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Physical Chemistry

In clouds, cubic-structured ice

X-ray scattering studies of water nanodroplets close gap between experiments and simulations

by Jyllian Kemsley
July 24, 2017 | APPEARED IN VOLUME 95, ISSUE 30

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Credit: Phys. Chem. Chem. Phys. 2015, DOI: 10.1039/c4cp02893g
Although the hexagonal structure of ice is most stable (left), supercooled nanodroplets can freeze with nearly 80% cubic structure (right). Hydrogens are omitted for clarity; bonds between oxygen atoms follow the path of an O–H bond and the sequential hydrogen bond to the adjacent oxygen.
Credit: Phys. Chem. Chem. Phys. 2015, DOI: 10.1039/c4cp02893g
Although the hexagonal structure of ice is most stable (left), supercooled nanodroplets can freeze with nearly 80% cubic structure (right). Hydrogens are omitted for clarity; bonds between oxygen atoms follow the path of an O–H bond and the sequential hydrogen bond to the adjacent oxygen.

The phase of ice formed from water droplets in clouds can affect clouds’ vapor content and optical properties. Although ice is most stable when its water molecules arrange through hydrogen bonding into a hexagonal structure, supercooling processes in clouds could allow formation and trapping of metastable structures such as cubic or “stacking disordered” mixtures of hexagonal and cubic layers. New research suggests that ice formed from supercooled water nanodroplets includes more cubic structure than previously determined experimentally, confirming molecular dynamics simulations (J. Phys. Chem. Lett. 2017, DOI: 10.1021/acs.jpclett.7b01142). Barbara E. Wyslouzil of Ohio State University, Claudiu A. Stan of SLAC National Accelerator Laboratory, and their colleagues produced and then froze supercooled water nanodroplets and used wide-angle X-ray scattering to study their structures. The free-electron laser setup at SLAC allowed the researchers to work with smaller droplets and probe the ice structures faster than in previous experiments. The scientists found that the ice crystals’ structure was stacking disordered with 78% cubic structure. Prior studies, which focused on micro​meter-sized droplets, indicated that they had about 50% cubic structure, whereas simulations of nanodroplets suggested as much as 70%.

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