It’s not all about high-speed imaging (well, outside CavLab, anyway!)…
In most applications, direct optical imaging of cavitation just isn’t feasible – good luck trying to get a microscope objective lens into brain tissue for observation of bubble-mediated blood-brain barrier disruption!
Most ‘end-users’ of cavitation are reliant on a less invasive approach; some kind of acoustic detector, or listening device, for measuring the (ultra-) sounds that driven bubbles generate. A key objective for CavLab research, that has been occupying us for the last year or more (!) has been matching what bubbles do, via high-speed imaging, to the sounds they make. Why? Because if those noises can be correlated to specific bubble activity, then that may/should allow the optimisation and refinement of cavitation applications (precision cleaning, sonochemistry and medical therapy, for example) which is one of CavLab’s core goals.
After much chin-scratching, and ‘highly charged’ group meetings (yes, I’m looking at you, Kristoffer – and thanks to Jae, for keeping the peace), we think we’re making some progress….
In a paper just recently accepted to the prestigious Journal of the Acoustical Society of America(or JASA, to those in the know), and a sister paper accepted to Ultrasonics, we offer an explanation for something known as the ‘cavitation spectrum’. There are hundreds… no thousands, of spectra in the research papers describing cavitation use. They’re almost a pre-requisite to having a paper on cavitation published…
In essence, the spectrum tells us what frequencies (or notes, in the language of a musician) bubbles have generated during the time the cavitation is recorded. And the cavitation spectrum can be a complicated thing, with all sorts of noises being made, many of which have defied explanation for many decades.
<—- This high speed data (captured a 5 million frames per second!) underscored what we did for this study. We use the precision of our laser-nucleation technique to put cavitation as up close and personal to the tip of hydrophone (the black tapered rectangle thing, at the bottom of the images) as we dared, for extremely detailed acoustical measurements. This was scary data to collect – if the cavitation had made contact with the tip, it would have seriously compromised our hydrophone. Even worse, if a big laser bubble had formed, the tip would have been smashed (and although the hydrophone wasn’t overly expensive, the calibration was – about x10 the price of the hydrophone itself!!).
Anyway, it was worth it. We got some pretty sweet data. And once we’d figured out how to handle the acoustic measurements of the shock waves (the Ultrasonics paper), everything just fell into place (more or less).
To get a bit technical, we demonstrated that all the non-linearity of acoustically driven bubbles are contained within the shock waves emitted during the strong collapses. If you take the shocks away all you’re left with is linear, oscillation-generated single frequency emissions (at the frequency you’re driving the cavitation at).
So, what does cavitation sound like? Hum and clap your hands – that’s pretty much it (just at a note that only bats hear, and rather fast clapping)!