What Is a Non-Newtonian Liquid?

[TSN79]

I've heard of non-Newtonian liquids (and seen them on tv for entertainment value). I also hear that they keep referring to it as a material that changes its viscosity, and so turns from a liquid to a solid. However, I would never look at such a liquid and think of it in that way. What I see is simply a thick liquid that allows you to stride across it without sinking in, cause it's so thick, but the word solid doesn't come to mind. Isn't the viscosity the same all the time, just high enough that if you walk fast you'll make it across? And so can't all fluids become non-Newtonian just by adding something to them to rise the viscosity to the right level?

 

[FredGarvin]

You may need to review the definition of viscosity.

Viscosity is a relationship between the shear stress and velocity gradient in a fluid. If that relation is essentially linear, then the fluid is considered Newtonian. If that relationship is not linear, then it is considered non-Newtonian. Things like paint, toothpaste, peanutbutter, etc...are all good examples of non-Newtonian fluids. My father-in-law is a retired chemical engineer. I have talked to him a lot about what he used to deal with in his plants. He would have loved to have worked with Newtonian fluids.

Other types, such as a Bingham plastic change their behavior depending on the level of shear stress. At one point they behave like a solid, but at higher levels of shear stress, they act like a Newtonian fluid.

Just because you add something to a fluid to make it more viscous does not mean you definitely will change its behavior from Newtonian to non-Newtonian.

http://en.wikipedia.org/wiki/Non-Newtonian_fluid
http://en.wikipedia.org/wiki/Bingham_plastic

 

[siddharth]

To add to what Fred said, a Newtonian fluid is one where the shear stress is propotional to the velocity gradient perpendicular to the shear. ie, $$\tau = \mu \frac{du}{dn}$$

A fluid which isn't Newtonian is a non-Newtonian fluid, and they are classifed into various types. As Fred pointed out, a toothpaste is an example of a non-Newtonian fluid, and in particular, a bingham plastic.

You can have various other types of non-Newtonian fluids, where the relation between the shear stress and perpendicular velocity gradient is not linear (for example, a power law fluid), or a time varying viscosity.