Nav: Home

Penn researchers lay groundwork to better understanding optical properties of glass

September 13, 2017

Glass is everywhere. Whether someone is gazing out a window or scrolling through a smartphone, odds are that there is a layer of glass between them and whatever it is they're looking at.

Despite being around for at least 5,000 years, there is still a lot that is unknown about this material, such as how certain glasses form and how they achieve certain properties. Better understanding of this could lead to innovations in technology, such as scratch-free coatings and glass with different mechanical properties.

Over the past few years, researchers at the University of Pennsylvania have been looking at properties of stable glasses, closely packed forms of glasses which are produced by depositing molecules from a vapor phase onto a cold substrate.

"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."

In order to answer these questions, Fahkraai and Ph.D. student Tianyi Liu collaborated with chemistry professor Patrick Walsh who designed and synthesized a new special molecule that is perfectly round with a spherical shape. According to Fakhraai, these unique molecules can never 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, whether they are atoms, colloids or grains, are arranged in a way that has no overarching pattern or order.

Surprisingly, the researchers noticed that these stable glasses are birefringent, meaning the index of refraction of light is different in directions parallel and normal to the substrate, which wouldn't be expected in a round material. Their results were published in Physical Review Letters.

With birefringence, light shined in one direction will break differently than light shined from a different direction. This effect is often harnessed in liquid crystal displays: changing the orientation of the material causes light to interact differently with it, producing optical effects. In most deposited glasses, this is a result of molecules aligning in a particular direction as they condensate from the vapor phase into a deep glassy state.

The birefringence patterns of the stable glasses were strange, Fakhraai said, as the researchers did not expect any orientation of these round molecules in the material.

After teaming up with physics professor James Kikkawa and Ph.D. student Annemarie Exarhos, who did photoluminescence experiments to look at the orientation of the molecules, and chemistry professor Joseph Subotnick, who helped with the simulations aimed at looking at the crystal structure and calculating the index of refraction of the crystal which allowed them to work out the math of the degree of birefringence or ordering in the amorphous state, the researchers confirmed their hunch that there was no orientation in the material.

Despite 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 their experiments, they found that this is due to the layer-by-layer nature of the deposition that allows molecules to pack more tightly in the direction normal to the surface during the deposition. The denser the glass, the higher the value of birefringence. This process 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 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. 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."

Since the stressors are distributed differently in and out of plane, these glasses could have different mechanical properties, which may be useful in coatings and technology. It may be possible to manipulate the orientation of a glass or its layering to give it certain properties, 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."

According to Fakhraai, one of the most exciting pieces of this research is the fundamental aspect of now being able to show that there can be amorphous phases that are high density. She hopes she and other researchers can apply their understanding from studying these systems to what would happen in highly aged glass.

"This tells us that we can actually make glasses that have packings that would be relevant to very well-aged glass," Fakhraai said. "This opens up the possibility of better fundamentally understanding the process by which we can make stable glasses."

-end-

This research was funded by National Science Foundation grants DMR-11-20901, DMR-1206270, CHE-1152488 and DMREF-1628407.

University of Pennsylvania

Related Glass Articles:

Breaking glass in infinite dimensions
With the help of some mathematical wizardry borrowed from particle physics -- plus around 30 pages of algebraic calculations, all done by hand -- Duke postdoctoral fellow Sho Yaida has laid to rest a 30-year-old mystery about the nature of glass.
Nature: 3-D-printing of glass now possible
3-D-printing allows extremely small and complex structures to be made even in small series.
Making batteries from waste glass bottles
Researchers at the University of California, Riverside's Bourns College of Engineering have used waste glass bottles and a low-cost chemical process to create nanosilicon anodes for high-performance lithium-ion batteries.
Atomic 're-packing' behind metallic glass mystery
A new method uncovers a four-decade mystery about metallic glass that could allow researchers to fine-tune its properties to develop new materials.
Glass's off-kilter harmonies
The transport of heat in amorphous materials is largely determined by the behavior of phonons -- quasiparticles associated with the collective vibrations of atoms.  Researchers from Georgia Tech developed a new way to calculate the heat contribution of phonons using computer simulations.
Theory lends transparency to how glass breaks
Rice University scientists explain how and why shear bands form in metallic glasses and make them more prone to break.
Model could shatter a mystery of glass
Princeton University researchers have developed a computational model for creating a 'perfect glass' that never crystallizes -- even at absolute zero.
Graphene cracks the glass corrosion problem
Researchers at the Center for Multidimensional Carbon Materials (CMCM), within the Institute for Basic Science (IBS) have demonstrated graphene coating protects glass from corrosion.
Your brain on Google Glass
A group of Drexel biomedical engineers use functional near-infrared spectroscopy to measure mental workload as subjects navigate a college campus.
Glass now has smart potential
Australian researchers at the University of Adelaide have developed a method for embedding light-emitting nanoparticles into glass without losing any of their unique properties -- a major step towards 'smart glass' applications such as 3-D display screens or remote radiation sensors.

Best Science Podcasts 2017

We have hand picked the best science podcasts for 2017. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: Radiolab

Oliver Sipple
One morning, Oliver Sipple went out for a walk. A couple hours later, to his own surprise, he saved the life of the President of the United States. But in the days that followed, Sipple's split-second act of heroism turned into a rationale for making his personal life into political opportunity. What happens next makes us wonder what a moment, or a movement, or a whole society can demand of one person. And how much is too much?
Now Playing: TED Radio Hour

Future Consequences
From data collection to gene editing to AI, what we once considered science fiction is now becoming reality. This hour, TED speakers explore the future consequences of our present actions. Guests include designer Anab Jain, futurist Juan Enriquez, biologist Paul Knoepfler, and neuroscientist and philosopher Sam Harris.