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Optic Properties of Glass Revealed

Thursday, September 28, 2017

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Researchers at the University of Pennsylvania have investigated the properties of stable glasses—closely packed forms of glasses that are produced by depositing molecules from a vapor phase onto a cold substrate—with the assumption that they would have the same characteristics as naturally aged glass.

Their findings, however, were different than expected, and could pave the way for innovations in certain coating technologies such as anti-scratch coatings.

“There have been a lot of questions,” said Zahra Fakhraai, an associate professor of chemistry in Penn’s School of Arts & Sciences, “about whether this is analogous of the same amorphous state of naturally aged glasses such as amber, which are formed by just cooling a liquid and aging it for many, many years.”

Optical Properties of Glass

For this endeavor, Fahkraai collaborated with Ph.D. student Tianyi Liu and chemistry professor Patrick Walsh, who created a new, perfectly round, spherical molecule. Fakhraai noted that, as these molecules are deposited, they can never align themselves with a substrate. The expectation was for glasses to be amorphous and isotropic, meaning the molecules would have no pattern or order.

University of Pennsylvania

According to the university, despite measuring zero order in the glass, researchers still saw an amount of birefringence that was on par with having up to 30 percent of the molecules perfectly ordered. This is due to the layer by layer nature of the deposition.

What researchers discovered, however, was that these glasses are actually birefringent—the index of refraction of light is different in directions parallel and normal to the substrate—which happens when molecules align in a partiulcar direction as they condensate. 

However, researchers noted that the pattern was strange, and teamed up with physics professor James Kikkawa and Ph.D. student Annemarie Exarhos, who conducted photoluminescence experiments to study molecule orientation.

Those tests proved the original hunch: that there was no orientation, despite the birefringence, because of the layer-by-layer nature of the deposition. The process can be controlled by changing the substrate temperature, which in turn 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 very 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.”

Manipulation & Coatings

Fakhraai went on to add that this teaches researchers about the process of actually accessing the lower state phases, along with providing a way to engineer optical properties without inducing a structure in the material.

Since the stressors are distributed differently, these glasses could have different mechanical properties, which may prove useful in the development of products such as anti-scratch coatings.

“We expect that if we were to indent the glass surface with something,” Fakhraai said, “it would have different toughness versus indenting it on the side. 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.”


Tagged categories: Glass; Research; Research and development

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