While toothpaste might possess some mild adhesive properties, it is also
only mildly viscous. Under even a small stress, it should pull apart.
Under the article for Thixotropy in Wikipedia, toothpaste is
specifically mentioned as doing just the opposite - thinning in response
to shear stress.
I think you are seeing an air pressure effect. Assume a six inch
diameter funnel with a wall thickness of 1/16 inch. The area of the rim
that you are sealing up is over two square inches, which amounts to 30
pounds of air pressure - quite enough to support a plate. I am only
counting the rim area, not the entire funnel area, which would be on the
order of 100 square inches.
The next experiment would be to try this in a vacuum somehow. In my own
lame effort to try this, I put some toothpaste on my thumb and
forefinger, and felt a mild stickiness that held them together. But is
that from air pressure or adhesion? Then I stuck my two fingers into the
hose of a Shop Vac and tried to get a good seal. Perhaps this is
experimenter bias, but it did feel like less adhesion when in the lower
pressure air stream.
I'm not sure what kind of substance would give enough of a seal to be
resistant to the water creeping underneath, yet not possess any
significant "stickiness". Maybe a teflon grease.
I think that we are both seeing air pressure effects dominating what we
are actually looking for. We're going to have to find an evacuated room,
and give our lives for science to see what is really going on (or in a
less newsworthy experiment, just set something up in a vacuum chamber).
A good experiment (for the toothpaste, not the pole in the water) could
be done in space. Stick two objects together with toothpaste, pull on
one with different amounts of force, and measure the maximum force or
acceleration before they separate. Do this inside the ISS, and then outside.
Back to "buoyancy" - I just don't see how still water can push upward on
the smooth side of a box or cup. Only the vertical force gradient can be
responsible for traditional buoyancy.
Is everybody familiar with the effect that happens when you have a
helium balloon in your car and you go around a turn? It goes in the
opposite direction that you would expect because of a horizontal force
gradient as the air is pushed to one side of the car. If you turn left,
denser air piles up on the right, and the balloon is pushed to the left.
If you open the window on the right side, it shouldn't happen (anyone
want to try this?).
And for John C., the cup is in the water with its open end up - there's
no air compression as it is lowered into the water.
Scott
* /From/: John Denker <jsd@av8n.com <mailto:jsd%40av8n.com>>
* /Date/: Thu, 04 Nov 2010 22:55:43 -0700
Then, having done the experiment, it is important to correctly
interpret the results.
Consider the three assertions that "Toothpaste is not glue,
and it's not particularly sticky. ... Therefore, there is no
net tensile force." The first assertion is literally true
in some narrow pedantic sense, but it is highly misleading,
and the second and third assertions are not true in general.
They are not true of any commercial toothpaste I have ever
seen.
The fact is, typical toothpaste is sticky, especially in thin
layers. It took me two minutes to design and carry out an
experiment to show an example of this. I put a thin layer of
toothpaste on the wide end of a funnel and used that to glue
the funnel to a plate. I was able to pick up the system
using the funnel as a handle, thereby demonstrating that the
toothpaste has significant tensile strength and stickiness.
The funnel remains open at the small end, so there is no
possibility of any suction-cup effects. The plate is entirely
supported by tension in the sticky paste.
In fact, typical toothpaste is a semi-solid. It is not a very
_good_ glue, but it does exhibit , which is one of
the properties one expects a good glue to have.