by A collaboration between scientists at Cambridge and UCL has led to the discovery of a new form of ice that resembles liquid water more than any other and may hold the key to understanding this most famous of liquids.
The new ice form is amorphous. Unlike ordinary crystalline ice, where the molecules arrange themselves in a regular pattern, the molecules in amorphous ice are in a disorganized form that resembles a liquid.
In this article, published in Science, The team created a new form of amorphous ice in the experiment and created an atomic-scale model of it in a computer simulation. The experiments used a technique called ball milling, which uses metal balls to grind crystalline ice into small particles in a steel vessel. Ball milling is regularly used to create amorphous materials, but it has never been applied to ice.
The team found that ball milling produced a novel amorphous form of ice that, unlike any other known type of ice, had a density similar to that of liquid water and was in a state similar to solid water. They named the new ice medium-density amorphous ice (MDA).
To understand the process at the molecular level, the team used computer simulations. By mimicking the ball milling process through repeated random shearing of crystalline ice, the team successfully computationally modeled MDA.
“Our discovery of MDA raises many questions about the nature of liquid water, and thus understanding the precise atomic structure of MDA is very important,” comments co-author Dr. Michael Davies who did the computer modelling. “We found striking similarities between MDA and liquid water.”
A happy middle ground
Amorphous ice has been proposed as a model for liquid water. To date, there have been two main types of amorphous ice: high and low density amorphous ice.
As the names suggest, there is a large density gap between them. This difference in density, combined with the fact that liquid water’s density is intermediate, has been a cornerstone of our understanding of liquid water. This has led in part to the assumption that water consists of two liquids: one with a high density and one with a low density.
Lead author Professor Christoph Salzmann said: “The generally accepted wisdom has been that no ice exists within this density gap. Our study shows that the density of MDA falls right within this density gap, and this finding may have far-reaching implications for our understanding of liquid water and its many anomalies.”
A high-energy geophysical material
The discovery of MDA raises the question: where might it occur in nature? In this study, shear forces were found to be key in creating MDA. The team suggests that ordinary ice in the icy moons may experience similar shear forces due to tidal forces exerted by gas giants like Jupiter.
In addition, MDA exhibits a remarkable property not found in other forms of ice. Using calorimetry, they found that when MDA recrystallizes into ordinary ice, it releases an extraordinary amount of heat. The heat released during the recrystallization of MDA could play a role in activating tectonic movements. More broadly, this discovery demonstrates that water can be a highly energetic geophysical material.
Prof Angelos Michaelides, lead author from Cambridge, said: “Amorphous ice in general is considered to be the most abundant form of water in the Universe. Now it’s a matter of understanding how much of it is MDA and how geophysically active MDA is.”
