By Leah Crane
Freezing starts small. When a droplet of water goes from liquid to solid, it usually starts with a tiny particle called an ice nucleus on which the first ice crystals form before they spread to the rest of the droplet or through a larger body of water. Now we know exactly how big that particle needs to be to start the process.
“A bottle of very pure water remains liquid even below 0 °C, but quickly freezes if one slightly shakes the bottle,” says Xin Zhou at the University of Chinese Academy of Sciences in Beijing. That is because the shaking introduces irregularities in the water that act as ice nuclei and start the freezing process. But more commonly the nuclei are impurities.
To examine how big a particle must be in order to start ice formation, Zhou and his colleagues used trillions of tiny flecks of graphene oxide. They mixed water with sheets of this material that ranged from 3 square nanometres to 50 square nanometres in size, and then lowered the temperature.
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When they used graphene oxide flecks smaller than 8 square nanometres, ice only began to form on the edges of the nanosheets at extremely cold temperatures – around -27.5 °C. But at 8 square nanometres, the critical temperature where ice starts to form jumped up by about 10 °C. They concluded that a particle must be about 8 square nanometres or bigger to act as an ice nucleus at these temperatures. They tested this with graphene oxide because it is easy to make tiny scraps of it, but they also tested other nanoparticles and confirmed that the process was the same regardless of the material.
The effect works in reverse, too. When they tested a coating of nanoparticles smaller than 8 square nanometres applied to a surface, it impeded ice formation, so a similar material could be used on anything you don’t want to get icy, Zhou says. He hopes that this knowledge will allow us to design better antifreeze coatings for use on aircraft to prevent a potentially dangerous build-up of ice.
Journal reference: Nature, DOI: 10.1038/s41586-019-1827-6
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