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#FridayPhysicsFun: cavitation! Because of reasons, I have recently been looking for the physical limits of pumping. One of the trickier problems is cavitation. https://en.wikipedia.org/wiki/Cavitation
Cavitation happens when the local pressure in a fluid becomes smaller than the vapour pressure: it is favourable for the fluid to be vapour instead, so a bubble starts to grow. But if the pressure increases again the bubble implodes - with great force. https://fuckyeahfluiddynamics.tumblr.com/post/3880804506/gas-bubbles-can-form-in-a-flowing-liquid-in-areas
One way of thinking about it is that water is nearly incompressible. But it has only a finite tensile strength and can be pulled apart leaving an empty region that quickly fills with vapour.
As the bubbles implode they produce shock waves and high temperatures - several thousand degrees C. The total energy is often small, but it is concentrated and often next to a surface. Cavitation can erode metal and rock. https://commons.wikimedia.org/wiki/Category:Cavitation#/media/File:Cavitation.jpg
The implosion is hot enough to generate light, sonoluminence. https://en.wikipedia.org/wiki/Sonoluminescence
The term comes from Latin cavus, something hollow or concave. Named so for the pits it makes on surfaces; one can also imagine the bubble as a little cavelike cavity. In rapids and waterfalls cavitation can literally start making caves, helping other erosion.
In pumps, pipe bends & engines cavitation happens when local forces "tear apart" the fluid, making temporary bubbles. One can also make them by strong ultrasonic sound fields. Useful when you really want to erode or emulsify things, like dental plaque. https://commons.wikimedia.org/wiki/File:High-Speed-Imaging-of-Cavitation-around-Dental-Ultrasonic-Scaler-Tips-pone.0149804.s004.ogv
One cool demonstration is breaking bottles by causing cavitation at the bottom. See also XKCD What If: https://what-if.xkcd.com/6/
Cavitation is *of course* exploited by mantis and pistol shrimp, who hit targets with about 50kg of cavitation force *and* 40-150 kg of direct force about 1/2 ms apart. https://jeb.biologists.org/content/208/19/3655 Unsurprisingly, they have weird composite shells: https://arxiv.org/ftp/arxiv/papers/1604/1604.07798.pdf
Some ferns also use cavitation to disperse spores. The spore housing uses cavitation to close in 10 microseconds. https://hal.archives-ouvertes.fr/hal-00826001/file/1215985_maintext-resumitted2-for-HAL.pdf
Cavitation is also involved in cracking joints:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4398549/ Strangely, it doesn't seem to cause lasting damage in habitual finger-crackers.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4398549/ Strangely, it doesn't seem to cause lasting damage in habitual finger-crackers.
And of course, underwater explosions produce cavitation from the shock waves, that in turn generate other shock waves. http://www.am.chalmers.se/~thab/IMAC/2010/PDFs/Papers/s38p003.pdf The explosion bubble itself often implodes a few times.
There is a fair bit of research on using more stable cavitation bubbles to reduce drag on torpedoes or submarines. Another kind of stable-ish cavitation is "the singing vortex" that makes a loud, pure tone: https://royalsocietypublishing.org/doi/10.1098/rsfs.2015.0025
