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Panta Rei

Friday, April 18, 2008 - 06:00 AM

Greek philosopher Heraclitus said 'Panta Rei', which means 'all things flow'. Rheology ('flow'-ology) is the study of viscoelastic materials like Jello that are a little bit liquid and a little bit solid. But even the most liquid of liquids have some solid character. And even the most solid of solids have some liquid character. Take those beautiful stained glass windows in gothic cathedrals. For a long time it was thought that these windows are thin at the top and thick at the bottom as a result of centuries of slow viscous flow. As it turns out, it would take much more than centuries for glass to flow (see comments below).

But as we saw in our show (So-Called) Life, scientific observation soon turns to manipulation. In the world of biomechatronics, scientists are using 'magnetorheologic' fluids to emulate the complex motions of human limbs to help victims of trauma, disease or birth defects.

Watch the closely related 'ferrofluid' in action below.

Reflecting on Heraclitus' statement, Marcus Reiner, one of the fathers of modern rheology said 'Everything will flow. You just have to wait long enough.'

Tell us what's begun to flow in your life..

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Comments [3]

Justin Paul

The stained glass phenomenon was a convenient choice poetically but an apparently poor choice scientifically as it has never been adequately demonstrated that the thickness difference is due to viscoelastic flow. Glass of course, does have both liquid and solid material properties, but..

Taken from "Do Cathedral Glasses Flow?", Am. J. Phys. v66, pp 392-396, May 1998:
"The conclusion is that window glasses may flow at ambient temperature only over incredibly long times, which exceed the limits of human history"

Many thanks to "myth-busters" Brandon and JM for pointing out that it takes a LOT longer for glass to flow!

Apr. 19 2008 04:49 AM
JM

Brandon,
Wikipedia was correct, and glass-makers know this, too. The unevenness was due to how they spun out glass in discs. Newer techniques make glass uniform in thickness. A paper by Y.M. Stokes (Y.M. Stokes. 1999. Flowing Windowpanes: Fact or Fiction? Proceedings of the Royal Society of London Series A: Mathematical, Physical and Engineering Sciences, Vol. 455(1987): p. 2751-2756) calculated that it would take millions of years for us to detect a change in the thickness of glass due to gravity.

Apr. 18 2008 01:39 PM
Brandon

recently I had heard the statement that "glass is like a living object always changing, every day it flows a little bit more down itself making the glass thin on the top and thick at the bottom, that is why old window panes start rattling in the wind". Here you bring up this again. My theory was that this wasn't true because glass is a very hard surface, just brittle. Also that in that same theory if it does keep flowing that eventually (this should be true of the oldest glass we have available) the flow would go down like gravity so much that wouldn't it turn into a stalagmite? I turned to Wikipedia for the answer and found they would make these discs out of glass for the windows and that was the reason for the thickness differential.

I am not a scientist or any kind of expert, so I would like to know which is correct? Wikipedia or the myth?

You can find the info by looking up glass under Wikipedia and goto the "Behavior of antique glass" section

Apr. 18 2008 07:04 AM

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