The conventional view among scientists about how ice forms is that it begins from seeds in which water molecules are packed together in a hexagonal structure and maintains this structure as it grows. But a new modeling study suggests that the nanoscopic ice seeds actually begin as a mix of hexagonal and cubic layers known as a “stacking disordered” structure (Nature 2017, DOI: 10.1038/nature24279). These eventually grow to form bulk ice with the traditional hexagonal structure. The findings have implications for weather models that consider how fast ice forms in clouds.
“Classical nucleation theory assumes that the nucleus is like a piece of the bulk phase,” says Valeria Molinero, a chemistry professor at the University of Utah who led the study. Molinero’s group found, however, that stacking disordered ice seeds are entropically favored over the hexagonal form. “What we’re seeing is that cubic ice is less stable than hexagonal ice, but mixing of the two favors cubic ice,” she says.
According to Tianshu Li, an associate professor in the department of civil and environmental engineering at George Washington University who studies ice nucleation, researchers had experimentally observed these stacking disordered seeds before but assumed they formed because of kinetic effects and weren’t thermodynamically stable. “This finding has remained puzzling,” he adds, but this new modeling study “now shows that the observed behavior is in fact a natural outcome of thermodynamics rather than kinetics.”
The new simulations show that stacking disordered ice nucleation rates are more than three orders of magnitude as fast as those predicted by classical nucleation theory. And the effect is size and temperature dependent. The classical theory needs to incorporate that size and temperature dependence so that extrapolations from lab experiments will accurately predict conditions in clouds, Molinero says.
The new starting point will have a significant effect on cloud models, Molinero says, especially in remote areas without aerosols that aid ice seed growth. Although the amount of radiation that clouds absorb depends strongly on how much ice they contain, she cautions that these new findings may not have a large impact on climate models.