Scientists vibrating normal frozen water in a pot of ultra-cold steel balls have discovered a previously unknown form of ice, closer to liquid water than any other ice to date.
It is amorphous ice, a form not found in nature on Earth. This is because its atoms are not arranged in a neat repeating crystalline pattern, but pell-mell, in atomic disorder.
But the amorphous ice that emerged 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, which is 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 (MDA) ice.
“Water is the foundation of all life. Our existence depends on it, we launch space missions in search of it, but 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 been discovered before, known as high-density and low-density amorphous ice. There is a huge density gap between them and the accepted wisdom is that no ice exists in this density gap,” says Salzmann.
“Our study shows that the density of MDA lies precisely within this density gap and this finding may have far-reaching implications for our understanding of liquid water and its many anomalies.”
Water, not beating around the bush, it’s just weird. Because it’s so ubiquitous and necessary for our survival, we don’t think much about it, but it doesn’t follow the same rules as other liquids.
It is a universal solvent; that is, many other substances dissolve in it very 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 strangest thing of all: as most fluids freeze, their density increases. Water does the opposite: it becomes less dense, which means that water ice is generally less dense than water. That’s why ice cubes float in your drink.
But not all ice cream is created equal. Here on Earth, ice naturally takes on a crystalline form, with its atoms arranged in a repeating hexagonal pattern. This is why snowflakes tend to be hexagonal. In the near-vacuum of space, however, ice is generally amorphous, as the atoms do not retain enough thermal energy to wiggle into a crystalline structure.
The density gap in amorphous ice was quite fundamental to our understanding of water. In fact, previous research and simulations have revealed that the split could mean that water exists as two separate liquids at very cold temperatures, even coexisting like oil and, well, water rather than to mix if the conditions were met. Hey, the water did stranger things.
But then Rosu-Finsen and his colleagues got their hands on steel balls. Ball milling is an industrial technique used to grind or mix materials. The researchers used liquid nitrogen to cool a grinding 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,” says Rosu-Finsen. “Rather than ending up with smaller chunks of ice, we realized we had found a whole new kind of thing, with some remarkable properties.”
The meaning of these properties is still not entirely clear. MDA could be a “glassy” state of liquid water, the researchers suggest. Although amorphous ice does not form in nature, other amorphous solids exist; glass is one of them, and it is simply a solid form of liquid silicon dioxide. But MDA could also just be highly sheared crystalline ice.
This suggests that our existing water models need to be reexamined to determine where MDA fits into the picture. But already, it looks promising for explaining some of the ways water ice behaves in the Universe.
The researchers experimented to see what happens when MDA recrystallizes, compresses it, and heats it up. They found that this process releases a surprising amount of energy, suggesting that MDA may play a role in tectonic activity on ice-encrusted worlds like the Jovian moon Ganymede.
And the discovery also shows potential for future experiments and probes of special properties of water.
“We’ve shown that it’s possible to create what looks like a sort of stop-motion water,” says chemist Andrea Sella of University College London.
“This is an unexpected and quite astonishing finding.”
The research has been published in Science.
