Re: Why does a cornstarch and water slurry harden when compressed?

Your question addresses an area of rheology, ie, the study of moving fluids.

Let's set a baseline at Newtonian rheology: The stress [viscosity, 'thickness']
is directly [linearly] proportional to the strain placed on it. Water is a good 
example of this behavior.

Another common rheology is thixotropic, one in which the stress decreases with 
strain [i.e., the harder you stir, the 'thinner' it gets].

The phenomenon you are asking about is called dilatent rheology [the harder you 
stir, the thicker it gets]

A cornstarch/water slurry is not a solution, but a suspension [or dispersion, 
if you prefer] of a solid in a liquid. At a certain ratio of cornstarch to 
water, the critical PVC is approached. PVC is an abbreviation for "Pigment 
Volume Concentration", a term widely used in the paint industry, and meaning 
simply the ratio of pigment {starch} in the total volume.

Exactly at the CPVC, there are no voids [the starch particles are in contact 
with each other and the water fills the interstitial space].

At this point, it cannot be said if you have a 'solid' or a 'liquid' the 
application of pressure [motion, stress, stirring] perturbs that and the flow 
characteristics change -- the water is 'squeezed out' [into adjacent areas]
leaving the starch behind as a damp solid.

This is the same phenomenon that you see when walking along wet sand at the 
beach: your footsteps look dry behind you, even though they were made in wet 
sand.

Other than simple water solutions, most systems exhibit thixotropic
behavior. There is little commercial appeal for dilatent ones.

Send me a note if you'ld like more info and refs

Ken Johnsen @ MSN