Laser-nucleated clouds

CavLab research is based on direct observation of cavitation activity – however, this is easier said than done!

Acoustic cavitation is an extremely rapid phenomenon; bubbles respond to an acoustic field on the timescale of the frequency, which for therapeutic ultrasound ranges from a few 100’s of kHz to several MHz.

The quiescent size (i.e. if it wasn’t being driven by pressure fluctuations) of acoustic cavitation bubbles is also determined by the frequency, via a resonance condition. Our bubbles are typically a few 10’s of μm.

So you have rather small objects, moving rather rapidly. To make things worse, you don’t know exactly where or when cavitation is going to start. Todays’ high speed cameras are just about fast enough, and microscope objective lenses are strong enough – but if you don’t know where to point them, and when to start the image capture, you’re not likely to see much.

We’ve therefore developed a laser-nucleation technique, to predetermine the instant and location of cavitation occurrence. This allows us to image bubble activity at the spatial and temporal resolution required to make sense of it.

Here’s some recent sample data:

cloud gif

This is a single cavitation cloud, imaged at 1 million frames per second, developing over 50 cycles of 254 kHz high-intensity focused ultrasound, in water. You can see that, very quickly, a cloud of bubbles form that respond to the ultrasound in unison, such that the cloud oscillates as a single entity. What we really like about this data, is that you can see shockwaves being emitted each time the cloud collapses.