Imagine you are standing in a freezer. You look at an ice cube. "Ah, a solid, yes, this makes a lot of sense to me," you think. But suddenly, it begins to melt. "Did someone turn up the temperature?" No. The laws of physics you remember from high school tell you this is impossible. You lose your goddamned mind.
This actually happened in a lab somewhere, kind of. It's fairly easy to cool liquids below their freezing points and keep them in a liquid state. But in a recent demonstration, researchers turned solid bismuth already below its freezing point and into a liquid — like the ice cube example above. This is the first time that a solid has been turned into a liquid below its freezing point in a lab, and it could help us better understand the most fundamental processes behind why and when things are solids or liquids.
"We experience phase transitions almost all the time," study author Guoyin Shen from the Carnegie Institution for Science told Gizmodo, "but we actually know very little about them."
So, remember back to chemistry — the melting point represents the temperature and pressure combination where solids turn into liquids. For ice, that temperature is 0C, at air pressures we experience. But you can also cause some materials to melt not only by raising the temperature, but by lowering the pressure.
That's what the researchers did. They held super-pressurised bismuth at 215C, then slowly decompressed it. In so doing, the bismuth started to melt, creating a so-called metastable liquid at a temperature and pressure combination where bismuth should be a solid. The researchers published their work in the journal Nature Communications this morning.
Image: Lin et al
Metastable liquids differ from stable liquids in that, once you have a stable liquid, you can slosh it around all you'd like and it will stay liquid, said Shen. Metastable liquids have found a little nook in the laws of physics where they can stay liquid-like — but like a spinning plate balancing on a stick, any perturbation and the atoms zip back into a solid form.
Scientists haven't turned a solid into a liquid below its freezing point before, explained Shen, making this research important for the most fundamental reasons. Moving forward, Shen hopes to experiment with other materials in the same way. Being able to form liquids in this new manner could open the door to materials that crystallise in ways we've never seen before.