GoKawiilBeneath a suburb of Hamburg in Germany, an underground particle accelerator propels electrons at close to the speed of light through a slalom course of magnets. Racing through the twists and turns, the electrons emit bursts of radiation that produce one of the world’s most powerful X-ray laser beams.
This prized machine, the European X-ray Free Electron Laser Facility (XFEL), has helped researchers to make ultrafast movies of chemical reactions and to map the atomic structures of viruses. Now, they are using it to crack the secrets of a seemingly simple process that has bedevilled scientists for decades: how water and other liquids freeze.
For 150 years, theorists have been trying to explain the process that turns pure liquids into solids. But their models of how quickly this happens are often wildly inaccurate when compared with experiments — results can be off by as much as 20 orders of magnitude.
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It’s hard to resolve this problem, says theorist Michele Parrinello at the University of Italian Switzerland in Lugano, Switzerland. “Experiments are very difficult,” he says. “And theory is difficult, and computer simulations are also difficult.” Tiny errors in modelling or in experiments can lead to huge changes in outcomes.
The mystery behind the freezing of liquids — including molten metals — is not just an esoteric puzzle. More accurate insights into freezing would help in understanding how ice develops in clouds high in Earth’s atmosphere and, in turn, would improve the models that are used to forecast how quickly the world will warm because of greenhouse gases. Better theories would also provide information for geophysicists on how Earth’s solid inner core formed and what happens inside other planets.
At the European XFEL and other laboratories with similar capabilities, researchers are finally starting to make headway in cracking the freezing problem. Using innovative experimental designs, they have captured the first few microseconds of the process. These and other developments are helping them to close the gap in understanding how this happens. And it turns out that disorder plays a bigger part in freezing than scientists had thought.
Pure problems
Modern theories about freezing have their roots in the work of physicists Daniel Fahrenheit in the early eighteenth century and Josiah Willard Gibbs more than a century later. Gibbs used statistical mechanics to describe the freezing process of a pure liquid — that is, one that doesn’t contain any contaminating particles.
In nature, most freezing isn’t so pure. Take what happens when you put a glass of water in a freezer. Water molecules first start to crystallize on the surface of the glass and on impurities in the water. This ‘heterogeneous nucleation’ happens much more readily than the ‘homogeneous nucleation’ that takes place in a pure liquid. In most clouds, for example, water freezes on particles of dust and other contaminants through heterogeneous nucleation. But at high altitudes, homogeneous nucleation often occurs because the air is extremely cold and comparatively free of impurities.
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