Substrate-enhanced supercooling in AuSi eutectic droplets

Supercooling on contact The local atomic arrangement in liquids differs substantially from the order in crystals. But when in contact with a crystal, the neighbouring atoms in the liquid can mimic the crystalline order, often triggering solidification of the liquid. This 'seeding' effect is a familiar one, seen in water as it nears its freezing point and in crystal nucleation. More surprising is the discovery, presented in this issue, of an opposite effect: if the atomic structure of a surface is similar to that of the contacting liquid phase, instead of crystallizing, the liquid remains liquid far below its melting point and supercooling is achieved. The phenomenon was observed in gold-silicon eutectic droplets in contact with gold atoms in a pentagonal arrangement layered onto a silicon substrate. This finding has implications for fundamental studies of freezing and also for the practical control of phase transitions. For instance, the containerless techniques often used to obtain supercooling might conceivably be avoided by coating containers with an icosahedrally ordered surface.