Q & A: Non-newtonian fluids

Q:
what is a non-neutonian fluid. I don’t think I spelled it right, but I think they exist. Are they fluids/solids that can exist in both states dpending on how much the system is stressed? Isn’t toothpaste one of those fluids?
- dan vidakovich (age 19)
U of I, AAE
A:
Hi Dan,

Yeah, they aren't solids, yet they also don't follow Newton's definition of fluids either. Basically, a fluid is called non-Newtonian if its viscosity depends on the force that is applied to it.

As you might know, viscosity is a measure of how easily a fluid flows (the higher the viscosity, the harder it would be to stir a bowl full of it). For example, water has a lower viscosity than syrup. For an ordinary fluid (like water) the viscosity wouldn't depend on how fast you were stirring it, but for a non-Newtonian fluid it would. Depending on the specific non-Newtonian fluid you had, it could get easier or harder when you tried to go faster. It's very cool actually... imagine something that would bounce a little if you dropped it (a very sudden impact force when it hits the floor), but that would get sort-of watery if you just held it in your hand. I had never heard that toothpaste was a non-Newtonian fluid, but if you do an experiment, definitely let us know the result. You can make your own non-Newtonian fluid by blending cornstarch and water together until it forms a syrupy mixture (try it). Let us know if you find some other examples.

Cheers Jesse

(published on 10/22/2007)