To degass the water I used the “boil the heck out of it” technique in an erlenmeyer flask. To my minor annoyance, the flask I had was just too big for the rubber cork, so innovation involving a rubber band and a bit of plastic bag had to suffice. And suffice it did..
After vigorous boiling for 10 minutes or so, I put the plastic bag over the top and secured it with the rubber band. Boiling like this removes most of the dissolved air in the water. The solubility of air in water decreases with temperature and having a neck that is small relative to the flask means that air doesn’t easily re-dissolve in the water.
This water is cooled and then carefully poured into the spherical flask. Now bubbles should not form all over the place under cavitation – which upsets the acoustic resonance of the flask, now single bubbles can be injected into the water for more fine control.
The first attempt with this resulted in an interesting failure, acoustic resonance was located and all was going well with no ultrasound induced streamers or bubbles appearing. A bubble was introduced to the water by disturbing the surface (design change pending), and stabalised- but not in the center of the flask. A small cloud of bubbles began to form over the surface of the primary bubble, rather like an eclipsing moon, until the primary bubble was made opaque by this tiny foam. This state remained for a few seconds and then suddenly the cloud of tiny bubbles was blown off the primary bubble in a beautiful “smoke” ring. Quite fascinating to watch – almost like the formation of a mini planetary nebula…
The tiny bubble ring then faded and dissolved leaving the primary bubble intact. This was not what I was expecting at all, now looking for some description of this effect. Unhappily it was at this point that the epoxy gave out and the flask detached from the ultrasound transducer. I should like to reproduce the effect and film it for the next experiment.