…and ‘covert cavitation’

Cavitation is an energetic and destructive phenomenon – not the sort of thing you want happening in your brain, without knowing about it. It could also deliver life-saving drugs there, so don’t rule it out, but in any case, you’d want to know if it’s there or not.

The most common method for detecting it is acoustic ‘listening’ devices. In researching cavitation applications, the amount of cavitation that has occurred is often assessed from a representation (called the cavitation spectrum) of the acoustic emissions collected by the acoustic detector device, during the time the cavitation existed. Specific features within the spectrum are linked to the cavitation-mediated effect being investigated – if one was being cynical, one might say that the feature selected just happens to link well with the effect, rather than being chosen for any more objective reasons, but that’s another story….

CavLab has spent quite some time trying to figure out the connection between specific spectral features and the actual bubble activity, supposing that this should help refine cavitation applications. And we made some progress in 2016, showing that all features could in fact be explained by periodic shockwaves generated by acoustically driven bubbles; a bubble emission that was perhaps previously underappreciated…

Anyway, we did this (and pretty much all our previous work) with single bubbles, or single clouds of bubbles, generated with single laser pulses. In our latest paper (Song et al JASA-EL 2017), we went crazy and introduced a whole other single cloud into the system.

“Cavitation is easy” ….said nobody, ever!

The cool thing about this is we can put the two clouds anywhere we want, relative to each other, the ultrasound and whatever acoustic detector we’re using. For the paper, we put the clouds in a very specific configuration (about one wavelength apart, in a direct line with the detector) – and demonstrated that this cavitation system effectively fools the detector into thinking there is no cavitation there at all!! That’s why we called it ‘Covert cavitation’ (catchy, huh?)

Luckily, this slightly contrived configuration is unlikely to occur in ‘real-life’, and you’ll be relieved to know that most cavitation doesn’t sneak about unnoticed – but the message stands, which is be careful how you measure bubbles!