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Finding the Viscosity of Pitch
ColdChrist writes "The University of Queensland has a page about a 72-year-old experiment on the fluidity of pitch. There's a webcam where you can try to become the first person ever to see a drop of the pitch fall; eight drops have fallen since 1930 and the ninth is now forming. The experiment 'demonstrates the fluidity and high viscosity of pitch, a derivative of tar once used for waterproofing boats. At room temperature pitch feels solid - even brittle - and can easily be shattered with a blow from a hammer', but it does flow, as the pictures demonstrate." I know this is going to bring up glass comparisons, so we'll head those off: glass is not a fluid.
The experiment is sitting in a glass cabinet just outside one of the lecture theatres used for a lot of first and second year engineering and science lectures.
When I started in first year (1999), the pitch had formed into an interesting drop, and it provided students with a pretty geeky talking point while waiting for lectures to start.
I remember when we went for holidays one year, and came back to find that the drop had fallen! Everyone was a bit pissed (understandably) that it had fallen during uni hols.
Apparently the rate of drop formation is slowing down due to the air conditioning in the building. Or at least thats a rumour circulating around UQ.
Well, from that very link one can glean: 'There is no clear answer to the question "Is glass solid or liquid?".'. Of course, that does not absolutely preclude the possible truth of michael's assertion, but it does make it seem a little ambigous. Oh, the semantics!
Money for nothing, pix for free
Ya know, ppl sitting around eating pizza waiting for this to happen is how they get too fat to work for the FBI! ;-)
Lord Kelvin (William Thomson) created an older pitch experiment: one which had a variety of objects lying on a tray of pitch that are slowly sinking in.
Its usually on show in either the Hunterian Museum or the Department of Physics and Astronomy at Glasgow University.
As I recall, this is considered the oldest continuously running scientific experiment, with the exception only of a wheat-breeding experiment in England? (I can't find references on that, just remember it from back in the mists of time)
BTW: it is more fun to watch paint dry - its faster...
It is pitch black. You are likely to be eaten by a grue.
Reminds me of a cornstarch and water experiment we used to do. Mix it together and you get a weird substance that exhibits properties of solids and liquids. Try it if you're bored...
Sorry, but I just couldn't resist the pun...
Must be a slow news day.
Yeah, I haven't slept in 32 hours. That's funny to me.
--
http://nemilar.net - Not your grandmother's soup kitchen
...who when reading the article - and looking at the picture of the smashed pitch - finds it hard to get images of a slow motion T-1000 out of my head?
Well, then. In light of the confusion about how many drops have actually fallen, and when they fell, I propose the experiment be restarted.
-This sig intentionally left blank
In reality God is a hacker who rooted the Universe.
So yes, it's a big experiment (read: Honeypot project)
Has anyone tried something like this with a quicker (but not too quick) fluid?
This would make an excellent Calendar type device - a glass funnel full of SOMETHING (my rubber bible is at home - anyone got one handy???) that would drip through in about a year.
Great for lecturing opportunities when people say 'what the fuck is THAT' and point at your bell jar full of brown gooey stuff!
I have a page about telescope making that should give you some jumping off points, but I haven't yet got to the polishing stage of the mirror I'm working on.
One reason for using pitch is that you can press a mirror into it and get a very close fit. Another is that if the mirror is not perfectly spherical, the pitch will flex as the mirror moves across it. And finally, the polishing abrasive (ferrous oxide or cerium oxide) will set in the pitch and have a planing action rather than rolling around and chipping little flakes off as in ordinary grinding.
Pitch is nasty stuff to work with. It takes a lot of practice before a novice telescope maker can make a pitch lap they're happy with.
-- Could you use my software consulting serv
The whole "glass is a liquid" thing is a classic example of one of thos things that people say without really understanding understanding what they mean. This article, which is well written, addresses the two main points that you need to prove that glass isn't a "liquid".
It then refutes the common and to my knowledge ONLY evidence for glass "flowing" on human timescales, the thickness difference in the top and bottom of old windowglass. Windows that are OPPOSITE what one would expect to find and the fact that hanging the windows with the thick edge down was common practice neatly debunks this evidence.
So, READ the whole article before you quote without understanding context...
+++ ATH0 +++
Now THAT's how I like my coffee...
