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| Q: | I am doing a report on non-Newtonian fluids. I saw your answer to the other question on non-Newtonian fluids, but it doesn’t tell me about the fluids themselves. How do they work? Why do they behave the way they do, instead of like Newtonian fluids? What is the difference that changes them? Thanks for any help you can give me.
-Megan (age 15)
Mt Si High School, Wa, USA |
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| A: | Hi Megan,
Excellent question! As you read in the other answer,
a non-Newtonian fluid is a fluid whose viscosity depends on the force
applied (and sometimes time and temperature as well!) and therefore has
very interesting mechanical properties. Non-Newtonian fluids get pretty
complicated, and I’m no expert on fluid dynamics (so this should all be
taken with a grain of salt), but here’s an explanation for one type of
non-Newtonian fluid which I think is pretty good.
You can
easily make a non-Newtonian fluid by mixing cornstarch and water
together until it forms a syrupy mixture. If you stir it around slowly,
it will flow like a thick liquid, but if you move it suddenly or poke
it very hard and very fast, it will feel like a solid! When you mix the
cornstarch and water, you create a colloidal suspension, which means
that on the microscopic level, you have a bunch of solid particles
floating around and suspended in a liquid. Because your
cornstarch/water mix is very thick, the particles of cornstarch
floating around in the water are packed very close to each other (so
that they are actually touching), but they are still able to slip past
each other. This is what happens when you move the mixture slowly --
the suspended particles have time to move and slowly slip past each
other, so the mixture acts like a liquid that can easily flow. However,
when you suddenly poke the mixture, the particles do not have enough
time to move out of the way (they are fairly massive), so they stay
where they are and the mixture feels like a solid. (This is probably a
somewhat simplified picture of what is actually happening, but is
hopefully a useful way to think about it).
Quicksand is
another example of a non-Newtonian fluid and it works much like the
cornstarch/water mixture -- instead of cornstarch particles floating in
water, there are individual grains of sand suspended in water. This is
why if you ever fall in a pit of quicksand, you should slowly try to
swim out instead of violently thrashing around. If you move slowly, the
quicksand will act like a liquid and you will be able to get out, but
if you move around very quickly, the quicksand will act like a solid
and you will get stuck!
I hope this helps! For a more detailed explanation and some more terminology, check out this site.
(republished on 07/25/06) |
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