We learn in grade school that water freezes at zero degrees Celsius, but that’s seldom true. In clouds, scientists have found supercooled water droplets as chilly as minus 40 C, and in a lab in 2014, they cooled water to a staggering minus 46 C before it froze. You can supercool water at home: Throw a bottle of distilled water in your freezer, and it’s unlikely to crystallize until you shake it.
Freezing usually doesn’t happen right at zero degrees for much the same reason that backyard wood piles don’t spontaneously combust. To get started, fire needs a spark. And ice needs a nucleus — a seed of ice around which more and more water molecules arrange themselves into a crystal structure.
Valeria Molinero, a physical chemist at the University of Utah, builds computer simulations of water to study ice nucleation.
The formation of these seeds is called ice nucleation. Nucleation is so slow for pure water at zero degrees that it might as well not happen at all. But in nature, impurities provide surfaces for nucleation, and these impurities can drastically change how quickly and at what temperature ice forms.
For a process that’s anything but exotic, ice nucleation remains surprisingly mysterious. Chemists can’t reliably predict the effect of a given impurity or surface, let alone design one to hinder or promote ice formation. But they’re chipping away at the problem. They’re building computer models that can accurately simulate water’s behavior, and they’re looking to nature for clues — proteins made by bacteria and fungi are the best ice makers scientists know of.
Understanding how ice forms is more than an academic exercise. Motes of material create ice seeds in clouds, which lead to most of the precipitation that falls to Earth as snow and rain. Several dry Western states use ice-nucleating materials to promote precipitation, and U.S. government agencies including the National Oceanic and Atmospheric Administration and the Air Force have experimented with ice nucleation for drought relief or as a war tactic. (Perhaps snowstorms could waylay the enemy.) And in some countries, hail-fighting planes dust clouds with silver iodide, a substance that helps small droplets to freeze, hindering the growth of large hailstones.
But there’s still much to learn. “Everyone agrees that ice forms,” said Valeria Molinero, a physical chemist at the University of Utah who builds computer simulations of water. “After that, there are questions.”
You can read the full story in Quanta magazine. Read the published research @PNAS.
Nathen Patchen
Mathematics