Michael Banks on fri 1 oct 99
Yes, mercury is cool stuff, only beaten by liquid helium which will climb
out of an open beaker against gravity. Freaky....
The schools should get into gallium. It melts at warm room temperature and
has low vapour risk I understand, compared to mercury. Otherwise it
exhibits all the interesting behaviour of "quicksilver".
Michael Banks,
Nelson,
New Zealand
>----------------------------Original message----------------------------
(Clip)
>If you really want to demonstrate surface tension effects look at metallic
>mercury. You can drill small holes in a container of mercury and its
surface
>tension is so high that it won't drain out. It's unfortunate that mercury
is
>poisonous and has been removed from schools. It is a most incredible
>substance to observe. (Snip)
>
>Bruce "they always take away the fun stuff" Girrell
>
Gavin Stairs on fri 1 oct 99
The surface tension discussion is only part of the point. The other part
is wetting, or adhesion of the liquid to the solid. This is what drives
capillary action, or the wicking effect. Really what we are talking about
in these surface effects is surface energy. The surface acts to minimize
its free energy. So if it can do so by covering a little surface as
possible and rolling up into a ball, it will do so. If, on the other hand,
the lowest combined energy come when the liquid covers as much of the
surface as possible, then it will spread out all over the surface, down to
a single atomic thickness. Oil tends to do this on many surfaces. One of
the problems of designing lubricants is to adjust these tendencies so the
oil stays put in the bearing, and doesn't spread all over. Used to be a
material called porpoise oil, or whale oil, also known as pivot oil, that
was derived from sperm whale oil, and later porpoises, which had that
admirable property. Nowadays, synthetics are designed for this by careful
modification of the long chains, introducing enough polarity that they tend
to stick to the surface, but not too much. Anyway, the whole thing is a
dance between the attraction of the liquid molecules to each other
(cohesion, surface tension) and to the substrate molecules (adhesion,
wetting, capillarity), and the special conditions at the surfaces.
One interesting measure of the balance of surface tension and wetting is
the contact angle of a droplet on a surface. If the age is less than
90degrees (rounder), the liquid is said not to wet the surface. If it is
greater than 90 degrees (flatter), the liquid is said to wet the surface.
This is one way to test how clean a glass (or a glazed pot) is: water will
wet the clean surface, but tends to ball up on a dirty surface. If the
surface has been in contact with animal fat or wax, it takes a surprising
amount of cleaning to get the surface clean, sometimes.
Another classic experiment is capillary action, by which it is possible to
measure the surface tension and the cohesion between the liquid and the
solid. Take a set of tubes of varying diameter and insert one end in the
liquid. Measure how much the liquid rises (or falls) in the tubes. I
won't go into the calculations, but by comparing the results for various
tubes, you can calculate some of the forces responsible for these wicking
effects.
And one final note: Water does indeed leak out of pottery containers. It
just doesn't do so in a visible manner. Water will tend to fill the voids
in any container it can wet. So it will occupy all the micro fissures it
can find. But when it gets to the outside, it tends not to wet that
surface. Why? Because to do so it would have to expand its free surface,
which costs a lost of energy, since water has a high surface tension. So
the water reaches the outer surface, but doesn't leak. Except it does
evaporate. This evaporation happens at a rate proportional to the free
surface, so in porcelain, it is negligible. But in earthenware, the void
fraction, and therefore the exposed water surface, is large, and
evaporation is significant. Enough to cool the whole pot by a few degrees.
Which is why water pots and flower pots are traditionally made of earthenware.
Gavin
At 11:59 AM 10/1/99 -0400, you wrote:
>----------------------------Original message----------------------------
>Yes, mercury is cool stuff, only beaten by liquid helium which will climb
>out of an open beaker against gravity. Freaky....
>
>The schools should get into gallium. It melts at warm room temperature and
>has low vapour risk I understand, compared to mercury. Otherwise it
>exhibits all the interesting behaviour of "quicksilver".
>
>Michael Banks,
>Nelson,
>New Zealand
>
>>----------------------------Original message----------------------------
>(Clip)
>>If you really want to demonstrate surface tension effects look at metallic
>>mercury. You can drill small holes in a container of mercury and its
>surface
>>tension is so high that it won't drain out. It's unfortunate that mercury
>is
>>poisonous and has been removed from schools. It is a most incredible
>>substance to observe. (Snip)
>>
>>Bruce "they always take away the fun stuff" Girrell
>>
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