Part of the set-up for creating medium-density amorphous ice. Credit: Christoph Salzmann
Key points:
- Researchers discovered a new type of ice that more closely resembles liquid water than any other known ices.
- The new amorphous ice, named MDA, has an unexpected, never-before-seen “medium-density.”
- The team says this discovery could rewrite our understanding of water.
UK researchers have discovered a new form of ice that more closely resembles liquid water than any other and may rewrite our understanding of water and its many anomalies.
In a new paper, published in Science, the researchers created a new form of amorphous ice in experiment and achieved an atomic-scale model of it in computer simulation. Unlike ordinary crystalline ice where the molecules arrange themselves in a regular pattern, in amorphous ice, the molecules are in a disorganized form that resembles a liquid.
The team found that ball-milling— vigorously shaking ordinary ice together with steel balls in a jar cooled to -200 degrees Centigrade—created a novel amorphous form of ice, which unlike all other known ices, had a density similar to that of liquid water and whose state resembled water in solid form. They named the new ice medium-density amorphous ice (MDA).
“We know of 20 crystalline forms of ice, but only two main types of amorphous ice have previously been discovered, known as high-density and low-density amorphous ices. There is a huge density gap between them and the accepted wisdom has been that no ice exists within that density gap,” said senior author Christoph Salzmann, professor at the University College London. “Our study shows that the density of MDA is precisely within this density gap and this finding may have far-reaching consequences for our understanding of liquid water and its many anomalies.”
The density gap between the known amorphous ices has led scientists to suggest water in fact exists as two liquids at very cold temperatures and that theoretically, at a certain temperature, both of these liquids could co-exist, with one type floating above the other, as when mixing oil and water. This hypothesis has been demonstrated in a computer simulation, but not confirmed by experiment. The researchers say that their new study may raise questions about the validity of this idea.
“Existing models of water should be re-tested. They need to be able to explain the existence of medium-density amorphous ice. This could be the starting point for finally explaining liquid water,” said Salzmann.
The researchers proposed that the newly discovered ice may be the true glassy state of liquid water – that is, a precise replica of liquid water in solid form, in the same way that glass in windows is the solid form of liquid silicon dioxide.
However, another scenario is that MDA is not glassy at all, but is in a heavily sheared crystalline state.