Delayed freezing in a supercooled liquid metal

In the presence of a suitable nucleating agent, a liquid in a metastable state below its thermodynamically defined melting point freezes. That’s what happens when atmospheric aerosol particles in clouds cause supercooled water droplets to form snowflakes. Researchers have suspected that the atomic surface structure of the seeding particles acts as a template, inducing local order in the disordered liquid and catalyzing its crystallization. Conversely, a solid with a different structure can inhibit crystallization, as has now been observed at the European Synchrotron Radiation Facility in Grenoble, France, by Tobias Schülli and his colleagues. The researchers coupled x-ray scattering data with molecular-dynamics simulations to study supercooled goldsilicon droplets on a silicon substrate, a system that is used to grow Si nanowires. Surprising results emerged when they heated the AuSi alloy above 676 K: As it cooled, the Si atoms leached onto the substrate and, as the figure shows, rearranged its surface atoms into pentagonal clusters. The alloy’s atoms near the interface mimicked the substrate’s surface structure (see inset), but the resulting local order did not promote crystallization in the droplets, which froze at 513 K, about 120 K below the freezing temperature for the AuSi alloy. Apparently, the pentagonal geometry inhibits freezing because it is not conducive to crystal packing. That finding suggests that substrates with such atomic structures offer a simpler method of maintaining and observing the supercooling process than techniques like magnetically levitating or otherwise suspending the liquid droplet. (T. Schülli et al., Nature 464 , 1174, 2010 http://dx.doi.org/10.1038/nature08986 .)