traditional analysis of the local structure of liquids are instead readily picked up by tracking the knots and links in the bond network of the liquid. The idea of searching for such intricacies in the somewhat abstract space of pathways running along transient molecular bonds is a very powerful one, and I expect it will be widely adopted to study complex molecular systems,” Micheletti says.

Source: University of Birmingham, UK