Glass has been around for at least 5,000 years. It is used in so many things that people use every day. But there is still a lot that is unknown about glass, like how some glasses form and how they achieve certain properties. A better understanding of the nature and properties of glass could lead to many innovations in technology, like scratch-free coatings and glass with mechanical properties.
Researchers at the University of Pennsylvania have been looking at the properties of stable glasses over a few years. Stable glasses are produced by depositing molecules from a vapor phase onto a cold substrate.
"There have been a lot of questions about whether this is analogous to the same amorphous state of naturally aged glasses such as amber, which is formed by just cooling a liquid and aging it for many, many years," said Zahra Fakhraai, an associate professor of chemistry in Penn's School of Arts & Sciences.
To answer these questions, Fahkraai and Tiany Liu, a Ph.D. student, collaborated with Patrick Walsh, a chemistry professor, who designed and synthesized a new special molecule that is perfectly round. These unique molecules can’t align themselves with any substrate as they are deposited. Because of this, the researchers expected the glasses to be amorphous and isotropic--meaning that their constituent particles are arranged in a way that has no overarching pattern or order.
The researchers noticed that the stable glasses are birefringent -- meaning the index of refraction of light is different in directions parallel and normal to the substrate. This wouldn’t be expected in a round material.
With birefringence, light shined in one direction will break differently than light shined from a different direction. The effect is often harnessed in liquid crystal displays: changing the orientation of the material causes light to interact differently with it, which produces optical effects. In most deposited glasses, this is a result of molecules aligning in a particular direction as they condense from the vapor phase into a deep glassy state.
The birefringence patterns of stable glasses were strange and the researchers didn’t expect any orientation of the round molecules in the material.
The researchers teamed up with a few physics and chemistry professors and students and they confirmed their hunch that there was no orientation in the material.
After measuring zero order in the glass, the scientists still saw an amount of birefringence analogous to having up to 30 percent of the molecules perfectly ordered. Through the experiments, it was found that this is due to the layer-by-layer nature of the deposition that allows molecules to pack tightly in the direction normal to surface during the deposition. The denser the glass, the higher the value of birefringence. This can be controlled by changing the substrate temperature that controls the degree of densification.
"We were able to show that this is a unique kind of order that is emergent from the process," Fakhraai said. "This is a new sort of packing that's unique because you don't have any orientation, but you can still manipulate the molecular distances on average and still have a random but birefringent packing overall. And so this teaches us a lot about the process of how you can actually access these lower state phases but also provides a way of engineering optical properties without necessarily inducing an order or structure in the material."
The stressors are distributed differently in and out of the plane so these glasses could have different mechanical properties. This could be useful in coatings and technology. It might be possible to manipulate the orientation of a glass or its layering to give it certain properties, like anti-scratch coatings.
"We expect that if we were to indent the glass surface with something it would have different toughness versus indenting it on the side," Fakhraai said. This could change its fracture patterns or hardness or elastic properties. I think understanding how shape, orientation, and packing could affect the mechanics of these coatings is one of the places where interesting applications could emerge."
One of the more exciting pieces of this research is the aspect of now being able to show that there can be amorphous phases that are high density. The team hopes they can apply their understanding from studying the systems to what would happen in highly-aged glass.
A paper on this research was published in Physical Review Letters.