May 15, 2013
Fossil Amber Overturns Long-Held Theory Of Glass Formation
April Flowers for redOrbit.com - Your Universe Online
In a new study from researchers at Texas Tech University, scientists have discovered that the structure of amber barely changes even after tens of millions of years. A report of the research, published in Nature Communications, could help answer fundamental questions about how glasses form."What we found was that in 20 million years, the amber changed density by only 2.1 percent. What we found challenges the way we look at glasses," explained Gregory McKenna, professor of chemical engineering at Texas Tech University.
National Geographic News´ Ker Than reports that the team´s findings are further evidence that the stained glass windows in many medieval cathedrals aren´t thicker at the bottom because glass flows like a liquid, despite what first year chemistry students are taught.
"Those windows aren't flowing," McKenna said. "The glass makers were just smart enough to put the thicker ends at the bottom."
The research team chose to focus on amber, which is fossilized tree resin, because its atoms are not arranged in any regular order. "In a crystal, everything is periodically arranged. If you know what's happening in one little bit, you can predict where the atoms are going to be everywhere else. In glass, things are much more disordered," explained Mark Ediger, an experimental chemist at the University of Wisconsin, Madison. Because amber is a noncrystal and has chaotic atom placement, it is a good analog for studying glasses.
The team was specifically interested in a process called glass transition — or the temperature at which a material transforms from a soft and flexible rubber-like state to a hard and brittle one. Glass transition is not well understood, despite decades of study. Scientists still struggle to answer such basic questions as, "what causes a liquid to slow so rapidly as it becomes a glass?" and "what's the best way to think about it?"
Glass transition is closely connected to the performance of materials, making it of practical as well as academic interest. The properties of glass transition are important for the design and manufacture of a host of glassy materials. Many modern technologies, such as airplane construction, rely on glass, or glass-like resins and plastics.
"The current planes are probably fine because they have relatively high glass-transition temperatures and the airplanes don't get very hot, but imagine if you are building a supersonic transport and the whole airplane gets hot and remains hot for several hours," McKenna said.
"At that point, you're pushing the limits of the materials, and working fairly close to the glass-transition temperatures. As the material changes, it could get more and more brittle, and you could conceivably have issues if you don't model them properly."
The team performed experiments on 20-million-year-old Dominican amber to better understand glass transitions. One of the tests was called a stress-relaxation experiment, involving stretching out strips of amber at different temperatures then measuring the rate at which they relaxed back to their original states. The results provided clues about how the amber´s molecules behave.
It takes a certain amount of force to distort the amber strips. By measuring the time it takes for that force to recede, the team learned how fast the molecules inside the amber could move. The experiment provided a rare opportunity to study glass transition in slow motion and at ambient temperatures because the temperature at which a material becomes frozen into its glassy state depends, in part, on how long it has to cool.
"When you cool a liquid, the reason it becomes a glass is because the molecules are moving so slowly that at some temperature they get stuck, and then they cannot reach the state they should have at such low temperatures," Ediger explained.
The lower the temperature when a material turns to glass, the longer it took to cool. "If I cool a liquid ten times more slowly, I'll get to a slightly lower temperature before I get stuck," Ediger said.
By using the fossilized amber, McKenna´s team had something impossible to replicate in the laboratory — a glass that took 20 million years to cool. This allowed the team to bring the temperatures down to the level of ambient air, when amber would normally be frozen in a glass-like state, and still have a liquid. Understanding the properties of amber will allow the team to glean important insights into how glass forms.
Ediger said using fossil amber was a creative idea in the pursuit of glass transition, and called McKenna´s experiments “beautifully done.” "I'm not sure there's another lab in the world that could do the experiment with the needed precision," he added.
McKenna´s team is continuing their studies by repeating the experiments with 220-million-year-old amber from the Triassic period.
"We are in the very early stages," McKenna said in a statement. "However, our research definitely is 'to be continued.'"