Solid or liquid

You don’t always need a big laboratory for exciting experiments... Using cornflour and a little bit of water you can discover where the boundary lies between a solid and a liquid.

When is something solid and when is it liquid? It might sound like a bit of a silly question at first, but the answer is more complicated than you think. If you’re no stickler for detail, you could summarize the distinction between the various physical states as follows (see also Episode 1: ionized gas in the microwave ): if you can walk on it it’s a solid, if you can swim in it it’s a liquid and if you can breath it it’s a gas.

In practice things are rather more complicated - they always are. There are substances that are literally more difficult to pigeonhole. So-called non-Newtonian fluids, mixtures of liquid and solid matter, are liquid under certain conditions while in others they appear firm and solid. You can easily create a non-Newtonian fluid yourself with only cornflour or potato starch and water.

Carefully mix three parts cornflour with two to three parts water. If the mixture is right, you will notice that it becomes increasingly difficult to stir. If you apply pressure, a filler-like substance appears to form. If you leave the mixture alone it reverts to a smooth watery paste. You can repeat the process indefinitely; as soon as you apply pressure to the mixture it appears to become a solid substance, left to rest it eventually becomes liquid again.

Make a big bowl of this mixture and spend an afternoon messing about! Take a big handful of the mixture and knead it into a ball. Or fill a saucer and place it on a good-sized speaker box. Put on a CD (preferably something noisy like Rage Against the Machine) and watch what the non-Newtonian fluid does. You should see tendrils and lumps rising out of the mixture which – if you use your imagination – resemble cartoon figures. If nothing happens, try blowing into the saucer with a straw to get the mixture moving.

The starch mixture is a simple example of a material with characteristics that can adapt to the surrounding environment. Such ‘smart’ materials, also known as shear thickening fluids (STF), can be used in armour plating and bullet and shrapnel-proof vests. The British army launched a trial with such liquid armour last autumn. Happily, less violent applications are conceivable too. You could, for example, use STF in ski suits and football socks, or produce feather-light bicycle helmets.

You can scale up this experiment fairly easily. The only real limitation is your imagination; you can buy potato and corn starch in bulk relatively cheaply and any waterproof container is suitable. If it’s mixed in the right proportions, the material can support the weight of an adult. A plastic tub or paddling pool filled with non-Newtonian fluid could be a good way to liven up a party.

Or you could go really big... There’s a film on YouTube of some kind of motivational happening where people run across a big tub filled with liquid. Tip: make sure the tub is not too deep. It’s tough going swimming in a liquid that hardens as soon as you exert pressure on it. If you fill a swimming pool with this starchy goo and dive in there’s a good chance you will not be able to rise up from the bottom.

Don’t forget to explain the theory of the non-Newtonian fluid to your guests. A spectacle is more exciting if you understand what’s behind it; that’s where science comes in. Wow… Aha.

Author: Ernst Arbouw