"Long before it's in the papers"
August 03, 2010

RETURN TO THE WORLD SCIENCE HOME PAGE

Planes with glass wings?

June 22, 2008
Courtesy University of Bristol
and World Science staff

Airplanes with glass wings might become a possibi­lity, thanks to a new in­sight into the na­ture of glass, some sci­en­tists say.

Al­though it looks sol­id, glass is ac­tu­ally a “jammed” state of mat­ter, ac­cord­ing to re­search­ers at the Uni­ver­s­ity of Bris­tol, U.K. and else­where. Un­like or­di­nary sol­ids, whose mo­le­cules crys­tal­lize in­to sta­ble grid-like ar­range­ments, atoms in a glass can’t reach their des­tina­t­ion be­cause the route is blocked by their neigh­bors. Thus glass nev­er quite be­comes a “prop­er” sol­id.

Colloidal particles mi­mick­ing atoms form a gel that sci­ent­ists say has the same struc­ture as glass. (Cour­tesy Pad­dy Roy­all)

Imagine a plane with glass wings. Thanks to a new analysis of the nature of glass, scientists say this is a possibility. Although it looks solid, glass is actually a “jammed” state of matter, according to the University of Bristol, U.K., researchers. Unlike ordinary solids, whose molecules crystallize into stable grid-like arrangements, atoms in a glass can’t reach their destination because the route is blocked by their neighbors. Thus glass never quite becomes a “proper” solid. For over 50 years scientists have tried to understand just what glass is. Work so far has focused on trying to understand the traffic jam. But the university’s Paddy Royall, with colleagues in Australia and Japan, found that the problem really lies with the destination, not with the jam. The scientists concluded that glass fails to be a true solid because of special atomic structures that form in a glass when it cools, as it the atoms approach their destinations. “Some materials crystallize as they cool, arranging their atoms into a highly regular pattern called a lattice. But although glass ‘wants’ to be a crystal, as it cools the atoms become jammed in a nearly random arrangement,” said Royall. “Back in the 1950s, Sir Charles Frank in the Physics Department at Bristol University suggested that the arrangement of the ‘jam’ should form what is known as an icosahedron, but at the time he was unable to provide experimental proof. We set out to see if he was right.” The problem is that you can’t watch what atoms do as they cool because they are just too small, Royall explained. So using special particles called colloids, that mimic atoms, but are just large enough to be visible with newer microscopes, Royall cooled some down and watched. What he found, he said, was that the gel these particles formed also “wants” to be a crystal, but fails to do so due to the formation of icosahedra-like structures—just as Frank had predicted. The formation of these structures is what leads to “jammed” materials and explains why a glass is neither liquid nor solid, he added. The findings are published in the June 22 issue of the research journal Nature Materials. Knowing the structure formed by atoms as a glass cools represents a major breakthrough in our understanding of so-called “meta-stable” materials like glass, Royall said. It will also allow further development of new materials such as metallic glasses, he added. Metals normally crystallize when they cool. Unfortunately, stress builds up along the boundaries between crystals, which leads to metal failure. The world’s first jetliner, the British built De Havilland Comet, fell out of the sky due to metal failure. If a metal could be made to cool with the same internal structure as a glass and without crystal grain boundaries, it would be less likely to fail, Royall claims. In other words, he added, “metallic glasses” could be suitable for a whole range of products that need to be flexible such as aircraft wings, golf clubs and engine parts.