Tiniest cubic ice crystals: X-rayed

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  • Published: Jul 15, 2017
  • Author: David Bradley
  • Channels: X-ray Spectrometry
thumbnail image: Tiniest cubic ice crystals: X-rayed

Near-perfect cubes

A new X-ray study of the tiny, almost perfect ice cubes formed in the chilly realm of high altitude clouds but extremely difficult to make at ground level could open up studies on how sunlight and air interact with clouds to improve our models of climate change. Credit: Wyslouzil et al

A new X-ray study of the tiny, almost perfect square ice crystals predicted to form in the chilly realm of high altitude clouds but extremely difficult to make at ground level could open up studies on how sunlight and air interact with clouds thereby improving our models of climate change.

An international team of scientists has set a new record for creating ice crystals that have a near-perfect cubic arrangement of water molecules. This ice polymorph has proved largely elusive in the laboratory despite the urgency with which we need new insights into its properties for atmospheric and climate science, says Barbara Wyslouzil of The Ohio State University a professor of chemical and biomolecular engineering who led the project.

The anomalous properties of water are well known as is water's critical importance to life on earth. Textbooks describe frozen water as having hexagonal symmetry and the existence of octagonal snowflakes and other horrors in popular culture will always raise the hackles of any chemist or crystallographer. Nevertheless, a slight rearrangement of the way in which water molecules hook together in the solid state can give rise to an intriguing cubic form.

All hail the halo

It has been assumed that cold clouds containing cubic ice, or ice with triangular symmetry, high above the earth's surface are what give rise to stunning atmospheric phenomena including halos around the sun. However, scientists have struggled for many years to produce cubic ice under laboratory conditions simply because it is so unstable. The closest any laboratory has got was to make hybrid crystals that contain around 70 percent cubic and 30 percent hexagonal forms of ice.

Now, writing in the Journal of Physical Chemistry Letters, Wyslouzil, graduate research associate Andrew Amaya and their collaborators describe how they have pushed this ratio closer to the cubic, freezing water droplets to 80 percent cubic crystals.

"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," Wyslouzil says. "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 order to generate cubic ice crystals, the team blasted nitrogen gas and water vapour through a nozzle at supersonic speed. As the gas mixture expands, it cools dramatically and tiny droplets form a hundred thousand times smaller than the average raindrop. These droplets are highly supercooled, in other words, they remain in the liquid state but are well below water's normal freezing point even as cold as -48 degrees Celsius. Within a microsecond these liquid droplets freeze solid trapping the water in what the team hoped would be a cubic state rather than the more common hexagonal phase.

International science

Indeed, when they carried out X-ray laser diffraction studies on their frozen droplets at the Linac Coherent Light Source (LCLS) at the SLAC National Accelerator Laboratory in Menlo Park, California, they obtained a diffraction pattern indicative of a mostly cubic structure; 80 percent cubic, in fact.

The team explains that the extremely low temperature to which they could take the water in the liquid state using supersonic jets and the subsequent rapid freezing were crucial to forming cubic ice. "Since liquid water drops in high-altitude clouds are typically supercooled, there is a good chance for cubic ice to form there," Wyslouzil explains. The team does not yet know the detailed explanation for obtaining such a high ration of cubic to hexagonal ice in their experiments. That said, science does not actually have a precise theory as to how water freezes at the molecular level under more everyday conditions either!

"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," explains team member Claudiu Stan, of 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."

"The next step is to look at the ice formation rates and to try and understand why what we measure in our nano-scaled droplets is so different than what others measure in bigger, micron-sized droplets,." Wyslouzil told us. "In the long run, we want to go beyond characterizing the properties of ice formed under these special condition, and also see and understand how ice forms," Stan adds.

The international team working on this project also included scientists from the National University of Singapore, Stockholm University, KTH Royal Institute of Technology, Brookhaven National Laboratory and the National Science Foundation BioXFEL Science and Technology Center.

Related Links

J Phys Chem Lett 2017, online: "How Cubic Can Ice Be?"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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