Glass seems pretty solid, right? Well, maybe not so much. The question whether glass is a liquid or a true solid has vexed scientists for years, but now researchers say they may have settled the debate.
Hot glass is certainly liquid, flowing like taffy or thick syrup. However, once it cools it seems a complete solid, solid enough to be a window pane or a water glass.
Solidification, on a microscopic scale, is generally defined as a state where all the molecules in a substance have permanently settled down into a fixed crystalline structure. That's where the problem comes in; seemingly solid glass, when viewed at the microscopic level, appears to not have settled down and might keep flowing.
The flow is a bit slow, though; you'd have to watch a windowpane for more than 10 million years to see any perceptible change.
That hasn't stopped researchers at the University of Bristol in England and Kyoto University in Japan, armed with supercomputer simulations and information theory, from declaring glass to be a true solid.
At very low temperatures, the size of the solid-like regions of glass -- completely crystalline -- increases over time, and given enough time the material would become truly solid, say Bristol physicists Paddy Royal and Karoline Wiesner, working with Kyoto's Ryoichi Yamamoto.
Atoms in the solid regions organize themselves into fixed geometrical shapes, such as icosahedra, something that was predicted 60 years ago and which the researchers have now confirmed.
But it happens so slowly, how do you predict when -- or even if -- the complete mass of glass would be considered a solid?
Time to turn to the mathematics, the researchers say.
"Information theory provided us with the mathematical tools to detect and quantify the movements of atoms, which turned out to move as if they were in communication with each other," Wiesner says.
Given enough time all the atoms would move into a fixed position, the researchers say in their study published in Nature Communications, meaning glass is -- or eventually becomes -- solid.
"We found that the size of the solid regions of icosahedra would grow until eventually there would be no more liquid regions and so the glass should be a true solid," Royall says.
So glass is a liquid until it completely stops flowing, the researchers say-- but don't hold your breath waiting for it to do so.
