What is cavitation?

What do beer bottles, the mantis shrimp and propellers have in common?

They are both good way to see a phenomenon called cavitation in action.

Cavitation happens when the pressure in a liquid suddenly drops. The drop in pressure is caused by pushing a liquid quicker than it can react, leaving behind an area of low pressure often as a bubble of gas.

Full video from Slow Mo Guys

In the above example the glass bottle is hit from the top, the glass moves down faster than the water, causing air bubbles and low pressure to form at the bottom. Once the bubbles collapse a shock wave breaks the bottle of the glass.

In general, objects like propellers are designed to avoid cavitation and the damage it can cause. However there is one animal that exploits the effect. The Mantis shrimp uses a punch strong enough to cause cavitation to stun prey. It can also be used to break down kidney stones, sterilise water, or break up dirt.

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