Glass conundrum answered

Glass – is it a liquid or a solid? The question that has stumped scientists for years may now have been answered thanks to research from Queen Mary, University of London. Glass has always sat in an unknown classification territory, somewhere between a liquid and a solid. The molecules are jumbled up randomly like a liquid, but move incredibly slowly, more like a solid.

“It is difficult to think of glass as a liquid when it displays all the qualities of a solid – it is hard and it shatters when it breaks,” said Dr Kostya Trachenko from the school of physics.

“However, contrary to what has been previously thought, we propose that glass is not different from a liquid from a physical perspective, in that the differences between the glass and the liquid are quantitative but not qualitative.”

Stained glass windows are often thicker at the bottom than at the top, thought to be so because glass flows over time. Trachenko and his colleague Professor Vadim Brazhkin from the Russian Academy of Science argue that this is incorrect from a quantitative point of view, but the qualitative idea is correct – glass is just a slow flowing liquid from a physical point of view.

Many theorists believe that glass must enter a phase transition at some point, like water changing from liquid water into ice.

“There has been no evidence to support the existence of a distinct glass phase: we know that the glass and liquid are nearly identical in terms of structure,” Trachenko said. “It was this simple yet persisting controversy that was at the heart of the problem of glass transition.”

A similarly important change is the jump of heat capacity which happens during the liquid-glass transition, but the scientists argue that a new phase or phase transition to explain this jump is not necessary.

They liken this theory of glass being a liquid to that of pitch or bitumen – which is very similar to glass as it shatters when broken with a hammer, but pitch actually drips every 10 years or so.