Bioinspired Mechanical Device Generates Plasma in Water via Cavitation
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Laboratory and industrial plasmas in liquid are conventionally generated by electric fields. In nature, one verified plasma generation method is the hydrodynamic flow induced cavitation by snapping shrimp. Collapsing cavitation is an effective energy focusing process to generate high-temperature and high-pressure energy states inside the cavitation, indicating of plasmas formation. The shrimp’s method of plasma generation has potential to be a high efficient technique due to the species evolution. Therefore a bioinspired mechanical device mimicking snapping shrimp snapper claw was designed to explore the possibility of producing plasma mechanically. Due to the complex fluid flow during the shrimp snapping process, a morphologically accurate claw is essential for hydrodynamic flow induced cavitation. Based on micro X-ray computed tomography (μ-CT) of a shrimp claw molt, a device was designed with torsion springs to actuate the snap. The major parts of the bioinspired device inherited claw morphology were only rendered feasible using additive manufacturing. Spring fixtures were designed to reliably actuate the claw with appropriate force and velocity to produce a high-speed water jet for inducing the cavitation. Different parameters such as torsion spring constants and releasing angles are explored to search for suitable parameters for larger cavitation size. And the underwater shock wave propagation and light emission during the cavitation collapse were recorded with an intensified charge-coupled device (ICCD) camera. The images of the luminescing cavitation at their first singularities were the direct proof of the “shrimpoluminescence” phenomenon which is similar to the sonoluminescence. Light emission evidences were verified in argon or air doped distilled water and saline water doped with air (the shrimp living condition), demonstrating the mechanical device can reproduce the plasma generation technique of the shrimp. The scale of the plasma ranges from 10 μm to 242 μm, and the time duration detected by the photomultiplier tube is ~15 ns. For this type of thermal plasmas, the plasma generation efficiency is directly coupled with the cavitation conversion efficiency. After comparing to other cavitation generation techniques, the cavitation conversion efficiency of the bioinspired mechanical device is ~3-2000 times more efficient. In order to characterize the plasma, a customized optical system is established for spectroscopy analysis. With the blackbody radiation assumption, the estimated plasma temperature was around 12,000 K for argon doped distilled water. Based on scaling laws such as matching the cavitation number, the 5 times scale-up device can operate in different liquids and a 25 times scale-up device was designed and generated cavitation in water successfully under different pressures to show the cavitation behavior difference. For future work, an automatic snapping shrimp robot can be used for plasma characterization and other practical applications. More distilled design can be explored with the aid of simulation. Additionally, hydrodynamics instabilities for jet-induced cavitation collapse could also lead to new research directions.
Tang, Xin (2020). Bioinspired Mechanical Device Generates Plasma in Water via Cavitation. Doctoral dissertation, Texas A&M University. Available electronically from