Re: What is cornstarch and water. I know it is non newtonian but what is that?

Ryan,
I'll try to simplify the complex subject of rheology, that is, the study of 
moving fluids.

Newtonian rheology is one in which the resistance to deformation [flow]is 
directly proportional to the force of that deformation [shear]. Water is the 
usual example: if you stir water very slowly, you don't feel much resistance 
against the spoon. If you stir faster, you'll notice more resistance against 
the spoon. If you could measure both forces with a machine called a viscometer 
[from 'viscosity' a word used to describe the force of the resistance to flow] 
you would see that the viscosity of water is directly proportional to the force 
applied trying to make it flow; The plot of viscosity versus stirring force 
would be a straight line. 

Any flow behavior that does not result in a straight line plot is called non-
Newtonian.

Some solutions and fluids resist less the faster they are stirred; this is 
called thixotropic [or shear thinning] rheology. Latex wall paint is a good 
example of this. It looks really thick in the pail, but glides onto the wall 
very easily because the motion of the brush is the same thing as stirring, so 
it 'thins out' during application but goes back to its thick state when the 
force is removed. This is why it doesn't run down the wall after you paint it. 
The plot of viscosity versus stirring pressure is a curve heading downward.

Your water-cornstarch slurry is not a colloid, but a dispersion of a solid 
[starch] in a fluid [water]. Colloids are similar, but much smaller and is 
beyond todays question. You've already observed that at a certain starch to 
water ratio that the dispersion flows like something slimy when you stir it 
very slowly but that when you stir real hard [or punch it] it hardens up. This 
non-Newtonian rheology is called dilatent [or shear thickening]. Another 
example of this is the wet sand at the edge of a beach that flows through your 
hands easily but hardens when you take a step on it. A plot of this dilatent 
viscosity versus force curves upward, i.e. a little more force will give you a 
lot more viscosity.

There are at least three or four other sub-categories of rheology, but I hope 
this helps to explain your observations.

Ken