January 7, 2013
Tetris-Like Molecular Structure Is Key To Making Ultrastable Glass
Brett Smith for redOrbit.com - Your Universe Online
Like a fine wine or aged cheese, ultrastable glass takes a long time to make, needs special conditions and is considered quite valuable. Unfortunately, manufacturers who want to take advantage of the strengths of ultrastable glass don´t have the luxury of waiting hundreds of years for it to develop.
While exploring ways to create this valuable material on a shorter timetable, researchers from the University of Wisconson-Madison have gained some key insights into the bizarre structure of glasses as well as how it affects their properties. A report of their research titled “Ultrastable glasses from in silico vapour deposition” appeared this week in the new edition of the journal Nature Materials.
"In attempts to work with aged glasses, for example, people have examined amber," said study co-author Juan de Pablo, a University of Chicago professor of molecular theory. "Amber is a glass that has been aged millions of years, but you cannot engineer that material. You get what you get."
In many laboratories, scientists use a technique called vapor deposition to create specialized materials. Previous research by another of the new study´s co-authors, Mark Ediger, found that glasses grown in this manner — within a certain temperature range and on a specially prepared surface — are far more stable than ordinary glasses.
Ediger determined that in order to achieve this degree of stability, molecules in the glass are arranged in a tightly-packed manner like the multi-shaped objects in the popular videogame Tetris.
"This is a little bit like the molecules in my deposition apparatus raining down onto this surface, and the goal is to perfectly pack a film, not to have any voids left," Ediger said. "The difference is, when you play the game, you have to actively manipulate the pieces in order to build a well-packed solid. In the vapor deposition, nature does it for us."
But just like when the blocks start falling too quickly in Tetris, the results of molecules that descend too quickly is a sloppily packed, unstable material that´s full of holes.
"In the experiment, if you either rain the molecules too fast or choose a low temperature at which there's no mobility at the surface, then this trick doesn't work," Ediger explained. “Then it would be like taking a bucket of odd-shaped pieces and just dumping them on the floor. There are all sorts of voids and gaps because the molecules didn't have any opportunity to find a good way of packing.”
Ediger speculated that certain conditions allow the molecules more room to arrange themselves into a more stable configuration. He collaborated with de Pablo and graduate student Sadanand Singh to create computer simulations designed to test his working theory that his new material was indeed glass and not another unknown material.
"There's interest in making these materials on the computer because you have direct access to the structure, and you can therefore determine the relationship between the arrangement of the molecules and the physical properties that you measure," explained de Pablo, a former UW-Madison faculty member who now works for the University of Chicago.
Accoding to de Pablo, researchers have long believed that there was no relationship between the mechanical properties of a glass and its structure at the molecular level. What his team´s study indicates, however, is that there are in fact clear “structural signatures” that can give clues about a glass´s properties.
"What we found here is that there are actually differences, it's just that you had to create better glassy materials. Once you create these materials, you see that the structure, the differences between ordinary and stable glasses are clearly there and are actually pronounced."