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Re: [Phys-l] buoyancy on a submerged pole



>> I claim that in thin layers, the toothpaste exhibits tremendous shear viscosity.

I disagree. Put some on the floor. It's like a banana peel - slippery. You wouldn't want to walk down steps coated in toothpaste.

Anyway, that's a side issue. We're trying to figure out if it is an adhesive or just a sealant. Someone needs to hang one thing from another thing using toothpaste as the sealant, and put them in a vacuum bell to see how much weight can be supported. I don't think it's much of an adhesive.

* /From/: John Denker <jsd@av8n.com <mailto:jsd%40av8n.com>>
* /Date/: Fri, 05 Nov 2010 09:24:02 -0700

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On 11/05/2010 08:02 AM, Scott Orshan wrote:

While toothpaste might possess some mild adhesive properties, it is
also only mildly viscous. Under even a small stress, it should pull
apart.

What means "should" pull apart? Experimentally, I observe it
does not pull apart! Maybe it depends somewhat on the brand of
toothpaste; I haven't done an exhaustive survey.

I think you are seeing an air pressure effect. ...

The next experiment would be to try this in a vacuum somehow.

An easier experiment is to see whether the toothpaste will
support a load per unit area exceeding the ambient air pressure.

=============

I think you are seeing an air pressure effect.

That may be. I don't know ... and for present purposes it doesn't
matter much, because the viscosity issue is more important. See
below.

I was including air pressure effects in my previous statements
about tensile strength. I probably shouldn't have, since the
physics is different, even though operationally it looks the
same at first glance. I stand corrected. There may be some
tensile strength there, or there may not; I don't know.

This is another example of the difference between doing the
experiment and interpreting the experiment.

it is also only mildly viscous.

Now that's where we disagree.

Almost by definition, a solid has infinite viscosity. I observe
toothpaste to be a semi-solid. An extruded cylinder of toothpaste
keeps its shape, which is something a fluid would not do.

I claim that in thin layers, the toothpaste exhibits tremendous
shear viscosity. That is, it resists shear to a very high degree,
in the sense that if you apply a moderate amount of shear it
responds with a flow rate that is very small, /disproportionately/
small, possibly zero. (Again: a solid exhibits infinite shear
viscosity in this sense: zero flow rate in response to moderate
shear stress.) Of course anything will break if you apply an
immoderately large stress, but that's not the point.

The suction-cup effect depends on resistance to shear stress.

Note that in thin layers, even a genuine fluid has tremendous
resistance to shear. In many cases, moistening a rubber suction
cup makes it work /better/. This is partly a dynamic effect,
in the sense that the thin layer of fluid will flow eventually,
just not soon.

The suction-cup effect is central to this entire thread.
If the suction-cup effect is operating, the pole will be in
compression (as observed on a strain gauge). If/when the
fluid leaks into the suction-space, the pole will be in
tension (again observed on a strain gauge).