Why does starch paste thicken when stirred?

I am studying bonding properties in various compounds in my high school A.P. chem course. Our instructor conducted an experiment using corn starch and a small amount of water. The resulting substance had a fairly liquidy consistency however when pressure was applied to the material, it hardened momentarily before returning to it's original state. While handling the material quickly it remained a solid, but liquified once at rest. What causes this phase change phenomenon in corn starch?

Cornstarch/water paste is an example of a non-Newtonian fluid, that is, a liquid whose viscosity changes, depending on how much force you use to get the liquid to flow. Viscosity is a measure of the liquid's resistance to flow (molasses has high viscosity; water's viscosity is much lower).

Pressing on the cornstarch suspension squeezes the water from between the particles. It resists flow because now the particles are grinding against each other. Releasing the pressure allows the water to seep between the particles again and the suspension flows more easily.

You can see the same effect with wet sand. The sand hardens under your feet. But if you lean over and draw a line in the sand with your finger, you'll see that it's rather soft.

Author: Fred Senese senese@antoine.frostburg.edu

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