The title says it all. And forgive in advance if it’s been asked before
Of course I googled the question before I came here but I wanted the straight dope.
I also search in Cecil’s column but nothing came up.
Thanks
It’s a solid. In the olden days the way they manufactured glass was such that the panes did not have a very even thickness, and people would put the thick side on the bottom for stability. Some wiseacre then imagined that the panes had started out with even thickness and “flowed” into their current state because glass was really a very viscous fluid, and a legend was born.
Yes.
Well, here’s what Cecil has to say about it. Notice that he somewhat corrects himself in his second reply.
You didn’t specify the temperature… it is also a gas/vapor.
Brian
It is entirely a question of semantics, of no importance. Cecil summarizes it nicely in the answer linked above:
According to my father the chemist, glass, like the vinyl they used to make LP records with, is a very thick viscous liquid. I didn’t think to ask him how “liquid” is defined. But I believe it has to do with lability - ability to move. At what rate of movement a liquid is determined to be a solid, I don’t know.
Defining vinyl or glass as a liquid doesn’t describe the material in any useful way. One could not observe either flowing over an entire lifetime (at least near room temperature) They are both better defined as amorphous solids.
My college geography professor – yes, geography – claimed it was liquid. Said it keeps flowing, and the bottoms of windows grow slightly thicker over time as a result.
Yes I remember being told that also.
Hmmm. My abdomen must be liquid too then.
Is this different from other more traditional solids?
It’s a myth; Does Glass Flow | Glass Notes, Version 4.0
“…if the windows found in early Colonial American homes were thicker at the bottom than the top because of “flow” then the glass found in Egyptian Tombs should be a puddle.”
It is a solid.
The molecules in glass would normally form a crystalline structure, but after melting, glass is cooled so quickly that the molecules (melted quartz, usually, SiO2, though technically glass can refer to a wide array of amorphous solids) do not have time to arrange themselves into a crystalline formation.
At the end of the article Cecil does come out and say that glass does indeed flow (just not very fast). Is he off-base here then?
From here ("Does Glass Flow?):
"Estimates of the viscosity of glasses at room temperature run as high as 10 to the 20th power (1020), that is to say, something like 100,000,000,000,000,000,000 poises. Scientists and engineers may argue about the exact value of that number, but it is doubtful that there is any real physical significance to a viscosity as great as that anyway. As for cathedral windows, it is hard to believe that anything that viscous is going to flow at all.
It is worth noting, too, that at room temperature the viscosity of metallic lead has been estimated to be about 10 to the 11th power, (1011) poises, that is, perhaps a billion times less viscous—or a billion times more fluid, if you prefer—than glass. Presumably, then, the lead caming that holds stained glass pieces in place should have flowed a billion times more readily than the glass. While lead caming often bends and buckles under the enormous architectural stresses imposed on it, one never hears that the lead has flowed like a liquid."
Cecil says:
The book “Basic Optics and Optical Instruments”, by the Naval Education and Training Program Development Center says it doesn’t take quite that long:
Cecil is unfortunately wrong, as are many of the professors cited above. Ordinary (silica) glass is a solid, not a liquid. Nor is this just a matter of semantics, because there is a way to clearly distinguish amorphous solids (like glass) from extremely viscous liquids.
The first important thing is to keep in mind questions of phase are thermodynamic, not kinetic. That is, the rate at which the liquid flows, or the amorphous solid would deform under pressure (or gravity) are irrelevant. These are questions of kinetics – of nonequilibrium transitions. What phase you are in is a question of equilibrium thermodynamics.
For one thing, even a perfect crystalline solid will flow under sufficient pressure, given sufficient time. Think of glaciers, for example. Ice is obviously a crystalline solid, but under sufficient pressure – two miles down under an ice sheet, for example – it will flow just like an extremely viscous liquid. (To be sure, the* mechanism* of flow may well be distinct, at the atomic level, but the point here is that macroscopic observations of flow say exactly nothing about the thermodynamic state of the material.)
How then, do we determine the thermodynamics state of the material? The answer is that we consider whether, on average, the atoms, ions or molecules making up the material are in stable mechanical equilibrium. That is, if one of them is momentarily perturbed from its initial position, is there a strong restoring force from its interactions with all its neighbors?
In a crystal, there obviously is. If a sodium cation is perturbed from its position in a NaCl crystal, there wil be very strong forces from neighboring ions pushing it back into position. It lies, one may say, at the bottom of an energy valley, and any momentary excursion, in any direction, takes it uphill in energy, so that there is a restoring force pushing it back to where it was.
Surprisingly, perhaps, it turns out that most particles in a liquid much of the time are also in mechanical equilibrium – if they are perturbed from where they are, there are also restoring forces that tend to push them back. This is indeed why liquids are resistant to high deformation rates, e.g. why water feels hard when you slap into it at high speed. Very high deformation rates try to make all of the molecules move at once, and quite large number of them are in mechanical equilibrium, so they strongly resist any such motion.
But the critical issue is that many of the particles in the liquid, above some critical fraction, are, for some critical fraction of the time, not in mechanical equilibrium. If they are perturbed from their initial positions, they do not experience restoring forces – in fact, they may experience forces that tend to accelerate the perturbation, like a ball at the top of a hill that starts rolling down one side. This is, ultimately, what allows a liquid to flow.
It is possible to take a snapshot of the instantaneous positions of all the particles in a material and determine, for each particle, whether it is in mechanical equilibrium or not. One way this is done is to calculate the frequency of oscillation around the initial position for small perturbations. A high frequency means strong restoring forces (like a strong spring oscillates fast). A low frequency means weak restoring forces. A frequency approaching zero means restoring forces tending towards zero. And, mathematically, forces that tend to accelerate deviations from initial position turn out to give you imaginary frequencies (in the sqrt(-1) sense), with larger imaginary values corresponding to stronger anti-restoring forces.
If you examine the complete spectrum of frequenies for the snapshot, you can tell whether you are looking at a crystalline solid, an amorphous solid, or a liquid. Essentially the crystalline solid will have a pattern of real frequencies and no imaginary frequencies, the amorphous solid will have no pattern, but lots of real frequences and very few imaginary frequencies, and the liquid will have lots of both real and imaginary frequencies. Notice, by the way, that this is a study of a *snapshot * – you do not need to observe the material over time, e.g. study dynamical processes like flow. That is as it should be if is a matter of thermodynamics you are considering, not kinetics.
I don’t know that there is any simple criterion for saying, when you look at the spectra, which belongs to a solid and which to a liquid. But they are distinct.
So, to summarize: the difference between an amorphous solid and a liquid is that the particles in a solid, amorphous or crystalline, are generally in mechanical equilibrium, while the particles in a liquid are not.
What, then, is the difference between something that stretches under its own weight, and something that is “flowing like a liquid”? With that nails-on-a-wall experiment, you could see similar results with any number of materials that inelestically bend under their own weight. But we wouldn’t call those materials “liquid”, we would call them “ductile” or “plastic”.
ETA: It seems that Carl Pham answered my question (quite thoroughly!) before my post.
I was thinking along the same lines. I understand a steel bar can be made which will bend under its own weight, and no one contends that it’s a liquid.