Oh glass, you beautiful amorphous translucent mystery, tell me your secrets.
Hello everyone, Julian back again for DNews.
Physicists from the University of Waterloo, McMaster University, ESPCI ParisTech, and
Université Paris Diderot all joined forces to answer a long standing question about glass,
and that question was: how does glass form?
Yes even though we’ve been making it for over three and a half thousand years, we still
weren’t sure how or why it formed until physicists from four separate institutions
solved the puzzle just last month.
Apparently the answer was a lot simpler than anyone expected, combining two older models
for how glass molecules behaved.
One is molecular crowding, that states that the molecules on the surface of the glass
can keep moving while the more crowded molecules on the interior can’t unless the molecules
around it cooperate.
This cooperative movement is the second puzzle piece, and it’s both helped and hindered
by the weak bonds between glass molecules.
Taken together, these ideas explain how glass behaves at a molecular level, and believe
it or not, it’s more tricky than you think.
Glass isn’t technically a solid, and you may have been told it’s a liquid but that’s
not true either.
It’s an amorphous solid, meaning it lacks the crystalline or polycrystalline repeated
structures of other solids.
This lack of a pattern and the weak chemical bonds explains how glass can behave like a
brittle solid, and also accounts for the glass-liquid transition, which is when heated glass is
between a supercooled liquid state and solid state and it behaves like a rubbery viscous
liquid.
The molecules on the surface of glass never actually stop moving though, even when cooled.
They do move EXTREMELY slowly, but they move nonetheless.
A common myth about glass is because of this movement, antique glass has sagged over the
years and that’s why many old building and church windows have glass that’s thicker
on the bottom.
That’s likely just an imperfection in how the glass was made, because for a flat pane
of glass to sag at room temperature would take longer than the age of the universe,
and I don’t think churches have been around that long.
Plus even older glass from egypt doesn’t have that sagged quality to it, so there goes
that myth.
The lack of a crystalline structure is often used to explain why glass is transparent,
and I’d like to debunk that while we’re at it.
You may have heard that light passes through glass because there’s space between the
disorganized molecules and photons can get through.It’s not the space between the molecules,
but the way the molecules react with photons.
Which is to say, they don’t.
Photons in visible light don’t have enough energy to excite the electrons in glass, so
instead they pass through it unchanged.
Ultraviolet light however does have enough energy to excite the electrons, so when UV
light hits oxide glass like in your car’s window, the photon is absorbed and the UV
light is stopped.
It’s why you don’t get sunburned sitting inside your car with the windows rolled up.
And if you were some hypothetical creature that could only see ultraviolet light, you
wouldn’t be able to see through a window.
This glass stuff man, it’s cooler than you think.
Case in point, we’re trying to search for life on Mars in glass.
Dr. O’Neill explains here.
Have you heard the “glass is a liquid” myth before?
It’s pretty common and it drives me mad.
If there are other myths or legends you want us to science, let us know in the comments
or on facebook or twitter, subscribe so you can see when we answer it, and I’ll see
you next time on DNews.