Cavitation could generate ultraviolet “micro-flashlights” for clean water treatment

We have already discussed in this blog how cavitation bubble collapse can be used to target and destroy cancer cells. On a similar biological theme, I recently found a paper that utilises the collapse of nanobubbles to disinfect water, although with a slightly different approach.

Chlorination is the most common technique for disinfecting water, although some pathogens, such as Cryptosporidium, require stronger treatment with exposure to short-wavelength ultraviolet light (UV-C). An interesting phenomenon known as sonoluminescence has previously been observed, where light can be emitted during cavitation, due to the localised high temperatures at the centre of the bubble collapse. However, this light is radiated isotropically, and the strength, or fluence, of the radiation would not be strong enough to disinfect the pathogens (Figure (a)).

(a) Spontaneous bubble collapse releases isotropic short-wavelength ultraviolet (UV-C) radiation which is not strong enough to disinfect pathogens; (b) gallium-alloy microparticles reflect the UV-C radiation from bubble collapse into a more directional and concentrated beam which is strong enough for disinfection.

Researchers from University of Canterbury, Texas A&M University, Swinburne University of Technology and RMIT University have proposed a novel method for utilising sonoluminescence by employing liquid gallium-alloy microparticles in the water. The group performed multiphase, compressible fluid dynamics simulations for the bubble collapse, coupled with a finite-difference time-domain scheme for the UV irradiation. The bubbles collapse as before, although are predominantly directed towards the metal particles. The cavitation jet deforms the metal particle surface, while also generating the UV light. The deformed shape of the microparticles reflect the generated UV light into a more concentrated, directional beam, which is claimed would provide high enough fluence to disinfect the pathogens (Figure (b)).

The concept still seems in the early stages, and so far, there are no attempts at replicating the results experimentally, however, I thought this was a very novel application of cavitation bubble collapse that was worth sharing. The research has been published in Nature Scientific Reports https://doi.org/10.1038/s41598-020-5818.