Any solid without an ordered, crystalline structure -- metal, plastic, a polymer -- that forms a molten liquid when heated above a certain temperature is a glass. The theory relies on two basic concepts: molecular crowding and string-like co-operative movement. Individual molecules within a glass aren't able to move totally freely. They move with, yet are confined by, strings of weak molecular bonds with their neighbours.
"We were surprised -- delighted -- that the model turned out to be so simple," said author James Forrest, a University Research Chair and professor in the Faculty of Science. "We were convinced it had already been published."
Molecular crowding describes how molecules within glasses move like people in a crowded room. As the number of people increase, the amount of free volume decreases and the slower people can move through the crowd. Those people next to the door are able to move more freely, just as the surfaces of glasses never actually stop flowing, even at lower temperatures.
The more crowded the room, the more you rely on the co-operative movement with your neighbours to get where you're going.
Theories of crowding and cooperative movement are decades old. This is the first time scientists combined both theories to describe how a liquid turns into a glass. The paper published by physicists from the University of Waterloo, McMaster University, ESPCI ParisTech and Université Paris Diderot appeared in the prestigious peer-reviewed journal, Proceedings of the National Academy of Sciences (PNAS).