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.

Comments

Kelvin's experiments

[Bazzargh]

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...

Cornstarch and Water

[FatAlb3rt]

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...

Pitch is used for polishing optics

[goingware]

Telescope makers and opticians use pitch for polishing glass.

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.

non-Newtonian fluid

[Kris Warkentin]

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.