CHaOS Science Roadshow

Cornflour

To make the stuff slowly add water to cornflour until it works - ask a committee member for help if you're getting stuck.

Basically get them to have a play with the stuff, it's great to play with.

Try and ask them whether it is a solid or a liquid (you may want to get them to come up with the terms solid and liquid first - eg what do we call hard things...)

It is a solid and a liquid - solid when you move (more correctly, shear) it fast and liquid when you don't.

Explanation

This is a picture of cornflour under a confocal microscope (here)
(Bromley & Hopkinson) which takes a 2D slice through an image rather than looking at the surface. Cornflour is lots of irregular shaped particles that are seperated by water normally so are lubricated and can move. If you squash them together it will push the water sideways a little bit and let them touch - now they lock together and behave as a solid.

It's like a room full of people and when you try and make it move quickly, everyone tries to move at once and they all get in each other's way and so no one can move anywhere. And when you do stuff to it slowly, everyone has time to move out of the way and file out.

Thickening soups is different as then the cornflour grains open up when heated and release long starch molecules that tangle together forming a gel like substance.

Industrial uses

It is a problem in the oil industry, as when they are drilling they are getting rock fragments in the mud coming back up, if there are too many they behave similarly, with catastrophic results to pumps.

Some people are talking about making liquid body armour using this effect, to make the body armour more comfortable.

Toothpaste is an example of something that does the opposite to cornflour - when you shear it by squeezing it out of the tube, it flows, but when there's no shear, it sits quite happily on the toothbrush without flowing anywhere.

Science background

Corn flour is shear thickening. This means the higher rate of shear, the higher the viscosity (i.e. the thicker it is).

The opposite of shear thickening is shear thinning. Many substances are shear thinning because the higher rate of shear can break up interparticular interactions and reduce the viscosity - e.g. shampoo.

Thixotropy - a health warning

Thixotropic / thixotropy is concerned with time related effects. The longer you shear a thixotropic fluid the lower the viscosity (the thinner it becomes) - e.g. paint - as you progressively break up interparticular interactions. Many fluids that are shear thinning are also thixotropic. However, this experiment has nothing to do with thixotropy. Thixotropy is a long word and shouldn't be used with children. Adults should be politely and gently explained the difference!

Rheoplexy / Anti-thixotropy is the opposite - i.e. the longer you shear a fluid the higher the viscosity (the thicker it becomes). I think that xanthan gum might do this under certain conditions, but it's very rare for substances to do this.