The pysics behind snapping shrimp
Scientists have always believed that the the sound of snapping shrimp is generated when the two claw halves hit each other on claw closure.
We have shown that the sound production mechanism is very different and that fluid dynamics is involved.
The shrimp can close its claw at amazing speeds, up to 30 000 rpm. The tip of the moving part of its claw moves at a speed of 20 m/s. The water that is located between the claw halves is squeezed out and forms a water jet with a speed of 30 m/s, or 100 km/h. The speed of the water jet is so high that we get a drop of the pressure to below the vapor pressure of water, resulting in cavitation.
Cavitation occurs when the water velocity is very high, in which case the pressure drops considerably because of the Bernoulli principle. The pressure drops even below the vapor pressure of water and the water will vaporize (or boil so to say) at ambient temperatures. Because of the low pressure the vapour bubble that is formed will grow to about 1 cm. But when the pressure rises again this bubble will experience a very strong collapse.
To record the sound of the snapping shrimp we used a hydrophone, an underwater microphone. The data were analyzed using a spectrum analyzer and stored on a computer.
We have used a high-speed digital video camera that records 40000 frames per second, i.e. every 25 microseconds the camera takes a snapshot. In this way we could visualize the claw closure, the bubble growth and the bubble collapse. The snap of the shrimp triggered the camera and because the camera also records pre-trigger frames, we could see in super slow-motion what happened in the short instance just after claw closure. The images were downloaded to a computer for image processing.
The sound recording and the video images were correlated in time. It showed that there was no sound during claw closure and that a very strong signal was observed precisely during bubble collapse. The claw is closed long (long on the time scale of the high-speed camera, 600 microseconds to be precise) before the bubble collapses, where we see the very loud bang of 200 dB.
In ship propulsion cavitation erosion is a major problem. When the water flows around a ship propellor at great speeds, an underpressure is created and the cavitation bubbles generated this way damage the ship propellor. So, even these big metal blades are damaged by collapsing bubbles generated around the propellor. As a matter of fact the shrimp uses its cavitation bubble to damage, stun or even to kill its prey. To the right you see a poor crab being shot at.
