Tag: science

“There’s a myth about glass you might have read about in high school: If you go to a church that’s hundreds of years old and look at the glass windows, you’ll find that the panes are thicker at the bottom of the frame than at the top. That’s because, according to lore, glass is actually a liquid, just one that flows very slowly.
This is a myth for a lot of reasons. The simplest is that the thickness of glass at the base of the windows can be explained simply by how glass panes were manufactured in the olden days. Back then, flat windows were made by spinning a glass form into a flat disc, which left the finished product with uneven thickness.

But also as a scientific explanation, the myth does not do glass justice. Glass is so much weirder than a very slow-moving liquid. In fact, even though glass is one of the most common, most useful materials in the world — lining our windows, covering our phones, delicately holding our stems of roses — scientists still have deep questions about what it fundamentally is.

“It defies the very simple categories we have of liquid, solid, and gas,” says Camille Scalliet, a theoretical physicist at the University of Cambridge. She’s not the only scientist flummoxed by glass. All over the world, physicists, chemists, and other specialists are trying to unlock its secrets.

It’s true that glass does have some liquid-like properties. But remarkably, rather than flow, glass doesn’t move very much at all. In 2017, scientists analyzed the church glass myth in a paper, determining that, over a billion years, church windowpanes would flow a single nanometer. (That is one-billionth of a meter; it’s infinitesimally tiny. A piece of paper is around 100,000 nanometers thick.)

And this finding gets us closer to the deepest mystery of glass. The question scientists grapple with isn’t “why does it flow.” Instead, “we don’t really know why it’s solid,” Scalliet says.”

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“Solids and liquids are both made up of atoms and molecules. Temperature changes how these components are arranged. Cooler temperatures solidify molecules; warmer temperatures make them juicy.

The important differences are seen on the microscopic scale of molecules. In liquids, the molecules are very disordered; they move around each other and flow. “If you could zoom in and see individual molecules, they would be packed randomly and they would be moving around very fast,” Scalliet says.

I think of a liquid like a crowd of people dancing at a club. They’re energetic, packed in, vibing. They can move around each other, bump and grind, dancing to the music. If you took a snapshot of the dancers, it would look like a chaotic, jumbled mess. That’s a liquid.

Solids are much more tame. As we typically think of them, they are made up of crystals, which are structured, orderly patterns of molecules. When the temperature cools down, the atoms and molecules line up in a regular geometric pattern. In the dance club metaphor, instead of undulating past each other, these ravers stop dancing and sit down in concert seats. They can still squirm a bit in those seats (as long as the thermostat in the theater isn’t set to absolute zero), but they’re mostly locked in place.”

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“The simplest explanation for how glass forms is that it’s a liquid that cools too quickly for those crystals to form. So the molecules get locked in place in a chaotic liquid-like arrangement.

Imagine you’re in the crowded dance space, and you decide you need to use the bathroom. But when you try to get there, a lot of the dancers decide to stop moving. When that happens, it becomes harder and harder for you to navigate across the dance floor. “If you’re with your partner and you want to just trade places, you can’t do it because you’re so jammed, you need to get other people to move,” David Weitz, a Harvard physicist, says.

And when you can’t move, it makes it harder for other people to move around you. So gradually, and then very suddenly, the whole dance floor seizes up. You’re locked in place, and not in an orderly geometric pattern. It’s a mess. It’s glass. And you’re not going to make it to the bathroom in time (again, it might take some billions of years to move just nanometers).

This is the basic definition of a glass: a liquid that has been locked in place. Or, in science-speak: an “amorphous solid.” And it applies to a lot of materials, not just the silica-based glasses that hang in our windows or cover our phones.”

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“Some plastics are considered glasses, as are natural materials like amber. And some parts of your cells are considered to be glass-like. Even foams like whipped cream can be described as glass-like, Weitz says. Finding out the underlying mechanics that connect all these forms of glass, that’s “the real challenge to me, the beauty of the whole science.””

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“If you take a picture of the molecular structure of a glass and the molecular structure of a liquid, they look the same. So why does one flow and another is locked in place?

“There are currently different ways to explain this, why the glass is not moving,” Scalliet says. But no theory is universally agreed upon.

The various explanations involve some very math-heavy invocations of thermodynamics. But in short, scientists are in search of a deeper order to this system that we can’t see just in a snapshot — something to explain glass’s solidness like you could explain the solidness of table salt by pointing to its crystal structure. The secret is likely in the collective action of the molecules over time, and how they influence one another as the liquid seizes up.”

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“In practical terms, it matters that scientists don’t have a complete theory of glass. For one, it means they simply don’t understand glass as well as they do crystalline solids.

With a crystalline solid, you can predict many of the properties of the solid just by looking at its simple crystal structure. Just by knowing the arrangement of the molecules in the crystalline solid, “you can understand, for example, how the solid will absorb heat,” Scalliet says, or “where it will break.” But in the case of glass, “you have basically an infinite number of arrangements. You don’t have this well-known underlying structure.””

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” it’s hard to predict the properties of glass. We learn how glass breaks by breaking it, how it holds on to heat by heating it. That leaves the manufacturing of new types of glass to be a bit of trial and error. But the lack of a complete theory also leaves scientists with some fundamental — even existential — questions about what glass truly is.”

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“glass will still flow a tiny bit over millions and billions of years. If we lived for that long, and experienced the passage of time more quickly, we might not think glass is very mysterious at all. We might think it was a liquid.

It could also be that, also over an immense period, glass will eventually crystallize and become a typical solid. In this light, glass is just liquid “that’s sliding on its way to being a crystal,” Mark Ediger, a chemistry professor at the University of Wisconsin Madison, says.”

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“there’s another exciting possibility here: that instead of crystallization, over very long periods, glass can inch closer to the state of “perfect disorder,” as Ediger describes.”

https://www.vox.com/the-highlight/23850787/what-is-glass-scientific-mystery