Sample Signals of Cavitation Experiment

We used light scattering method to detect cavitation events. We shine a He-Ne laser beam (632 nm) through the windows of our experimental cell. The cell is filled with liquid helium. The laser is aligned carefully so that it goes through the acoustic focus generated by the hemispherical transducer. If cavitation event occurs, bubble(s) (with slightly different refractive index from liquid) would scatter light off its incident beam. We use photomultiplier tube (PMT) to detect the scattering light and use digital oscilloscope (LeCroy 9310A) to record and count the signals.

Without having cavitation events, we may only detect the light scattered by the density fluctuation due to the big pressure swing at focus if the fluctuation is big enough, or just detect the background of light. The figure shown left is the scattering light of density fluctuation. The duration of the scattering light is exactly the same as the duration of the driving voltage on the transducer. Typically we use quite short pulse (about several microseconds long) to drive our transducer. The time scale for the figure is 0.2 minisecond per division. You can see the width of the pulsed signal on the left upper corner of the figure is very short.(5 microseconds)

Once a bubble of macroscopic size (typically bigger than 10 microns) formed within the acoustic focal region, much more light would be scattered by the bubble. If the laser beam is appropriately focused, hugh scattering signal can be detected. For really big bubbles, the scattering light even can be seen by bare eyes. The lifetime, proportional to the maximum size of the bubble, depends on liquid temperature, static pressure and driving voltage on the transducer. The figure shown right is the scattering light of a bubble detected by PMT. We can see the formation, growth, and collapse of the bubble. The time scale for the figure is the same as the first figure. (0.2 minisecond per division) We can clearly see the difference between the cavitation events and non-events. One important thing I want to point out is that the acoustic sound lasts only a few microseconds. The bubble grows abruptly under negative pressure caused by the gigantic sound pulse. After that, the bubble grows only due to its inertia. That's why the scattering light signal increases rapidly at the very beginning. If the bubble is so big (like the one shown right) that secondary bubbles can form. When the big bubble collapses, a lot of energy would be released and deposited into small region so that a second bubble would be created by those energy. In the figure, the second and third bubbles can be seen.

The figure shown left is the typical signal applied on the transducer for generating sound pulse in liquid helium. The time scale for the figure is 1 microsecond per division. The resonance frequency of the transducer (in thickness resonance mode) is about 1.4 MHz.

Sample signals recorded from my cavitation experiment