How can corn starch and water both flow and be firm?

17 October 2010

Question

This is Steve from Thailand.  I am wondering how the properties of a corn starch and water mixture change so that it flows slowly like thick maple syrup when poured from a bowl, and if you hit a bowl of it with your fist, the impact causes it to harden.

Thanks
Steve Albrecht
Rayong Thailand

Answer

Cornstarch, or corn flour, is basically made up of lots of tiny, tiny particles. They really like water so they wet very, very well. So you get lots of tiny particles surrounded by water and that water sort of lubricates them and means they can move past one another if you apply slow forces and it flows like a liquid. If you hit it quickly, you squash the whole material very fast and particles don't have time to move out of the way, so they all kind of jam into each other and lock together, and so, you get a solid line of particles which locks it all up and it behaves like a solid. And so, it's a solid when you hit it fast. When it flows slowly, it's behaving like a liquid. Of course when you cook it, it's entirely different. It's no longer particles it's polymers. They just turn into gloopy liquid which is just kind of sticky and gloopy.

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