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To GQ.
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Now, this is interesting, because while I am now convinced that glass does not flow under its own weight below its transition temperature of several hundred degrees Celsius, I distinctly remember seeing a photograph–perhaps in my high school physics textbook–showing how two differently tinted panes of glass, lain flat one upon the other, would lead to the pigments mixing. Obviously either the temperature was significantly higher than usual room temperature, or the pigments were lead-based, or something else that dramatically decreased the viscosity. I’m inclined to go with the latter.
The Master explains that glass is a liquid–and then explains that glass is a solid.
Then the Teeming Millions nip at the Master’s ankles in several threads:
February 2000
Spilling into March 2000
And again in December 2000
Returning in November 2002
Once more in January 2003
And once again in January 2003
In the theory of simple continuous materials, a fluid is a non-solid material with no preferred configurations.
The isotropy group g of such a material satisfies
g=HgH[sup]-1[/sup]
for all unimodular H. A theorem of group theory states that since the simple unimodular group U satisfies the above condition, the only groups that satisfy the condition are U and {1,-1}. {1,-1} is a solid, so the only materials that are fluids are those whose isotropy group is the unimodular group.
As corollaries, every fluid is isotropic and every configuration of a fluid is undistorted.
This definition leaves open the question of whether there are non-solid materials with preferred configurations. The answer is yes: the liquid crystals. Fluids, liquid crystals, and solids exhaust the possible types of simple, continuous materials.
What if quantum mechanics turns out to be wrong, or only an approximation to a deterministic underlying theory?
For the record, that’s a nice variant of an experiment started by Lord Kelvin in Glasgow: a lump of pitch placed at the top of a little staircase, which over the century-plus since has then flowed down it. It’s a few years now since I was last in the university’s physics department, but they certainly used to have it on display in the lobby, alongside various other Kelvin artefacts.
At one stage there was a lengthy correspondence in Nature about what was the longest running scientific experiment. Kelvin’s is not the oldest (that’s an agricultural trial at Rothamsted), but it’s up there.
I should add that what I gave is the only the definition. Whether a particular sample of material at a particular time satisfies the definition can only be answered experimentally. This is analogous to measuring a dish to see if it’s round. You start with the definition of what round is, then you see if real objects satisfy the definition. You will of course find that no real materials perfectly satisfy the definition of a liquid, just as no real objects are perfectly round.
Or Molasses!
Okay, glass is being handled elsewhere; I’ll take roadways 
Bituminous concrete roadways, or what may be called asphalt or tar or macadam, is a flexible pavement. It is different than a cement concrete roadway, which is considered to be rigid. Cement concrete can be used to structurally span a hole or void; in general bituminous concrete can not.
Bituminous concrete is similar to cement concrete in the sense that it contains a graduated range of aggregates. The greater the size of the aggregate, the greater the bearing strength. Typically, pavement design involves having a course of larger aggregates (the base or binder course) beneath a course of finer aggregates (the top or wearing course). These aggregates are coated in hot bitumen; this causes the aggregate materials to adhere. While the mixture is hot, it is spread on a prepared surface, and rolled repeatedly while the bitumen cools and the aggregates lock together. The bitumen becomes very stiff at normal temperatures, making the road seem rigid. Have you ever turned your wheels on pavement on a hot day, and caused ruts in the pavement? If so, too much bitumen was used, which becomes more fluid in the hotter weather. This is a shortcut used by unscrupulous driveway installers; they increase the bitumen to save on aggregate, to make it easier to install, and to keep it more fluid when it is cooling off.
Side note : (Okay, this is all a side note) One could drive directly on the base course if necessary if (and you may have to when the road is under construction) if bearing strength was all that was required, but the top course causes less tire wear, is more aesthetic, and is less susceptible to damage from freeze/thaw in colder climates.
All that said (and man does it look boring even to me :rolleyes: ), I have trouble seeing roadways as amorphous solids like glass and polymers, unless we were to also classify Rocky Road (sorry) Ice Cream in the same category. Perhaps the bitumen itself, but even then the molasses analogy seems closer to me…
best to all,
plynck
liquid crystals as a 6th state?
Interesting! Never seen that one. Although this indicates that the pigment diffuses from one to the other, and not necessarily that the glass itself is mixing. otherwise, non-pigmented glass should fuse to itself if you lay two panes on top of each other, and my box of microscope slides should have formed a glass block…
Atomic hydrogen is able to diffuse through steel very nicely, at room temperature. Hydrogen won’t diffuse from one steel block to another unless they are welded together - simple contact won’t do the job because of surface oxides and the discontinuity in the polycrystalline structure, among other things. Steel is of course a solid, by any definition.
However, glass is already oxidised and amorphous, which might permit diffusion of pigment atoms/ions across a simple contact interface. I’m speculating a bit - an entirely different mechanism may be at work here!