Scientists rattling normal frozen water around in a jar of ultracold steel balls have discovered a previously unknown form of ice, closer to liquid water than any other ice yet.
This is amorphous ice, a form not found in nature on Earth. That's because its atoms are arranged not in a neat repeating crystalline pattern, but jumbled up all higgledy-piggledy, an atomic omnishambles.
But the amorphous ice emerging from the team's experiments, a process called ball milling, is unlike any amorphous ice ever seen.
Amorphous ice is usually low density, around 0.94 grams per cubic centimeter, or high density, starting at 1.13 grams per cubic centimeter. The new ice has a density of 1.06 grams per cubic centimeter – clocking in incredibly close to the density of water, at 1 gram per cubic centimeter.
Researchers led by chemist Alexander Rosu-Finsen, formerly of University College London in the UK, have named the new form medium-density amorphous ice (MDA).
"Water is the foundation of all life. Our existence depends on it, we launch space missions searching for it, yet from a scientific point of view it is poorly understood," says chemist Christoph Salzmann of University College London.
"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," Salzmann explains.
"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."
Water, not beating around the bush, is just weird. Because it's so ubiquitous and necessary for our survival, we don't tend to think about it much, but it doesn't follow the same rules as other liquids.
It's a universal solvent; that is, many other substances dissolve in it really easily. Its surface tension is unusually high compared to other liquids, as is its boiling point.
And its density under cooling conditions is perhaps the weirdest thing of all: as most fluids freeze, their density increases. Water does the opposite: it becomes less dense, meaning water ice is generally less dense than water. This is why ice cubes float in your drink.
But not all ice is created alike. Here on Earth, ice naturally takes a crystalline form, with its atoms arranged in a repeating hexagonal pattern. That's why snowflakes tend to be hexagonal. In the near-vacuum of space, however, ice is usually amorphous, because the atoms don't retain enough thermal energy to wiggle around into a crystalline structure.
The density gap in amorphous ice was fairly fundamental to our understanding of water. In fact, previous research and simulations found the division could mean that water exists as two separate liquids at very cold temperatures, even coexisting like oil and, well, water rather than mixing if the conditions were right. Hey, water has done weirder things.
But then Rosu-Finsen and his colleagues got their hands on some steel balls. Ball milling is an industrial technique used to grind or blend materials. The researchers used liquid nitrogen to cool a milling jar to -200 degrees Celsius (-328 degrees Fahrenheit), added normal water ice, and shook things up.
"We shook the ice like crazy for a long time and destroyed the crystal structure," Rosu-Finsen explains. "Rather than ending up with smaller pieces of ice, we realized that we had come up with an entirely new kind of thing, with some remarkable properties."
What these properties mean is not quite clear yet. MDA could be a "glassy" state of liquid water, the researchers suggest. Although amorphous ice doesn't form in nature, other amorphous solids exist; glass is one of them, and it's simply a solid form of liquid silicon dioxide. But MDA could also simply be heavily sheared crystalline ice, too.
It does suggest that our existing models of water need to be re-examined in order to find out where MDA fits into the picture. But already it's showing promise for explaining some of the ways water ice behaves in the Universe.
The researchers experimented to see what happens when MDA recrystallizes, compressing it and warming it up. They found that this process releases a surprising amount of energy, suggesting that MDA could play a role in tectonic activity on ice-encrusted worlds like Jovian moon Ganymede.
And the discovery shows potential for future experiments and probes of the peculiar properties of water, too.
"We have shown it is possible to create what looks like a stop-motion kind of water," says chemist Andrea Sella of University College London.
"This is an unexpected and quite amazing finding."
The research has been published in Science.
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