With time on my hands today, I finally helped Brian make Oobleck, which he first read about in January. It was a heckuva fun afternoon, messy one though.
Isaac Newton thought that no matter how much force you applied to a liquid, it would either stay as runny or as thick, ie, its viscosity (the measure of a fluid's resistance to flow) is constant. For example; no matter how much you shake a bottle of water, its viscosity remains the same.
In the case of a non-Newtonian liquid, when force is applied to it, it either becomes thicker or more runny, eg quicksand becomes thicker and ketchup becomes more runny (that's why we shake a bottle of ketchup).
Oobleck is a non-Newtonian liquid. We made it by mixing 1 cup of cornstarch with 1/2 cup of water (we added food colouring too) to form what chemists call a colloidal suspension. What this means is that Oobleck is actually made up of tiny, solid particles (cornstarch) suspended in water.
When we left Oobleck alone or moved it slowly, it acted like a liquid, but when we applied force to it, it acted like a solid. Why?
Cornstarch is a natural polymer - a long, bulky chain of atoms. When no or little force is applied, these polymer chains have time to slide past one another along the water molecules, and Oobleck acts like a liquid. But when force is quickly applied, these polymer chains have no time to move out of the way, and run into one another, resulting in Oobleck acting like a solid.
This youtube video of people running on oobleck is funny.