Cecil wrote: “A supercooled liquid, the term applied to glass for many years, has been rapidly chilled past its normal freezing point and become apparently solid without assuming the regular crystalline structure typical of solids.”
Incorrect. A supercooled liquid is a material which has been cooled below the freezing point but which has not yet frozen. Or to be more clear about it, it’s still behaving like a liquid, but it’s in a metastable state. In order to solidify (i.e., crystallize) there needs to be nucleation sites for crystals to start forming. An example of a supercooled liquid would be extremely pure water (dust and other contaminants act as nucleation sites) which has been cooled to, say, 30 degrees Fahrenheit. You can drop a tiny piece of nearly anything into it and watch the water freeze (i.e., turn solid) VERY rapidly.
Tom, especially if you’re gonna disagree with Cecil, please post a link to the column you are referring to: http://www.straightdope.com/ubb/Forum6/HTML/000170.html (I assume) and also check to see if there is another thread on that particular topic on the message board, which there is. Thank you.
Jill
I posted a new topic because all of the messages responding to that column (those responses being at http://www.straightdope.com/ubb/Forum1/HTML/000491.html ) were discussing the “what is glass?” question. I wanted to bring up the “what is supercooled liquid?” question and try to provide an answer.
After seeing your post, Jill, I went and read that other set of messages. Those appear to be responses from a column from last fall and still mostly deal with the “what is glass?” question. One or two people did point out that “supercooled liquid” was a misnomer as applied to your everyday glass, but no one provided (as far as I could tell) a correct definition. So I still think this separate topic is worthwhile (or is the tradition here to post any and all responses to a column in a single thread, regardless of what’s being discussed?).
In any case, even after that earlier set of comments, Uncle Cece came out this week with his definition (quoted in my original posting) of supercooled liquids. I believe that it is an incorrect definition. I should have been less terse in my reply, though. I’d like to add that “supercooled liquid” can apply to materials which become glass as well as those which crystallize. In fact, one could argue that it’s the crystallizing materials which are more interesting when you can make them supercooled.
Sidenote: There was some science fiction story (by Heinlein, I think) that used supercooled liquid in a sub-plot as the way to kill someone without it looking like homicide. In the story, the governor of a large moon colony has an Olympic-sized swimming pool in his residence; it is of course super-pure water. A bad guy (who happens to be a house-guest if I remember correctly) gets in and sets the pool temperature to a couple of degrees below the freezing point. When the governor goes in for his solitary early morning dip, he dives in and the water instantly crystallizes around him (he provides tons of nucleation sites), and the whole pool freezes before he can surface. What a horrible way to die – some combination of shock (maybe leads in this case to a heart attack?) and asphyxiation. Then the bad guy turns the pool temp back up. Eventually (hours later) they find the guy’s body floating in the water, and it looks like “natural causes.” Cool (and sick), huh?
[Glass is a noncrystalline, or amorphous, material. Its atomic structure is one of disorder or randomness. to produce a glass, the ingredients must be present in the correct proportions. After these ingredients have melted and combined, the resulting fluid must be cooled rapidly enough so that randomness, which is chaacteristic of the fluid state, is frozen in. If the mixture of ingredients is not correct or if the cooling rate from the molten condition is too slow, the atoms will arrange themselves in a regular pattern that is charateristic of the crystalline solid state. Glass is sometimes called an undercooled fluid. Its structure is random like that of a fluid, but it is frozen and, therefore, has some of the properties of a solid, such as hardness and rigidity.]
The Volume Library The Southwestern company Nashville Tennessee
Somehow I thought that glass was in a clas of its own because of this
[So I still think this separate topic is worthwhile (or is the tradition here to post any and all responses to a column in a single thread, regardless of what’s being discussed?)]]
Usually, yeah. But if you read the other thread on the column and really feel your comments warrant another thread, that’s okay.
Jill
Can’t seem to find a definition of supercooled liquid.
I thought I had heard that glass was the only substance that was supercooled. Maybe I’m wrong. I have no experience in fluids.
My point here is only the definition of (supercooled) undercooled.
Yes, you’re wrong. In fact, multiple times over. Other liquids, as already described here, can be supercooled. Glass, however, also as explained several times over, is not a supercooled liquid but an amorphous solid.
Pick 1
From Websters
a mor phous adj. [<Gr.a-, without+morphe, form], 1. shapeless. 2. of no definite type. 3 in chem., not crystaline.
I know what it is called and it will not be changed here but it just doesn’t seem to fit the definition. It, except for the rigidity does not fit the solid definition.
Dunno what you were trying to say, but if you’re thinking we should check out Websters for our scientific definitions, you’re on the wrong track.
So who should we turn to? Hmmm. How about people who know what they’re talking about? I’ve posted this in other threads that were linked in the other glass-related thread here, but you apparently haven’t read them. So I’ll just post it here.
Referring to glass as a supercooled liquid, as Doremus points out in Glass Science (incidentally a book Copyrighted in 1973 – not exactly recent), “the difficulty with this view is that glasses can be prepared without cooling from the liquid state. Glass coatings are deposited from the vapor or liquid solution, sometimes with chemical reactions. Thus sodium silicate glass can be made by evaporating an aqueous solution of sodium silicate (water glass) and baking the deposit to remove water. The product of this process is indistinguishable from sodium silicate glass of the same composition made by cooling from the liquid.” (p. 1)
Interestingly, while he doesn’t speak specifically of the supposedly flowing windows, he does say the following, also on p. 1:
“Glass is an amorphous solid. A material is amorphous when it has no long-range order, that is, when there is no regularity in the arrangement of its molecular constituents on a scale larger than a few times the size of these groups. … A solid is a rigid material; it does not flow when it is subjected to moderate forces. More quantitatively, a solid can be defined as a material with a viscosity of more than about 10^15 P (poises).”
Alright you’ve worn me out. my finger is bruised. There is definitly something wrong with a definition that cannot be agreed upon in at least 35years That is when I first heard about the two definitions I think. I’m no scientist or obviously a typist either.
good luck