Quicksand

Procedure

• Put half a packet of corn flour in the container.
• Gradually add a little water while mixing until we get a not too thick mixture and let it rest for a while.
• Rest the spoon and observe how it sinks. Then, we tap the surface and see how it bounces.
• Do the same with our hand. We take a little and see how it slips between our hands. Then we try to punch it.

Theoretical explanation

As observed, the mixture made with water or cornstarch is a fluid with special properties. It is what we call a non-Newtonian fluid.

These fluids do not have a definite and constant viscosity value, hence their surprising response to external stimuli. When stirred slowly, they behave like a semi-liquid fluid, but when vigorously shaken, it becomes as hard as a rock.

There are plenty of substances besides cornstarch that exhibit these properties. Ketchup or toothpaste are examples of fluids where viscosity decreases under pressure, which is why when we vigorously shake the bottle, the content comes out more easily.

Did you know…? These non-Newtonian liquids are used in the production of bulletproof vests. When a bullet impacts at high speed, they become solid and act as shields, but under normal conditions, they are flexible for the comfort of the police officers.