Ode to Mr Wizard and Mr Atrocity: Non-Newtonian fluids

This afternoon, in lieu of doing ethics homework, I spent several hours reading blogs. I stopped at one in particular, as it piqued my scientific interest. Mr Atrocity posed a list of questions he could not answer, and as I read I realized nor could I. However, I thought I'd give one a try. He asked, "Why does cornflour do that weird liquid/crumbly thing it does?"
Feeling relatively confident that the answer was only a wiki away, I set off on a quest. I went to the kitchen and mixed up a batch of oobleck (the grade school science teacher's name for a mixture of cornstarch and water). After playing with it for longer than I can be made to admit, I began searching for what causes the crumbly phenomenon. While there wasn't a simple answer to be had, here's what I learned.

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A mixture of cornstarch and water forms a dilatant, or shear-thickening fluid. A dilatant substance is a type of Non-Newtonian fluid. Non-Newtonian fluids are fluids in which the viscosity changes as varying shear forces are applied. In a Newtonian fluid (such as water), the viscosity depends on temperature and pressure, but not forces acting on it. In a dilatant substance, the apparent viscosity increases with the rate of shear. So in the case of our cornstarch mixture, if you press on it quickly and firmly with a finger, the viscosity increases and the texture is firm. However, if you use the same finger to press gently and lightly, it will appear more fluid (have a lower viscosity), and your finger will sink in.

The most common theory of why this happens is as follows. Starch is not soluble in water, therefore the mixture of water and cornstarch is actually a suspension - the starch particles are suspended in the water, but are not broken down by it. Because of this, each particle of starch is surrounded by water. The water provides lubrication and allows the particles to slide past one another easily. However, if force is exerted on the suspension, the water is squeezed out from between the granules of starch and friction occurs.

For more fun with oobleck, check out this video showing what happens to cornstarch solution when shaken. It's fun!