"Glass is an amorphous solid that flows like a liquid", you'd probably say if you were asked to define the stuff. Is that true, though? What if I told you, for example, that the old myth, that old windows are thicker at the bottom because the glass has flowed a little bit, was false? Glass is an incredibly complex physical idea that avoids easy definition.
A pair of scientists is hoping to re-define what a "glass" is, so students can better understand what's going on with this strange state of matter. Others aren't sure if they're ready to accept a be-all, end-all definition. But one thing is for sure: Glass isn't a "solid" or a "liquid" in the way you might think.
"It combines features of solids and liquids," paper author John Mauro, Penn State scientist and inventor of Gorilla Glass for Corning Incorporated told Gizmodo. "We proposed a new definition of glass to accurately capture both features."
Mauro and Edgar Zanotto from the Federal University of São Carlo in Brazil propose a simple, official definition as follows: "Glass is a nonequilibrium, non-crystalline state of matter that appears solid on a short time scale but continuously relaxes towards the liquid state."
At the more advanced level, they suggest "Glass is a nonequilibrium, 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 is to solidify, i.e., crystallise." They recently published their explanation in the Journal of Non-Crystalline Solids.
The identity crisis begins with the fact that no matter how solid-seeming it is, glass always "relaxes," moving towards a state of supercooled liquid. These are liquids that have been cooled below their freezing points but are not allowed to arrange themselves into regular crystalline patterns. Some researchers consider glasses supercooled liquids because they, too, flow (incredibly slowly) and don't have regular crystalline structures.
But there's still a poorly-defined temperature — the glass transition temperature — at which glass becomes softer and some of its innate properties change. Glass also stores more built-in energy than a supercooled liquid at the same temperature.
The key difference between a glass and other amorphous solids is the presence of this glass transition temperature, one where the properties noticeably change from a glass to a supercooled liquid. Other amorphous solids don't have that.
Think of a theatre full of randomly placed humans. Everyone is told to grab the hands of someone nearby if they want, regardless of the seats. That's a glass — but it's a real mess and would be much easier if everyone was holding their neighbour's hands in the seat next to them. Someone begins to turn the thermostat up, and everyone is instructed that they're allowed to let go and take on a more comfortable hand-holding arrangement when they start to feel warm.
The glass transition temperature is just some best-fit temperature to describe when people have really started letting go of and re-grabbing someone else's hand — during that time, it's a supercooled liquid. There's a "glass transition time," which is just how long it takes to reach that transition temperature.
The glass researchers would then stop observing, but the assumption is that in all cases, eventually, everyone is holding their neighbour's hand and the solid becomes a crystal.
Ultimately the researchers' definition is meant to capture all of that, and they posit that prior definitions haven't quite done so. One researcher, Robert Brow at the Missouri University of Science & Technology, told Gizmodo the article was "great" and had no problems with its science. He still felt the researchers veered a little too close to calling glass a liquid, given just how solid it really is.
The amount of flow is incredibly slow — cathedral windows are thicker at the bottom because they're heavier at the bottom and were installed that way, for example. Glass's flow requires billions of years to move just a few nanometres, explained Brow. Essentially, some things just evade definition and glass is one of those things. "Unless your definition is four pages long, it can be misinterpreted. That's where I come from."
So, there's a new definition of glass in town — but as a reminder, there is still a whole lot that scientists don't know about glass and what it is. It's a little too complex in its behaviour to be easily and correctly captured by a high school chemistry textbook. And it's getting glass scientists talking.
"It's created debate in the community," said Mauro. "People appreciate the chance of discussing it."