Strange fluids

Chemical and Process Engineering: "Some Strange Fluids

Not all fluids are the same. Some are thick and some are thin. Some get thicker when you stir them, some get thinner. The thickness of a fluid is referred to as its viscosity which has units of centipoise (cP). The viscosity tells us how hard we have to push the fluid to get it to move. Here's some typical viscosities:

* Water 1 cP
* Air 0.001 cP
* Golden Syrup 25000 cP
* Engine oil 200 cP

You can have fun with corn flour or custard powder (some of the cheaper brands don't work so well).

Put some dry corn flour or custard powder in a bowl. Add just enough water so that you can pour it out (into another bowl). You've probably alread found that it gets harder (more viscous) as you mix it. The hard you try the harder it becomes. If you want to create a mess try rolling some into a ball between your fingers and then between your hands. As soon as you stop rolling it, or when you pass it to someone else it will start to flow. Yuk. We refer to this fluid as a shear thickening, or dilatant, fluid.

The opposite behaviour is so common you don't even notice it. Think of thick yoghurt, for example. When you are not stirring it is hard to pour but when you stir it, it seems relatively thin. A lot of foods, or fluids that contain long molecules are like this. we call them shear thickening, or pseudoplastic, fluids.

What about toothpaste or mashed potato or putty? If you don't push them they don't flow. They are solid until you push hard enough and then they become liquid. Yes they are solid and liquid. We call them Bingham Plastic fluids.

Golden syrup is quite boring by comparison. It seems thick and it is thick. When you stir it, it stays just as thick. Its a normal, Newtonian, fluid."