  Bubble dynamics and luminol emissions of cavitation in sub-millimeter-sized
PFA flow tubes, submerged in an ultrasonic bath reactor, are studied at 27 kHz
driving frequency. Nucleation of cavitation inside the tubes only takes place
via a free interface, realized here in form of an alternating water-air slug
flow. High-speed recordings show that cavitation bubbles in the water slugs
often develop localized structures in form of clusters or bubble plugs, and
that such structures can be seeded via a single pinch-off from the free
interface. Within the structures, bubbles strongly interact and frequently
undergo merging or splitting events. Due to the mutual interaction and
resulting motion, bubbles often collapse with a fast displacement, suggesting
jetting dynamics. Bubble compression ratios are estimated on basis of observed
individual bubble dynamics and numerical fitting by a single bubble model. The
resulting peak temperatures around 3000 K allow for dissociation of water
vapor. This is in accordance with observed sonochemiluminescence from luminol,
originating from active bubble zones in the tubes.
