Breaking a Beer Bottle: Cavitation

By Tracy V. Wilson, HowStuffWorks.com

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School yourself on the basics of beer at How Beer Works.

Learn more about the physics of propellers and their cousins, impellers, in How Airplanes Work and How Personal Watercraft Work.

It's a trick you might have seen on the Web or in a bar. Somebody fills a beer bottle with cold water, gives the top a sharp tap with an open palm, and watches as the bottom of the bottle -- and all the water -- fall spectacularly to the ground. You might guess that the moving water creates a vacuum, causing the bottom of the bottle to simply implode, but the real culprit is cavitation.

Cavitation is something you'd ordinarily hear about in conjunction with propellers. As a propeller spins in water, it creates areas of low pressure. If the pressure drops below water's vapor pressure, small pockets of vapor -- also known as bubbles -- form. It's like what happens when you bring water to a boil on the stove, but you don't have to raise the temperature to create the bubbles.

When you start boiling water for your spaghetti, though, the bubbles rise to the surface and escape. When they're being tossed around by a propeller, they have nowhere to go, and since they're surrounded by higher pressure liquid, they collapse. These tiny implosions can severely damage the propeller and surrounding surfaces, like pipes or walls.

This causes problems for engineers, who have to figure out how to get pumps and propellers to do their thing without collateral damage from cavitation. But it's a boon for armchair strongmen at the local pub. Here's a play-by-play of what happens:

The bottle breaker starts with a glass bottle that's filled with water to within an inch or two of its top.
Holding the neck of the bottle, he strikes its mouth hard with an open palm, leaving no room for air or water to escape from the top.
The bottle moves downward, but the water stays in roughly the same place due to inertia, or an object's tendency to resist changes in its motion.
As the bottom of the bottle moves away from the water, a low-pressure area develops, and tiny bubbles of water vapor start to form and collapse.
The collapse of a bubble creates a shock wave and noise. When lots of bubbles collapse at the same time, the shock and sound vibrations break the bottom of the bottle.