Glasses have been used for thousands of years, but some of the fundamental properties of these common household and industrial materials still remain unexplained. One such puzzle is why glasses become more viscous and rigid over time without major changes to their structure—a phenomenon known as aging. A new study conducted by physics doctoral student Peter Yunker, James M. Skinner Professor of Science Arjun Yodh, post doc Zexin Zhang, and Kevin Aptowicz of West Chester University sheds light on this process. The team’s findings, published this September in the journal Physical Review Letters, hold potential for applications ranging from improved pharmaceuticals to novel coatings based on polymer, ceramic, and metallic glasses.
“Glasses are composed of constituent particles in a disordered manner throughout the medium,” Yunker explains. “In a young glass, scientists have observed that groups of particles move quickly as the material ages, but that the distribution of particles in space remains essentially the same.”
“It has been hard to understand this process,” Yunker continues, “because to observe a glass system age, you have to somehow rejuvenate it. You have to form a glass, put it under the microscope, and then make it young again.” Yunker and his collaborators developed a novel optical heating technique that enabled them to melt glass into liquid and then rapidly decrease temperature to return the system to the glass state. These glasses were essentially colloids composed of micron-sized particles in water that are readily observed by microscopy.
"Many pharmaceuticals have a glass-like structure because such structures can provide advantages for dissolving and releasing medication into your body. However these drugs also must sit on the shelf for a long time. By understanding aging, pharmaceutical scientists can potentially design amorphous drugs that will have longer expiration periods." - Arjun Yodh
During periods of tens of seconds after initial glass formation, the researchers microscopically observed that the particles composing the glass experienced dramatic changes in their local environments while seeking to crystallize (attain an ordered configuration). But, because there is so little space for particles to rearrange in the glass, the motion of one particle requires the correlated motions of neighboring particles. In one class of these rearrangements, the rearranging domains comprise very big clusters of particles that become progressively larger and larger as the glass ages. As a result, the particle rearrangements become harder and harder to achieve, thereby slowing glass dynamics.
Yunker’s finding has potential implications in efforts to design more effective drug delivery vehicles. “Many pharmaceuticals have a glass-like structure because such structures can provide advantages for dissolving and releasing medication into your body,” Yunker says. “However these drugs also must sit on the shelf for a long time. By understanding aging, pharmaceutical scientists can potentially design amorphous drugs that will have longer expiration periods.”
Additionally, Yunker says his research provides insight into the glass transition. “The glass transition,” he explains, “is said to be one of the deepest mysteries in condensed matter physics because it can’t be understood in the way we understand other thermodynamic phase transitions, like crystallization. When a system crystallizes, order appears in the system. In a glass, however, the structure looks the same as the liquid from which it was made, but its macroscopic properties are very different. If you push on a glass, it pushes back; if you push on a liquid, it doesn’t. Relaxation processes, such as the correlated motions of clusters of particles, play a key role in these macroscopic properties.”
Yunker partly credits his findings to the freedom in Yodh’s lab to explore the mysteries of glass in an open-ended way. “When we got involved with these experiments, we weren’t trying to verify a particular hypothesis,” he says. “We had developed an interesting new experimental system with new kinds of control, and we set out to use it to learn about aging in glasses. Even though we didn’t have a final expectation, Arjun could see the value in the idea, and he gave us the leeway to try things that were more open-ended.”
