Killer bubbles

The terrible power of imploding water

If the loudest animal on Earth is the sperm whale, a giant squid killing deep sea monster, what is the second loudest?

A sperm whale can make a sound of about 230 decibels, which is louder than a jet engine. It needs to do this because it hunts giant squid in the blind depths of the deep ocean, and uses powerful sonar pulses to both locate, and stun, the Kraken while hunting.

But consider for a moment: What do you think the second loudest creature on the planet is?

You would be forgiven for thinking it was another whale, given that they are gigantic beasts that must traverse the inky waters, bellow songs over many miles and hunt with bursts of cacophonous sound… but you’d be wrong.

The second loudest creature in the world is a shrimp less than two inches in length. It’s called the pistol shrimp, and it’s an assassin.

The pistol shrimp can produce an acoustic pressure pulse of 189 dB (some studies have estimated over 200), which is louder than a large calibre rifle. It’s so loud, in fact, than in World War 2 the US Navy was reported to have used pistol shrimp colonies to hide submarines, as their background gunshots interfered with sonar detection. How does a tiny shrimp do such a thing?

It uses cavitation: The terrible power of imploding water.

1: Steam by other means.

Water has a boiling point, at normal sea level pressure, of 100 celsius, which is a temperature seldom reached in nature, and good thing too. What is not always appreciated to non-engineers is that water’s boiling point changes with pressure. As you may have noted if you’ve been following these articles, that can be useful: Raise the static pressure of water and you raise its boiling point, which is useful in power generation such as nuclear pressurized water reactors, where heating the water to over 300C would vaporise the liquid, were it not pressurised to an almighty 150 atmospheres. Likewise, it can be useful as well to inspire boiling: A multi-stage flash distillation unit (also covered in a recent article of mine) deliberately lowers the pressure in successive tanks of heated brine, to encourage the water to flash into clean steam.

But while the manipulation of boiling points through pressure is a useful trick, it can also be a dangerous one, that can wreck propellors, break valves, strip paint and, of course, let the pistol shrimp shoot things.

It’s called the Bernoulli principle: In an idealised inviscid fluid, kinetic energy, static pressure and potential energy are conserved, meaning that if you massively increase dynamic pressure (the energy required to bring a unit of fluid to a dead halt) while keeping potential energy the same, this is balanced by a reduction in static pressure. 

In short: If you use a venturi or the upper surface of a hydrofoil to locally accelerate water, you also locally reduce the static pressure of it. And if you do this too much…

Cavitation.

Spin a propellor too fast, or concentrate flow through a valve or a venturi too much, and local acceleration drops the pressure far enough for spontaneous vaporization to occur at ambient temperature.

Great. Lovely. So what has this got to do with propellors and pistol shrimp?

The pistol shrimp, tiny assassin, is easily identified by a single oversized claw. This claw can retract and ‘pinch’ at tremendous speed, squeezing water out of a special cavity, which accelerates through a narrow orifice at about 100 kph in an instant. The local pressure drop induced by this then causes cavitation, and the formation of a steam bubble. But this bubble doesn’t last long.

As soon as it has passed the low pressure zone, physics returns with a vengeance and the bubble implodes. With spectacular force.

The acoustic shockwave generated by the collapsing bubble will kill a pistol shrimp’s prey and deafen sonar systems, and it gets much worse than that…

The collapsing bubble causes a massive pressure spike within the steam bubble, just for an instant, that raises the temperature at its core to thousands of degrees. This spectacular pulse of heat and pressure, for the tiniest fraction of a second, causes trace gases to ionize, and in a process called sonoluminescence, produces a faint flash of light in the pico to nano-second range.

This tiny two inch shrimp can box its prey so hard that it actually creates light.

2: Pulverising Propellors.

You might expect such a violent process to be a design constraint on things underwater. You’d be right.