Scientists Make Progress Understanding How Ice Forms at the Molecular Level

Scientists using the European XFEL — an underground particle accelerator near Hamburg that generates one of the world's most powerful X-ray laser beams — have made significant progress in understanding how pure liquids transition into solids, a problem that has confounded researchers for over a century. By firing ultrafast X-ray pulses at high-velocity jets of liquid krypton and argon cooling in a vacuum, the team captured X-ray diffraction patterns showing the liquids developing crystal structure within just a few hundred micrometres and a few microseconds. Their combined experimental and simulation results brought predicted nucleation rates to within 100–1,000 times of measured values — roughly 100 times closer to agreement than previous experiments had achieved. The core theoretical framework, classical nucleation theory (CNT), is extraordinarily sensitive to small changes in variables like surface tension and viscosity, meaning even minor differences in assumptions or experimental conditions can shift predicted freezing rates by tens of orders of magnitude. Researchers chose simpler Lennard-Jones liquids like krypton and argon as a starting point precisely because their molecular interactions are less complex than water's hydrogen-bonding network, allowing more consistent theoretical predictions. A key emerging insight is that disorder plays a larger role in the freezing process than previously appreciated. Improved understanding of nucleation could refine climate models — particularly predictions of ice formation in high-altitude clouds — and shed light on how Earth's solid inner core formed and what occurs inside other planets.