The colder that water gets, the smaller this energy barrier gets. This makes it easier for random molecular motions to push a tiny embryonic ice structure over the critical size threshold. Ice forms and grows, and the lower-energy crystal structure stays stable.
Boosting Nucleation
Surfaces and impurities can dramatically lower the energy barrier for nucleation — and therefore raise the temperature at which ice forms. “Since the late 1970s, we’ve known that there are lots of aspects of a surface that are important,” said Miriam Freedman, an atmospheric chemist at Pennsylvania State University.
Like a microscopic construction scaffold, surfaces with the right structure make it easier for water molecules to arrange themselves into a crystal. Researchers have identified a few things that can make a surface better or worse at nucleating ice. A surface’s crystallinity, or structural orderliness, matters. And substances with chemical structures that mimic ice tend to be good at ice nucleation. Pores of a certain size confine water molecules in a way that helps ice form, too.
Meister and Molinero have been working together to unravel the secrets of nature’s best snow makers — bacteria and fungi whose proteins interact with water in ways that promote ice nucleation. Many of these organisms are plant pathogens, and it’s possible that their ice-nucleating proteins evolved to cause frost damage.
The best known ice nucleator is a bacterium called Pseudomonas syringae, which has a protein that can force water to freeze at around minus 2 degrees Celsius. “It’s so good that all the artificial snowmaking, at least in Utah, and some [other] places in the U.S., uses this bacteria to make snow,” Meister said.
Bigger proteins tend to be better for making ice, possibly because they act as a more effective template: Imagine trying to build a skyscraper with a scaffold just a few stories tall.
But with all they know, scientists still encounter surprises. Meister, Molinero and their co-authors recently discovered an exception to the bigger-is-better rule: fungal proteins that are great at ice nucleation despite being tiny. They get around the problem by clumping together into large, ice-nucleating aggregates.
Predicting Ice
Molinero develops theories and computational models that capture how ice nucleates, including its interaction with surfaces. In 2009, she and her colleague Emily Moore published a simplified model of water that treats each H2O molecule as a single, tetrahedron-shaped atom; surprisingly, computer simulations of this monatomic-water model accurately reproduce water’s large-scale properties, like its density. Then, in 2011, Molinero and Moore used the monatomic-water model to pinpoint a specific structural change in supercooled water that sets the lower limit of water’s freezing point. The model predicts that water must freeze at minus 48.15 C.
More recently, in computer simulations published in May in the Proceedings of the National Academy of Sciences, Molinero and her colleagues showed that ice crystallization happens fastest when water’s temperature and pressure are tuned to a point of transition between denser and less dense liquid phases. And in March, they presented a new model at the American Chemical Society conference that can predict the temperature at which ice will nucleate on a given surface. The model is informed by experimental data and considers a battery of factors, from the surface’s chemistry to the shapes of its defects.
Depending on their size and geometry, bumps and divots on a surface can squeeze water molecules into configurations that make it easier or harder for ice to form. As part of their model, Molinero’s team developed and tested a new formula for how the bump or divot’s angle affects ice nucleation. Using the formula, Molinero thinks it should be possible to design better ice-nucleating materials just by introducing defects of the right size and shape. “You can take a surface that is not so good and make it quite outstanding,” she said.
According to Molinero, the models atmospheric scientists use to predict cloud behavior don’t yet account for the nuances of ice nucleation. And it’s still unclear which particles are actually the most important for seeding clouds in nature. Mineral particles like Saharan dust are abundant in the atmosphere and can nucleate ice. But they’re not alone up there.
“Up in the clouds, you find some of these bacteria, some of these fungi, that are very good at ice making,” Meister said. “That completely raises the question: What makes it rain?”