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Int'l team makes "squarest" ice crystals

SAN FRANCISCO, July 10 (Xinhua) -- An international research team has set a new record for creating ice crystals that have a near-perfect cubic arrangement of water molecules.

Seen under a microscope, normal water ice is made of crystals with hexagonal symmetry, explained Barbara Wyslouzil, professor of chemical and biomolecular engineering at the Ohio State University. But with only a slight change in how the water molecules are arranged in ice, the crystals can take on a cubic form.

Researchers have used the presence of cold cubic ice clouds high above the earth's surface to explain interesting halos observed around the sun, as well as the presence of triangular ice crystals in the atmosphere. They have struggled for decades 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 around 70 percent cubic, 30 percent hexagonal.

"While 80 percent might not sound 'near perfect,' most researchers no longer believe that 100 percent pure cubic ice is attainable in the lab or in nature," said Wyslouzil, who worked with graduate research associate Andrew Amaya and their collaborators, including those in Singapore and Sweden, on the project. "So the question is, how cubic can we make it with current technology? Previous experiments and computer simulations observed ice that is about 75 percent cubic, but we've exceeded that."

In a paper published in the Journal of Physical Chemistry Letters, the researchers said they 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. The droplets were highly supercooled, well below the usual freezing temperature of 32 degrees Fahrenheit, or 0 degrees Celsius. In fact, they remained liquid until about -55 degrees Fahrenheit, around -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, Wyslouzil was quoted as saying in a news release: "Since liquid water drops in high-altitude clouds are typically supercooled, there is a good chance for cubic ice to form there."

To measure the cubicity of the ice formed, the researchers performed X-ray diffraction experiments at the Linac Coherent Light Source (LCLS) at the SLAC National Accelerator Laboratory in Menlo Park, California, where the droplets were hit with the high-intensity X-ray laser from LCLS and recorded the diffraction pattern on an X-ray camera. They saw concentric rings at wavelengths and intensities that indicated the crystals were around 80 percent cubic.

Exactly why it was possible to make crystals with around 80 percent cubicity is currently unknown and 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 Claudiu Stan, a research associate at the Stanford PULSE Institute at SLAC and partner in the project. "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."

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