Monday, December 05, 2016

Ask a Physicist: Introduction to Cavitation

Talitha, from Australia, writes:
My boyfriend insists that if something moving fast underwater, the water wouldn't be able to move behind the object at the same speed and would create an air bubble. This doesn't seem right to me—please help!

Talitha,
So here's the deal: your boyfriend is almost right, but it's not quite an air bubble—the process he's describing is called cavitation, a name which comes from the word cavity.

Have you ever seen cooking directions that need to be adjusted for different altitudes? The temperature at which water boils depends on the pressure around it. You can imagine all the individual water molecules in a mug of tea bumping into one another, repelling each other thanks to their negatively-charged electron shells—they want to get as far away from one another as possible. Remember, though, that here at Earth's surface, there's an enormous amount of weight sitting on top of the water molecules in your mug of tea—we're practically at the bottom of an ocean of air, and all that air pressure keeps the water molecules squished together in the cup. The higher up you go, though, the less air pressure there is—and as a result, it takes less energy for molecules to get free from the water's bulk, meaning it can boil at a lower temperature. In this video, a guy actually gets water to boil at room temperature, simply by hooking it up to a vacuum pump and pulling all the air out!

Now, let's say a submarine is racing through the water at an extreme velocity. As it goes along, it's knocking all the molecules in front of it out of the way. Once it passes by, those molecules need to rush back in to fill the space where the submarine was, and this process does take a certain amount of time. As a result, objects moving rapidly through any fluid (air, water, etc.) create a low-pressure zone in their wake. In some cases, the pressure in this zone gets low enough that water can "boil" off and form a cavity—a space that's mostly vacuum, but with a few molecules of water vapor in it. This is cavitation. That cavity usually slams shut very quickly—and when it does, watch out! Cavitation temperatures can reach upwards of 8000°C at the point of collapse, creating a tiny but intense shockwave. You might think this is the kind of thing that only happens in fighter jets or other speed record-setters, but in fact it's a major problem in the shipping industry: when a ship's propellers spin underwater, they often move fast enough that they create tiny cavities around them, a process which eventually wears them down until they need replacing.

A propeller creating cavities in its wake.
Image Credit: U.S. Navy, Public Domain.

While it takes some serious speed to cause cavitation, manmade objects aren't the only thing powerful enough to do so. There's an animal called the mantis shrimp that actually utilizes cavitation for self-defense and to hunt its prey. The mantis shrimp has an extraordinarily powerful claw, one that works a little like a catapult—the shrimp cranks it all the way open, creating extraordinary tension. Then, when it's ready, it releases that tension and the claw comes screaming shut through the water at an unbelievable pace—fast enough that the water around it doesn't have time to react. The sudden motion of its claw creates a cavity, and when the cavity collapses it creates a shockwave in the water that's powerful enough to stun nearby fish or even crack open a clam's shell after a few tries.

The mantis shrimp's powerful claw can clearly be seen creating cavitation bubbles at 1:58

But it's not just the mighty mantis shrimp that can create cavitation bubbles—you can too! This video by YouTube celebrities The Slow Mo Guys shows how to take advantage of cavitation for an amazing party trick:


Get your gloves and goggles (perhaps some adult supervision as well), and give it a try. Now that you know the physics of it, you can blow your friends' minds twice—once with the trick, and once with the explanation! (If you're using a glass soda or beer bottle, it's a good idea to empty it out first and refill it with water. Can you guess why?)
Stephen Skolnick

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