Physical chemists from Ohio State University and elsewhere have set a new record for creating ice crystals that have a near-perfect cubic arrangement of water molecules.
Amaya et al created 78% cubic ice crystals. Image credit: Ohio State University.
Seen under a microscope, normal water ice is made of crystals with hexagonal symmetry.
But with only a slight change in how the water molecules are arranged in ice, the crystals can take on a metastable cubic form, commonly known as ‘cubic ice’ or ‘ice Ic.’
Physical chemists have struggled for years to make cubic ice in the laboratory, but because the cubic form is unstable, the closest anyone has come is to make hybrid crystals that are 73.3% cubic, 26.7% hexagonal.
In a paper published in the Journal of Physical Chemistry Letters, Ohio State University Professor Barbara Wyslouzil and co-authors describe how they were able to create frozen water droplets that were 78% cubic.
“While 78% might not sound ‘near perfect,’ most researchers no longer believe that 100% pure cubic ice is attainable in the lab or in nature,” Professor Wyslouzil said.
“So the question is, how cubic can we make it with current technology?”
“Previous experiments and computer simulations observed ice that is about 75% cubic, but we’ve exceeded that.”
To make the highly cubic ice, the team drew nitrogen and water vapor through nozzles at supersonic speeds.
When the gas expanded, it cooled and formed droplets a hundred thousand times smaller than the average raindrop.
These droplets were highly supercooled, meaning that they were liquid well below the usual freezing temperature of 32 degrees Fahrenheit (0 degrees Celsius).
In fact, the droplets remained liquid until about minus 55 degrees Fahrenheit (around minus 48 degrees Celsius) and then froze in about one millionth of a second.
“The extremely low temperatures and rapid freezing were crucial to forming cubic ice,” Professor Wyslouzil said.
“Since liquid water drops in high-altitude clouds are typically supercooled, there is a good chance for cubic ice to form there.”
Exactly why it was possible to make crystals with 78% cubicity is currently unknown. But, then again, exactly how water freezes on the molecular level is also unknown.
“When water freezes slowly, we can think of ice as being built from water molecules the way you build a brick wall, one brick on top of the other,” said co-author Dr. Claudiu Stan, a research associate at the Stanford PULSE Institute at SLAC.
“But freezing in high-altitude clouds happens too fast for that to be the case — instead, freezing might be thought as starting from a disordered pile of bricks that hastily rearranges itself to form a brick wall, possibly containing defects or having an unusual arrangement.”
“This kind of crystal-making process is so fast and complex that we need sophisticated equipment just to begin to see what is happening.”
“Our research is motivated by the idea that in the future we can develop experiments that will let us see crystals as they form.”
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Andrew J. Amaya et al. 2017. How Cubic Can Ice Be? J. Phys. Chem. Lett 8: 3216-3222; doi: 10.1021/acs.jpclett.7b01142
