Date: Sat May 26 18:01:54 2007

Author: Urs Lauterburg

Subject: Re: a good diffusion lab or demo

Post:
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Tom,

This is some ongoing project of mine and I have,=20
up until now not found anything which would show=20
the process accurately and over a suitable amount=20
of time without being poisonous or hazardous.

However we would demonstrate the mechanism=20
beautifully by demonstrating heat conductivity.=20
Heat actually equilibrates the same way the=20
concentration of a given substance does in a=20
solvent.

I once did the famous diffusion of bromic gas,=20
one of the every few colored gases and the effect=20
has shown well. However, the aggressive bromic=20
gas dissolved the joints of my rectangular glass=20
container which was used to show the process in a=20
shadow projection setup. After some time the=20
container fell apart, the gas escaped and we had=20
to finish the lecture earlier than intended.

Then I experimented by just applying a single=20
drop of highly concentrated KMn in water in the=20
center of a flat round dish filled with just=20
enough distilled water to cover the bottom. The=20
dish was put onto a lite table with a video=20
camera on top to see how the strongly colored KMn=20
drop slowly spreads out into the water according=20
to the laws of diffusion due to the concentration=20
gradient. Qualitatively this worked fairly well,=20
but the idea was to also illustrate the=20
concentration profile to be instantaneously=20
displayed by a LabVIEW based image analysis=20
routine. Unfortunately the analysis has shown=20
that the profile does not behave according to the=20
theoretical velocity distribution of diffusing=20
particles. The concentration stayed pretty flat=20
and changed rapidly only at the edge of the=20
growing purple circle formed by the KMn. Also=20
disturbing convection sometimes made the circle=20
shift a bit instead of staying centered.

Anyway I still hope to get a good combination of=20
harmless watery substances of different colors to=20
get a nearly perfect diffusion profile. Maybe a=20
kind of jelly like solution with a drop of food=20
color would work better.

Does somebody have an idea? HCl and NH3 are no=20
option they just stink too nasty and are more for=20
the chemistry type of people who love that kind=20
of stuff ;-)

Just my 0.001=A4 worth of a comment to this topic

Urs

Urs Lauterburg
Physics demonstrator
Physikalisches Institut
University of Bern
Switzerland


>Hi everyone
>
>
>
>Does anyone know of a laboratory or demo
>
>that teaches the principles of diffusion.
>
>
>
>My prof prefers one that uses the motion of large
>
>objects that you can actually see and touch rather than
>
>the microscopic diffusion of molecules.
>
>
>
>Any suggestions, macro or micro, would be welcome.
>
>
>
>Thanks
>
>
>
>Tom Dershem
>
>Claremont McKenna College

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Tom,

This is some ongoing project of mine and I have, up until now not
found anything which would show the process accurately and over a
suitable amount of time without being poisonous or hazardous.

However we would demonstrate the mechanism beautifully by
demonstrating heat conductivity. Heat actually equilibrates the same
way the concentration of a given substance does in a solvent.

I once did the famous diffusion of bromic gas, one of the every few
colored gases and the effect has shown well. However, the aggressive
bromic gas dissolved the joints of my rectangular glass container
which was used to show the process in a shadow projection setup. After
some time the container fell apart, the gas escaped and we had to
finish the lecture earlier than intended.

Then I experimented by just applying a single drop of highly
concentrated KMn in water in the center of a flat round dish filled
with just enough distilled water to cover the bottom. The dish was put
onto a lite table with a video camera on top to see how the strongly
colored KMn drop slowly spreads out into the water according to the
laws of diffusion due to the concentration gradient. Qualitatively
this worked fairly well, but the idea was to also illustrate the
concentration profile to be instantaneously displayed by a LabVIEW
based image analysis routine. Unfortunately the analysis has shown
that the profile does not behave according to the theoretical velocity
distribution of diffusing particles. The concentration stayed pretty
flat and changed rapidly only at the edge of the growing purple circle
formed by the KMn. Also disturbing convection sometimes made the
circle shift a bit instead of staying centered.

Anyway I still hope to get a good combination of harmless watery
substances of different colors to get a nearly perfect diffusion
profile. Maybe a kind of jelly like solution with a drop of food color
would work better.

Does somebody have an idea? HCl and NH3 are no option they just
stink too nasty and are more for the chemistry type of people who love
that kind of stuff ;-)

Just my 0.001=A4 worth of a comment to this topic

Urs

Urs Lauterburg
Physics demonstrator
Physikalisches Institut
University of Bern
Switzerland


Hi everyone



Does anyone know of a
laboratory or demo

that teaches the
principles of diffusion.



My prof prefers one
that uses the motion of large

objects that you can
actually see and touch rather than

the microscopic
diffusion of molecules.



Any suggestions, macro
or micro, would be welcome.



Thanks



Tom Dershem

Claremont McKenna
College



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