Cavitation in a nutshell

The world is full of amazing and wonderful bubbles of all different kinds. The most familiar bubbles are created when a gas, such as air, is surrounded by water or another liquid. These bubbles make up soapsuds, sea foam, and the bubbles that children blow for fun. In contrast with these common bubbles where a gas is trapped in a liquid, there is another type of bubble with a more epic birth story. Bubbles can actually be created by pulling on water hard enough to rip a hole in it, causing a bubble to violently explode into existence. These bubbles are cavitation bubbles. Our hero, Brooke Bubble, is a cavitation bubble.

But what exactly does it mean to "pull on water hard enough to rip a hole in it”? When water moves very quickly or is exposed to very focused sound, the pressure in the water can drop dramatically. As a result, water molecules (H2O), normally pushed tightly together by this pressure, are able to spread out and move further away from each other. If the pressure drops below a certain point, the water molecules can spread out so much that the liquid water starts to evaporate into a gas. A bubble of water vapor is born. This is similar to what happens when water boils, but without the heat. The newly formed vapor bubble grows rapidly as more water evaporates. When the bubble grows, it pushes the water around it out of the way. But the water pushes back, hard. Eventually the bubble stops growing and collapses violently, concentrating all of the energy of the bubble into a much tinier space. All of this occurs in less than a blink of an eye. It can create pressures in the bubble as great as those found at the bottom of deepest parts of the ocean (Fujikawa and Akamatsu, 1980) and temperatures hotter than the surface of the sun (Flint and Suslick, 1991)​! When the bubble is all smashed down into this tiny, hot, compressed state, it is incredibly unstable and can't stay like this very long. After a few millionths of a second, it begins to grow explosively. This cycle of growth and collapse continues until the bubble’s energy dissipates into the surrounding water.
​​​During the collapse and explosion, some very destructive and interesting things can happen. When the bubble collapses, extremely high speed jets of water, sometimes reaching several thousands of miles per hour (Bourne and Field, 1992), can shoot into one side of the bubble and out the other. This water jet is capable of damaging nearby structures. The collapse can also produce shock waves (powerful bursts of energy) that can damage the bubble's surroundings.  These shock waves can also create even more bubbles which will undergo the same violent collapse and explosive growth. These clouds of cavitation bubbles can tear apart steel, cement, human tissue, and so much more! 

Cavitation bubbles occur in nature

  • The smasher type of mantis shrimp uses a special claw designed to move so fast (45 mph in water!) that it creates a cavitation bubble which, when it collapses, launches a powerful shock wave to crack shells or stun prey (Patek et al., 2004). If you want to learn more about this amazing animal, check out these videos, this comic and this TED talk:
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A mantis shrimp attacks a crab (more at https://www.youtube.com/watch?v=U11DgbefmQQ).
  • ​The speed of dolphins and several large fish like tuna is limited by cavitation bubbles near their tails. The pain caused by the collapsing bubbles force dolphins to swim slower than they normally would. For tuna, the cavitation cloud around their fins actually slows them down (Iosilevskii,and Weihs, 2008).
  • Thresher sharks stun fish through rapid slapping of their tails.  This violent motion also creates cavitation bubbles in the water (Oliver et al., 2013).  Check out the thresher shark in action below:
  • Cavitation damage has been proposed as one among many coastal erosion mechanisms. Bubbles entrained by the waves are forced into cracks in the rock. The pressures can be high enough to cause the bubbles to implode and damage the surrounding rock. (Young and Bryant, 1992; Sunamura, 1997)

Cavitation bubbles can cause problems in engineering


  • ​​Cavitation bubbles are created in the high speed water flow around ship and submarine propellers. On collapse, these bubbles damage the propellers. Cavitation damage therefore limits the speed and lifespan of marine propellers. Additionally, noise generated by the collapsing bubbles in the wake of a submarine compromises a submarine's stealth capabilities. (Kuiper, 1981; Carlton, 2011)
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Cavitating propeller in a water tunnel experiment at the David Taylor Model Basin.
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Cavitation damage on a properller. Picture by Axda0002 at Wikipedia.
  • Pump propellers are also susceptible to cavitation damage. Cavitation in the fuel pumps of early rockets are thought to be at the origin of the Pogo oscillations, vibrations in the rocket structures that led to catastrophic failures. (Ng and Brennen, 1978)
  • The spillways that let water through dams often have very high speed flows that frequently cause cavitation bubbles. Despite the fact that these spillways are made from tons and tons of steel reinforced concrete, bubbles have been known to carve away semi-truck sized holes in them over a matter of weeks. Modern dams are built to protect against this by pumping air into the flowing water, which helps to absorb some of the energy from collapsing cavitation bubbles. (Fortner, 2003)
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The Glen Canyon dam was nearly destroyed by cavitation bubbles in 1983.
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Over sixty thousand gallons of steel reinforced concrete were washed away.

