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 as well) and therefore has very interesting mechanical properties. Non-Newtonian fluids get pretty complicated, and Iím no expert on fluid dynamics (so this should all be taken with a grain of salt), but hereís 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 they form 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)