Science Puzzle
Solid, Liquid or Both?
Stir cornflour into water until it is thick like custard. Punch the surface hard and your fist bounces off something that feels like concrete. Lower your finger gently and it sinks straight through as if into cream.
Is it a solid or a liquid, and how can it be both?
The Answer
It is a liquid, but a strange one whose thickness depends on how roughly you treat it. Ordinary liquids like water or honey have a fixed thickness: honey is always thicker than water, whether you stir it fast or slow. This mixture breaks that rule.
Suspended in the water are millions of tiny cornflour grains, each with a thin slippery film of water between it and its neighbours. Press slowly and the grains have time to shuffle aside, the water films stay intact, and everything flows. Hit it hard and the grains are slammed together faster than the water can squeeze out from between them. For an instant they jam into a rigid crowd, and the mixture goes stiff. Ease off and the water seeps back, the jam releases, and it flows again.
Fill a paddling pool with the stuff and you can run across the surface, so long as you never stop moving. Stand still and you sink.
The principle: Non-Newtonian fluids. In a shear-thickening fluid, sudden force jams the suspended particles together and the material stiffens, while gentle force lets it flow.