Using ultracold temperatures and some steel ball bearings, scientists have created a brand-new, bizarre form of ice that has the same density as liquid water.
The ice, known as medium-density amorphous ice, fits into a gap in the annals of frozen water that scientists weren't sure would ever be filled. Unlike the crystalline ice that forms naturally on Earth, the newly created ice doesn't have an organized molecular structure. Instead, its molecules are in a chaotic mismatch, more like glass — a state known as amorphous. Other types of amorphous ice have been made before, but they've been either much less dense or far denser than liquid water. This new Goldilocks version of amorphous ice is right in the middle, almost exactly matching liquid water's density, researchers explained in a new study published in the journal Science (opens in new tab) on Feb. 2.
"It's something completely new," said study senior author Christoph Salzmann (opens in new tab), a professor of physical and materials chemistry at University College London.
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When ice freezes normally on Earth (opens in new tab), its molecules stack into an organized crystalline structure. This crystalline ice is one of the weird quirks of H2O, because it floats on liquid water in its solid state rather than sinking. This is due to the relatively big gaps in the crystal structure of water ice, compared with other materials that form denser structures when they crystallize.
When properly manipulated, though, liquid water can also freeze in a disorganized, amorphous state. The first of these states, low-density amorphous ice, was discovered in the 1930s. It's made by depositing water vapor on very cold surfaces. This process happens naturally in space, Salzmann said, so low-density amorphous ice may be the most common form of ice in the universe.
In the 1980s, researchers discovered that they could also make high-density amorphous ice by compressing regular ice at very low temperatures (opens in new tab). But no one had ever made amorphous ice with a medium density — that is, until Salzmann and his colleagues had a "crazy Friday afternoon idea." They decided to try ball milling ice.
A ball mill is a device kind of like a very advanced cocktail shaker. A material is put into a chamber with stainless-steel balls and shaken or turned until the material is ground up. Ball milling is used in many industries, but it's particularly good at creating amorphous materials and at grinding soft, frozen materials into powders, Salzmann said.
"We said, 'Why don't we ball mill ice and see what happens?'" Salzmann said.
The researchers expected that the ball mill would just break the ice crystals into smaller ice crystals. But that's not what happened. Instead, the tumbling steel balls sheared and compressed the ice crystals, shoving them into a new state of disorganization. The result? Medium-density amorphous ice.
Computer modeling showed that the ice starts in a nice, crystalline state, its hydrogen bonds forming a hexagonal lattice. The random shearing from the ball-milling pushes these hydrogen bonds this way and that, leaving them pointing up and down in a chaotic zigzag.
The new form of ice forms at 77 kelvins, or minus 321 degrees Fahrenheit (minus 196 degrees Celsius). It has some odd properties beyond its density of 1.06 grams per cubic centimeter (0.037 ounces per 0.06 cubic inches). (Water has a density of 1 gram per cubic centimeter, or 0.035 ounces per 0.06 cubic inches.) Among them, Salzmann said, is that when the researchers compressed the medium-density ice and heated it to minus 185 F (minus 120 C), the ice recrystallized, releasing a large amount of heat.
"With other forms of [amorphous] ice, if you compress them and you release the pressure, it's like nothing happened," Salzmann said. "But the MDA [medium-density amorphous ice] somehow has this ability to store the mechanical energy and release it through heating."
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Medium-density amorphous ice might occur naturally on the ice moons of gas giant planets, Salzmann said, where the gravitational forces of the enormous worlds compress and shear the moons' ice. If so, the mechanical energy stored in this form of ice could influence the tectonics on these Hoth-like moons.
Understanding medium-density amorphous ice could also help researchers understand liquid water better more generally, Salzmann said. Water is odd not only because its crystalline form floats but also because it has other unique properties, like high surface tension and high melting and boiling points. Scientists still debate the nature of water at extremely low temperatures. Any debate now needs to take into account medium-density amorphous ice, Salzmann said.
"A lot of our understanding of liquid water was built on the pillars that there is low-density and high-density amorphous ice," he said. "How does the medium-density amorphous ice fit into that picture?"
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