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William,

while the starch in cornflour is a long polymer, it is quite tightly locked
up in the grains, and so the long chain effects aren't in play in the usual
cornflour demos.

The long polymer effects are part of the underlying behaviour of slime (
ooblech and friends).

You can see the difference between the behaviour of bound starch granules
and long starch molecules if you heat it, as when thickening a source - but
this change is irriversible.

When penicillin was first purified it was almost impossible to inject,
because the crystals in suspension behaved just like cornstarch, and the
harder they pushed the worse the problem.
They eventually had to smash the penicillin crystals into less regular
particles before suspending them so they could be injected easily.

Try cornstarch in a syringe, you'll see the problem. Industrial chemists
spend much of their time alternatively trying to overcome or enhance these
behaviours.

Cheers
Stuart K






-----Original Message-----
From: William [mailto:[log in to unmask]]
Sent: Friday, 3 October 2003 12:26 AM
To: [log in to unmask]
Subject: Re: cornstarch as a non-Newtonian fluid


ISEN-ASTC-L is a service of the Association of Science-Technology Centers
Incorporated, a worldwide network of science museums and related
institutions.
****************************************************************************
*

Take a look at Kinetic Carnival of Physics - Jearl Walker.
In it he talks about Isotropic Fluids - the corn starch and water
phenomena.  Plus he seems to suggest that all mixtures of solids and fluids
(including sand in air), quicksand, beach sand that is water saturated and
even solid particles suspended in air exhibit these properties - but to a
greater or lesser extent depending on the materials.
My rather limited understanding is that essentially, when put under high
shearing pressures the particles make contact with one another in a jumbled
arrangement acting as a solid (essentially certain angles of the particle
make contact with other particles while other spaces are left free (and
water goes into those spaces)).  However when left alone the water allows
the particles to flow into more naturally occurring arrangements and the
particles slip over one another with a layer of fluid between them - acting
as a fluid.  So when not under pressure the fluid aspect takes over the
material and particles glide over one another in a more orderly fashion.
 By varying the degree of pressure you vary the solid/liquid
characteristics exhibited.  I have also been told that this is more
complicated with corn starch because it has long chain molecules (like
spaghetti) so exhibits the characteristics even "more so" do to its polymer
style nature.  Of course, I don't know if that's true or not (but it makes
sense to me).


William Katzman                    "The important thing is to never stop
questioning." -A. Einstein
Director of Exhibits               "Are you sure about that Al?" -His wife.
Catawba Science Center
ph: (828) 322-8169 x307
fax:(828) 322-1585
[log in to unmask]

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