On Demand Bulk Nanobubble Generation*

A critiical feature of bulk nanobubbles that distingusihes them from droplets and particles is their ability to expand under the action of a rarefaction wave. This feature has been probed  in experiments with two kinds of waves. 1) a short bi-polar pressure wave with sub-microsecond duration created with a laser-induced breakdown near a free interface and 2) a typical lithotripter shock wave trailed by a rarfaction wave. Both waves cause the expansion vapor bubbles only if the same volume of water is illumiated by a short and intense laser beam. We argue that the laser beam by some means nucleate nanobubbles that are then expanded into visible bubbles. By applying a suitable timing between rarefaction wave and laser beam and assume that classical diffusion holds we can measure the population of nanobubbles generated.