Liquid or solid?

Most things are easy to classify as either liquid or solid. But what about things that appear to be both?

Rheologists (scientists who study flowing liquids) call things like cornflour slurry a dilatant fluid. A dilatant fluid behaves like a liquid when moved slowly, but like a solid when hit hard. Why? Dilatant fluids often contain particles that, when moved slowly, will change shape and slide past each other. When they are moved quickly they lock up - a little bit like people trying to push quickly through an exit door and blocking it up.

Some fluids behave in the opposite manner - a thixotropic fluid tends to look like a solid until you stir it, and then it behaves like a runny liquid. Test this with a bowl of yoghurt or tomato sauce.

Let's experiment

1. Take the cornflour or cornstarch and slowly mix in the water (using fingers or a spoon) until it has the consistency of tomato sauce when you pour it.
2. Now that you have made your slurry, investigate it's ‘magical' properties! Try tapping it with your finger at different speeds - tap it slowly and you should be able to poke your finger right in. Tap it fast and it should be solid. Try picking it up and letting it run through  your fingers.

Disposal: Scrape the slurry into the rubbish bin.