Need to Build a Bubble Machine? Get Help From a Pistol Shrimp!

Featured Image Caption: Pistol shrimp and goby fish make great roommates. The pistol shrimp has poor eye sight, but can make burrows while the goby fish keeps an eye out for predators. Randall’s Pistol Shrimp (Alpheus randalli) (c) Dan Schofield some rights reserved CC by SA 4.0 Deed

Primary Source Article: Soyama H, Tanaka M, Takiguchi T, Yamamoto M. Development of a Cavitation Generator Mimicking Pistol Shrimp. Biomimetics. 2024; 9(1):47. https://doi.org/10.3390/biomimetics9010047

Secondary Source Articles:
https://www.shotpeener.com/library/pdf/2014024.pdf
https://web.tohoku.ac.jp/ism/menu4/cavitation_peening.pdf

Special thanks to Dr. Hitoshi Sayama from Tohoku University, Japan for permission to use cavitation peening images and providing links to help understand cavitation peening.

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Bubbles are fun to play with and capture the imagination.  You can set up a relaxing bubble bath, blow bubbles outside for playtime, or watch cartoons with characters who have bubble beam powers.  (Shout out to you Pokémon and Jo Jo Bizarre Adventure Fans!)   -However, bubbles are not all playtime.  Under the right conditions, bubbles are downright destructive, especially underwater.

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When bubbles pop under the pressure of water, they send out pressure waves that can affect nearby objects or living things.  The fancier word for this phenomenon is called cavitation bubbles.  Cavitation bubbles can damage hydraulic machinery such as boat propellers and pumps that handle a flowing water supply.  Over time, the material that is exposed to these bubbles erode and no longer works.

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The Positives of Cavitation Bubbles

Still, cavitation bubbles are not all bad news.  They are useful in unexpected ways.  Cavitation can be used to slow down the cracking and erosion process in materials or break up materials we don’t want. Cavitation is a process often taken advantage of in medical and industrial applications.  Got a kidney stone? A machine can deliver ultrasonic shock waves that cause cavitation bubbles to break up your stone without surgery.  The same principle applies to wastewater processing plants that break up clumps.  (Don’t ask what those clumps are.) Better still, if you need to make power plants last longer, you can strengthen the walls by giving it the bubble treatment.  Shooting bubbles at a material to increase its durability is called cavitation peening.

Shot Peening vs Cavitation Peening

There are two main methods to strengthen materials: shot peening and cavitation peening.  In the first scenario, imagine a person shooting a pellet gun on a surface area.  Doing this keeps the material from breaking down but makes the surface bumpy.  In contrast, cavitation peening is about creating a force strong enough to make an indent without using solid objects. Instead, the force creates bubbles, which then make an impact on the surface and produces an indent when it collapses. Between the two peening methods, cavitation peening creates better results. 

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Purpose of the Experiment

This experiment is an exciting example of biomimicry, the field of learning from nature’s adaptations to create designs . Dr. Soyama and colleagues wanted to make a compact and efficient bubble making machine.  The Japanese scientists envisioned developing a cavitation generator that doesn’t need a laser and instead uses a piezo actuator. A piezo actuator is a small cylinder shaped motor that can make linear movement when an electric field is applied.  Piezo actuators are typically used in devices such as electrical switches, micro pumps, inkjet printers, and anti vibration devices.  With the help of the pistol shrimp and a laser cavitation generator, Soyama and colleagues have developed a prototype generator that can be used for cavitation peening.

The Challenges of Cavitation Peening

Cavitation peening is a tricky technique.  The force involved in creating a compressive stress (indents) on a surface can come from the speed of air, water, lasers, or sound, e.g., water jet, water cavitation jet, laser cavitation, and vibratory horn technique, respectively.  All techniques have flaws.  For the jet technique, there isn’t enough force to make the desired intensity of cavitation.  Lasers can do the job, but it can take a long time to treat a surface and can be expensive. Scientists have proposed using sound to create cavitation bubbles but getting the right intensity is tricky. So, how do scientists work around this problem? Enter the pistol shrimp!

Pistol Shrimp, not Your Average Cocktail

The pistol shrimp, Alpheus randalli, is a red and white crustacean with a small and large claw.   At 1.2 inches long, the shrimp packs a mean punch.  When prey gets near, the shrimp delivers a wave of bubbles that produce an immense amount of pressure and sound that is out of this world.  Traveling at 60-70 mph, when a bubble pops on its target, it can deliver sound up to 210 dB (decibels). For reference, that is louder than a gunshot.   Japanese scientists decided to study the shrimp’s performance on bubble shooting in order to better develop machines that can better develop cavitation bubbles, i.e., cavitation generators.  

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How to Build a Bubble Machine

Scientists measured the dimensions of the claw of the pistol shrimp using a scanner called a uCT. Using this information, they were able to calculate how much volume of water is squeezed out of its claw. They then placed the pistol shrimp in an aquarium and measured the sound it made snapping its claw using a device called a hydrophone and then compared the noise made from the pulsed laser.  Then, scientists used a high speed video camera to determine the size and speed of an ideal cavitation bubble from a the pulsed laser generator.  Finally, after combining this information, they used this information to determine how to build the generator containing a piezo actuator and how well the prototype performed using a high speed camera.

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So, Can I Have My Bubble Machine Now?

When scientists tried producing bubbles from the generator with the piezo actuator, the volume of the cavitation was smaller than the pistol shrimp because the water jet velocity was not fast enough.  However, at the conclusion of the experiment, Dr. Soyama and colleagues were able to obtain useful information that could get manufacturers closer to having a working piezo cavitation generator. They obtained results on the speed and volume of water needed, as well as the the ideal diameter size of the bubble to create the cavitation force needed to make the generator effective. 

Additionally, their recommendation for future designs is to find a way to better design the geometry of the nozzle.  Overall, the results are promising.  If engineers can find a way to get the piezo actuator generator to make bubbles 3mm in size, manufacturers can move away from lasers and work with a design that is more compact, efficient, and less expensive. This experiment highlights how learning design from nature can create innovative solutions. Thank you pistol shrimp!

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Want to learn more? Check out this video on cavitation peening by Dr. Hitoshi Soyama!

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Additional References

APC International, Ltd. APC International Ltd. website, accessed 16 March 2024 <https://www.americanpiezo.com/standard-products/stack-actuators.html>

Destructive power of bubbles could lead to new industrial applications. (2015, June 30). Space Daily https://link.gale.com/apps/doc/A419921315/ITOF?u=txshrpub100594&sid=bookmark-ITOF&xid=33399504

Ramaswamy, K., Marx, V., Laser, D., Kenny, T., Chi, T., Bailey, M., Sorensen, M. D., Grubbs, R. H., & Stoller, M. L. (2015). Targeted microbubbles: a novel application for the treatment of kidney stones. BJU International, 116(1), 9–16. https://doi.org/10.1111/bju.12996

Versluis M. (2012, Oct 29). On the sound of Snapping Shrimp. YouTube. https://youtu.be/Zg10Et8FEWc?si=YGEh7k54KGSjVMi6

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