In fact, another page confirms that the 8th drop fell in November 2000, so it is indeed the 9th drop forming.
Patrick Doyle
I mod down every jackass who puts his moderation policy in his sig. Oh, wait a sec....
Robert H. Brill, Research Scientist
The Corning Museum of Glass
July, 2000
Early one spring morning in 1946, Clarence Hoke was holding forth in his chemistry class at West Side High School in Newark, New Jersey.
"Glass is actually a liquid." the North Carolina native told us in his soft Southern tones. "You can tell that from the stained glass windows in old cathedrals in Europe. The glass is thicker on the bottom than it is on the top."
Now, more than half a century later, that is the only thing I can actually remember being taught in high school chemistry. I didn't really believe it then, and I don't believe it now.
In the years that followed, I came across the same story every now and then. Most often it popped up in college textbooks on general chemistry. And now, thanks to the Internet, our Museum has received dozens of inquiries about whether or not this is true. Most people seem to want to believe it.
***
It is easy to understand why the myth persists. It does have a certain appeal. Glass and the glassy state are often described by noting their similarities with liquids. So good teachers, such as Mr. Hoke was, like to quote the story about the windows. As is the case with liquids, the atoms making up a glass are not arranged in any regular order-and that is where the analogy arises. Liquids flow because there are no strong forces holding their molecules together. Their molecules can move freely past one another, so that liquids can be poured, splashed around, and spilled. But, unlike the molecules in conventional liquids, the atoms in glasses are all held together tightly by strong chemical bonds. It is as if the glass were one giant molecule. This makes glasses rigid so they cannot flow at room temperatures. Thus, the analogy fails in the case of fluidity and flow.
***
There are at least four or five reasons why the myth doesn't make sense.
Some years ago, I heard a remark attributed to Egon Orowan of the Massachusetts Institute of Technology. Orowan had quipped that there might, indeed, be some truth to the story about glass flowing. Half of the pieces in a window arc thicker at the bottom, he said, but, he added quickly, the other half are thicker at the top. My own experience has been that for earlier windows especially, there is sometimes a pronounced variation in thickness over a distance of an inch or two on individual
fragments. That squares with the experience of conservators and curators who have handled hundreds of panels. Although the individual pieces of glass in a window may be uneven in thickness, and noticeably wavy, these effects result simply from the way the glasses were made. Presumably, that would have been by some precursor or variant of the crown or cylinder methods.
One also wonders why this alleged thickening is confined to the glass in cathedral windows. Why don't we find that Egyptian cored vessels or Hellenistic and Roman bowls have sagged and become misshapen after lying for centuries in tombs or in the ground? Those glasses are 1,000-2,500 years older than the cathedral windows.
Speaking of time, just how long should it take theoretically-for windows to thicken to any observable extent? Many years ago, Dr. Chuck Kurkjian told me that an acquaintance of his had estimated how fast-actually, how slowly-glasses would flow. The calculation showed that if a plate of glass a meter tall and a centimeter thick was placed in an upright position at room temperature, the time required for the glass to flow down so as to thicken 10 angstrom units at the bottom (a change the size of only a few atoms) would theoretically be about the same as the age of the universe: close to ten billion years. Similar calculations, made more recently, lead to similar conclusions. But such computations are perhaps only fanciful It is questionable that the equations used to calculate rates of flow are really applicable to the situation at hand.
***
This brings us to the subject of viscosity. The viscosity of a liquid is a measure of its resistance to flow-the opposite of fluidity, Viscosities are expressed in units called poises. At room temperature, the viscosity of water, which flows readily, is about 0.01 poise. Molasses has a viscosity of about 500 poises and flows like... molasses. A piece of once proud Brie, left out on the table after all the guests have departed, may be found to have flowed out of its rind into a rounded mass. In this sad state, its viscosity, as a guess, would be about 500,000 poises.
In the world of viscosity, things can get rather sticky. At elevated temperatures, the viscosities of glasses can be measured, and much practical use is made of such measurements. Upon removal from a furnace, ordinary glasses have a consistency that changes gradually from that of a thick house paint to that of putty, and then to that of saltwater taffy being pulled on one of those machines you see on a boardwalk. To have a taffy-like viscosity, the glass would still have to be very hot and would probably glow with a dull red color.
