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David Bradley ISSUE #72
January 2008
Ice Age the Movie

The ordered structure of ice dissolves little by little into disorder when a tiny burst of light hits an ice crystal. At least that's what computer simulations of the process carried out by Swedish researchers from the University of Uppsala would suggest. The study could have implications for understanding one of the most anomalous materials we know, water, H2O.

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Carl Caleman and David van der Spoel simulated what would happen if a burst of light lasting just a fraction of a second were to hit an ice crystal. Writing in the journal Angewandte Chemie, they suggest that the energy absorbed from the burst of light by the ice will first make the bonds between hydrogen and oxygen atoms in the frozen water oscillate. Within a few picoseconds (millionths of a millionth of a second) the energy of these oscillations is converted into twists and turns that break apart the previously locked together water molecules, causing the ice crystal to melt. However, some semblance of crystalline order remain visible for some time after this until the ice is completely melted.

The common form of ice crystals is known as hexagonal ice. In this form the oxygen atoms of the water molecules are arranged in a tetrahedral lattice. Each water molecule is bound to four neighboring molecules by means of bridging hydrogen bonds, leading to an average of two bridges per molecule. In water, there are, on average, only 1.75 bridging hydrogen bonds per molecule.

Caleman and van der Spoel's molecular dynamics movie of melting ice made it possible for them to simultaneously describe both the structure and the dynamics of the melting system with atomic resolution and with a time resolution (frame-rate) of the order of femtoseconds (10-15 s, billionths of a millionth of a second).

http://dx.doi.org/10.1002/anie.200703987 Angew Chem Int Edn, 2008, in press