Researchers from Sapienza Università di Roma and the University of Birmingham have discovered new evidence that water can transform into a denser type of liquid.
Researchers from Boston University initially suggested this “phase shift” in water 30 years ago in a study. But proving the transition’s presence has proven difficult because it has been projected to happen under supercooled settings. This is because water really prefers to quickly turn into ice at these low temperatures rather than remain a liquid.
Contrary to common examples of phase transitions in water between a solid or vapour phase and a liquid phase, this liquid-liquid phase transition is still largely unknown due to its hidden state.
The 1992 theory of a liquid-liquid phase transition is now significantly strengthened by the new evidence, which was published in Nature Physics. Co-author of this work and former member of the original research team at Boston University, Francesco Sciortino is currently a professor at Sapienza Università di Roma.
The team has utilized computer simulations to assist explain the characteristics that, at the microscopic level, separate the two liquids. They discovered that the water molecules in the high-density liquid organize themselves into configurations that are referred to as “topologically complicated,” such as a trefoil knot or a Hopf link (think of two links in a steel chain). Thus, it is argued that the molecules in the high-density liquid are entangled.
The molecules in the low-density liquid, in contrast, are unentangled because they primarily form simple rings there.
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.”
This beautiful computational work uncovers the topological basis underlying the existence of different liquid phases in the same network-forming substance. 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.
Professor Pablo Debenedetti
In their simulation, the researchers first used two popular molecular models of water, followed by a colloidal model of water. The size of a colloidal particle can be a thousand times that of a water molecule.
Colloids are utilized to monitor and understand physical phenomena that also happen at the much smaller atomic and molecular length scales because of their relatively larger size and slower movements.
Dr. Chakrabarti, a co-author, says: “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.”
Professor Sciortino says: “In this work, we propose, for the first time, a view of the liquid-liquid phase transition based on network entanglement ideas. I am sure this work will inspire novel theoretical modelling based on topological concepts.”
The team anticipates that the model they have created will open the door for fresh research that will support the hypothesis and broaden the definition of “entangled” liquids to include other liquids like silicon.
Pablo Debenedetti, a professor of chemical and biological engineering at Princeton University in the US and a world-leading expert in this area of research, remarks:
“This beautiful computational work uncovers the topological basis underlying the existence of different liquid phases in the same network-forming substance.”
He adds: “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.”
Christian Micheletti, a professor at International School for Advanced Studies in Trieste, Italy, whose current research interest lies in understanding the impact of entanglement, especially knots and links, on the static, kinetics and functionality of biopolymers, remarks:
“With this single paper, Neophytou et al. made several breakthroughs that will be consequential across diverse scientific areas. First, their elegant and experimentally amenable colloidal model for water opens entirely new perspectives for large-scale studies of liquids.”
“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.”
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.”
The research was supported by the Royal Society via International Exchanges Award, which enabled the international collaboration between the researchers in the UK and Italy, the EPSRC Centre for Doctoral Training in Topological Design and the Institute of Advanced Studies at the University of Birmingham, and the Italian Ministero Istruzione Università Ricerca Progetti di Rilevante Interesse Nazionale.
