Inside droplets that won’t freeze are crystals that won’t melt

Usually, it makes no difference whether you describe a solid–liquid transition in terms of its melting temperature or its freezing temperature. In pure, bulk substances, the two are identical. But at the nanoscale, where the effects of curvature and finite size loom large, things are different. A case in point is the gallium nanodroplets in the electron microscope image shown here. At each droplet’s core is a tiny gallium crystal (dark green) that remains solid up to 800 K, even though the surrounding liquid (light green) freezes at 180 K. Newly discovered by Maria Losurdo (CNR-NANOTEC, Bari, Italy), April Brown (Duke University), and coworkers, the droplets are thermodynamic oddities—pure substances in which solid and liquid stably coexist over a range of temperatures. Despite repeated cycling between room temperature and 380 K over the course of a year, the droplets never completely froze, nor did they fully melt. A team from Spain predicted in 2012 that such coexistence could occur, but the new study marks the first time it’s been seen in the lab. Losurdo and her colleagues suspect the unusual behavior has to do at least partly with the sapphire substrate on which the droplets were prepared. Because crystalline gallium and sapphire have closely matching lattices, the droplet can relieve interfacial stress—and lower its energy—by partially solidifying at the sapphire surface. Thus a solid appears at temperatures ordinarily too high for it to exist. The finding is more than just a curiosity; it could add a new wrinkle to ongoing efforts to use liquid metal nanodroplets in next-generation plasmonic devices. (M. Losurdo et al., Nat. Mater., in press .)