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Micheal,
the corn starch story ( as I understand it and not necessarily in terms
suitable for your target age group) is a like this.
The right type of cornstarch mix would be called a "dilatent" material.
which means that when stirred or subjected to "shearing" forces at actually
expands. Why so? the starch granules of the right type of corn stach stack
and settle rather like blocks of lego, you can fit more blocks of lego into
a container if you fit them neatly, throw them in roughly and they take up
much more space. If water is present between these grains, when the mix is
stirred, the mix expands and needs more fluid to fill the gaps, it is the
tension generated by this "water shortage" that pulls the grains together so
strongly.
Things to check to convince yourself ( others) - fill a drink bottle ( one
with a straw out the top, that comes from low in the bottle) with coarse
beach sand and water - leave an air space over the top. poor water into the
straw so you can see a level. gently squeeze the bottle, the water level
goes down, not up. The sand has expanded, and drawn more water in.
If you stand on the beach where the sand is wet, but not awash, you will
notice the sand around your feet swell up and dry out, rather than get wet
from squeezing water out of the sand under your feet.
This is why some cornstarches DONT WORK, because the are milled differently
( and often from other starch sources) and or rounder, and so the effect of
stirring dose not change the "attitude" of the grains with respect to each
other, and so doesn;t take up more space ( well not much more anyway) and so
doesn;t create this dilatent change....
SO for me the beach and a box of lego would be where I start.
Hope this helps
Dr Stuart Kohlhagen P.S.M.
New Concepts
Questacon - Australia
-----Original Message-----
From: Kerkman, Mike [mailto:[log in to unmask]]
Sent: Thursday, 2 October 2003 8:11 AM
To: [log in to unmask]
Subject: 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.
****************************************************************************
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Hello list.
So I was hoping someone could offer up a good explanation of why
cornstarch + water makes such a peculiar non-solid, non-liquid. The
explanation I need has to be appropriate for a group of students ranging
from 2nd to 6th grade and up. From what I can tell from an admittedly brief
internet search, the forces between the starch molecules are strong when
pressure is applied, and when there's no pressure, they relax.
thanks for any help you can give me.
Oh, and also this experiment (mix about 3pts cornstarch to 2 of water) might
be good for the Halloween-related stuff. I dunno, I guess just because it's
goop. But anyway, if any of you haven't tried this experiment, give it a
shot because it is so, so cool.
Mike
Michael Kerkman
Botanical Educator
The Huntington Library, Art Collections, and Botanical Gardens
1151 Oxford Road
San Marino, CA 91108
626-405-3586
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