Glass is a material that seems to have a split personality. Is it a solid? Is it a liquid? Could it be both?
A simple Google search brings up dozens of articles on the topic. And if you peruse the results, you encounter a variety of opinions on the subject.
Unlike liquids, atoms in most solid materials are arranged in an orderly crystalline structure. Atoms in liquids can’t seem to sit still—they’re constantly moving, like young children after a large dose of sugar. Glass’s atoms are not arranged like other neat and tidy solids.
That’s why glass is an amorphous solid—a solid that is not organized in a distinct pattern or shape. The glass transition from liquid to solid happens so fast that the atoms and molecules don’t have time to get their act together and line up in an orderly fashion.
What happens to glass over a long period of time? Researchers have already debunked the flowing glass window myth earlier this year by proving that the glass in medieval windows doesn’t succumb to gravity.
So who better to weigh in on the glass solid–liquid debate than people who study glass for a living?
Researchers Edgar Zanotto, professor in the Department of Materials Engineering at the Federal University of São Carlos, and John Mauro, professor of materials science and engineering at Penn State University, have authored a new definition of glass, recently published in the Journal of Non-Crystalline Solids.
“There are several definitions of glass, but most of them contain serious errors,” Zanotto explains in a news release on Agência FAPESP. “Many definitions say glass is a solid, and others say it’s an isotropic material [whose properties are the same in all directions], but many glasses are not.”
“Glass is neither a true solid nor a true liquid, but rather a unique hybrid phase that combines both solid-like and liquid-like qualities,” Mauro states in an email.
Credit: Amazing Compilations; YouTube
The duo proposed two improved definitions of glass—one for the layman, and an advanced, more detailed version for glass scientists and researchers who understand glass transition.
Glass is a non-equilibrium, non-crystalline state of matter that appears solid on a short time scale but continuously relaxes towards the liquid state.
Glass is a non-equilibrium, non-crystalline condensed state of matter that exhibits a glass transition. The structure of glasses is similar to that of their parent supercooled liquids (SCL), and they spontaneously relax toward the SCL state. Their ultimate fate, in the limit of infinite time, is to crystallize.
Their research was based on three different characteristics of glass, which distinguish it from solids, that were present in previously published articles, according to Zanotto:
Mauro, formerly of Corning Incorporated and an ACerS Fellow, spent years at the company as a glass researcher—and was one of the co-inventors of the company’s signature product, Gorilla Glass. Zanotto, also an ACerS Fellow, heads the São Carlos Center of Research, Technology and Education in Vitreous Materials (CeRTEV), a lab that conducts state-of-the-art research on glass and ceramics.
“We hope that our new definition of glass will help inspire students to learn more about this fascinating material,” Mauro says.
The paper, published in the Journal of Non-Crystalline Solids, is “The glassy state of matter: Its definition and ultimate fate” (DOI: 10.1016/j.jnoncrysol.2017.05.019).
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