At somewhat cooler temperatures, pieces of glass will still sag slowly under their own weight, and if they have sharp edges, those will become rounded. So, too, will bubbles trapped in the glass slowly turn to spheres because of surface tension. All this happens when the viscosity is on the order of 50,000,000 poises, and the glasses are near what we call their softening points.
Below those temperatures, glasses have pretty well set up, and by the time they have cooled to room temperature, they have, of course, become rigid. Estimates of the viscosity of glasses at room temperature run as high as 10 to the 20th power 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 the11th power, 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.
***
When all is said and done, the story about stained glass windows flowing-just because glasses have certain liquid-like characteristics-is an appealing notion, but in reality it just isn't so.
Thinking back, I do recall another memorable remark by Mr. Hoke. One day, our self-appointed class clown sat senselessly pounding a book on his desk at the back of the room. "Great day in the mawnin', son! " shouted Hoke. "Stop slammin' your book on the desk. Use your head!" That was good advice-no matter how you read it.
Reprinted with permission from Dr. Robert Brill, brillrh@cmog.org
And it's more fun to play with than pitch.
No Zen is good zen
I did read the whole article. I also studied the subject for a while when doing a physics degree at university so I am keenly aware of the context.
Michael was flat out wrong in that the article explained the debate, and the rather than supported one side of it. It is, as the article said, a matter of semantics.
Liquid means lots of things: the two most common technical meanings are 1) this flows and 2) this has no long range crystalline order. Hence by 2) glass is a liquid, and by 1) glass isn't. Hence the conclusion from the article that it is a matter of semantics.
As a glass scientist, I wanted to add my 2 cents worth. Almost any substance can occur in a glassy state if quenched fast enough. This includes most metals, plastics, and pitch. Below a critical temperature (the glass transition temperature Tg) a glass is a brittle, perfectly Newtonian solid. At temperatures above Tg, viscosity decreases to the point where relaxation can occur, and the substance becomes rubbery, then fluid. The apparent viscosity at Tg is ~ 10^13 poise. Real motion is observed at ~10^8 poise. The Tg of optical pitch is a bit below room temperature, and the room temperature viscosity is ~10^9 poise. The problem with the experiment cited is that temperature fluctuations change the viscosity exponentially. Droplet formation time will vary accordingly.
That is an example of a non-Newtonian fluid. Normal Newtonian fluids' viscosity is a function of temperature: the colder it gets, the thicker it gets. Non-Newtonian fluids' viscosity is a function of something else, in this case, force. That is, the more force you apply to it, the thicker it gets. If you want a really good and simple 'goop' recipe, try this:
-white glue, mixed with water, 50:50
-tablespoon of borax (from laundry section) in a few cups of water
-(optional) food coloring mixed with glue
pour the glue/water mix into the borax solution and it with thicken up. You'll pull out a slimy, goopy mass that is too watery to play nicely with but if you work it in your hands for a bit to get the excess water out, you'll have some fun. Bounce it around, slap it, tear it and it's more like a solid. Let it sit on your hand and it flows like a liquid. Plenty of fun.
In Soviet Russia, hot grits put YOU down THEIR pants.
Make sure that they've signed the personal injury waver first, of course...
Hmm pity, they don't have this demo on their web page Ontario Science Centre only a block away from me.
One line blog. I hear that they're called Twitters now.
I rememebr this, it used to sit at the front of the old Kelvin Lecture theater before the remodelled it, in fact it sat out in the open and it was pretty much gathering dust.
It was more like a little series of steps, pitch had been placed in a reservoir at one end and had flowed down the steps into the reservoir at the other end. In fact it had started overflowing at the bottom.
www.thinkgeek.com is reselling a goo they labeled "smart mass." The original product is Crazy Aaron's Thinking Putty. I'll leave it to google to provide links. Crazy Aaron has quite a few mpeg's of the product being shot from a potato gun.
It's similar to your cornstarch putty, though a bit more involved. It exhibits different properties on four different time scales. It will drip on its own weight slowly, will bounce firmly if dropped, will tear and shear if pulled too quickly, and will shatter if struck with a hammer.
Kinda like the force shields in the Dune movie and books. You can dent it easily with a fingertip if you move slowly, but it will repell your fist if you try to punch it.
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