Cool uses of cavitation bubbles

  • Supercavitation, where a cavitation bubble is large enough to surround an object, has been studied to reduce drag in fast moving water vehicles. Using supercavitation, torpedoes or submarines could move fast enough to create a bubble surrounding them, thereby reducing their drag significantly. (T. T. Truscott, et al., 2013)
  • Ultrasonic cleaning uses high frequency vibrations to generate cavitation bubbles near the surface that is to be cleaned. The ultrasound is carefully tuned to create bubbles that collapse in such a way that they break down dirt and grime on the surface without causing damage. Through this process, cavitation is used to clean teeth, surgical equipment, jewelry, lenses, tools, musical instruments, industrial parts, electronic equipment, and many other things.
  • ​Ultrasonically generated cavitation also has many surgical and medical uses. It is used to break up kidney and gall stones, blood clots, and cataracts, among other things. It has even been used to break apart cancerous tumors in animals and is being studied for this purpose in humans. (Roberts et al., 2006; more info)

References​

  1. Benjamin, T. B., & Ellis, A. T. (1966). The Collapse of Cavitation Bubbles and the Pressures thereby Produced against Solid Boundaries. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 260(1110), 221–240. http://doi.org/10.1098/rsta.1966.0046
  2. Bourne, N. K., & Field, J. E. (1992). Shock-induced collapse of single cavities in liquids. Journal of Fluid Mechanics, 244(-1), 225. http://doi.org/10.1017/S0022112092003045
  3. Bourne, N. K., & Field, J. E. (1999). Shock-induced collapse and luminescence by cavities. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 357(1751), 295–311. http://doi.org/10.1098/rsta.1999.0328
  4. Brennen, C. E. (1995). Cavitation and bubble dynamics. Oxford University Press , New York. http://resolver.caltech.edu/CaltechBOOK:1995.001
  5. Brennen, C. E. (2015). Cavitation in medicine. Interface Focus, 5(5), 20150022. http://doi.org/10.1098/rsfs.2015.0022
  6. Carlton, J. (2011). Marine propellers and propulsion. Butterworth-Heinemann.
  7. Flint, E. B., & Suslik, K. S. (1991). The Temperature of Cavitation. Science, 253(5026), 1397–1399. http://doi.org/10.1126/science.253.5026.1397
  8. Fortner, B. (2003). Water Vapor Almost Busts Dam. Retrieved April 23, 2016, from http://www.popsci.com/scitech/article/2003-03/water-vapor-almost-busts-dam
  9. Fujikawa, S., & Akamatsu, T. (1980). Effects of the non-equilibrium condensation of vapour on the pressure wave produced by the collapse of a bubble in a liquid. Journal of Fluid Mechanics, 97(03), 481. http://doi.org/10.1017/S0022112080002662
  10. Gaitan, D. F., Crum, L. A., Church, C. C., Roy, R. A., Felipe Gaitan, D., Lawrence Crum, B. A., … Roy, R. A. (1992). Sonoluminescence and bubble dynamics for a single, stable, cavitation bubble. The Journal of the Acoustical Society of America, 91(6), 3166. http://doi.org/10.1121/1.402855
  11. Iosilevskii, G., & Weihs, D. (2008). Speed limits on swimming of fishes and cetaceans. Journal of The Royal Society Interface, 5(20), 329–338. http://doi.org/10.1098/rsif.2007.1073
  12. Kuiper, G. (1981). Cavitation Inception on Ship Propeller Models. Delft University of Technology. Retrieved from http://www.marin.nl/web/Publications/Publication-items/Cavitation-inception-on-ship-ship-propeller-models.htm
  13. Ng, S. L., & Brennen, C. (1978). Experiments on the Dynamic Behavior of Cavitating Pumps. Journal of Fluids Engineering, 100(2), 166. http://doi.org/10.1115/1.3448625
  14. Oliver, S. P., Turner, J. R., Gann, K., Silvosa, M., D’Urban Jackson, T., Clua, E., … Motta, P. (2013). Thresher Sharks Use Tail-Slaps as a Hunting Strategy. PLoS ONE, 8(7), e67380. http://doi.org/10.1371/journal.pone.0067380
  15. Patek, S. N., Korff, W. L., & Caldwell, R. L. (2004). Biomechanics: Deadly strike mechanism of a mantis shrimp. Nature, 428(6985), 819–820. http://doi.org/10.1038/428819a
  16. Roberts, W. W. , Hall, T. J., Ives,  K., Wolf Jr., J. S., Fowlkes, J. B. , & Cain, C. A. (2006). Pulsed cavitational ultrasound : a noninvasive technology for controlled tissue ablation (histotripsy) in the rabbit kidney. Journal of Urology, 175(2), 734–738. http://dx.doi.org/10.1016/S0022-5347(05)00141-2
  17. Sunamura, T. (1977). A Relationship between Wave-Induced Cliff Erosion and Erosive Force of Waves. The Journal of Geology, 85(5), 613–618. http://doi.org/10.1086/628340
  18. Truscott, T. T., Epps, B. P., & Belden, J. (2014). Water Entry of Projectiles. Annual Review of Fluid Mechanics, 46(1), 355–378. http://doi.org/10.1146/annurev-fluid-011212-140753
  19. Young, R. W., & Bryant, E. A. (1992). Catastrophic wave erosion on the southeastern coast of Australia: Impact of the Lanai tsunamis ca. 105 ka? Geology, 20(3), 199. http://doi.org/10.1130/0091-7613(1992)020<0199:CWEOTS>2.3.CO;2