Oscillations, Waves, and Interactions - GWDG

174 R. Mettin
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Figure 2. Free nucleation into a cluster in a 20 kHz sound field (high-speed recording
with 1000 frames per second (fps), exposure time 1.5 µs). An object that appears as a germ
bubble (arrow first frame) develops into a conglomerate of bubbles that travels to the right.
The visibility of the cluster changes, because the relative phase of the sub-period exposure
time is drifting from frame to frame, and the cluster bubbles oscillate with strong volume
changes. On some frames they are close to collapse and too small to be resolved.
observed similar events [8] and termed them “comets”. Possibly they are triggered
by an advected microbubble, or ionizing particle radiation.
Another typical picture of nucleation is shown in Fig. 3. A quasi-continuous stream
of small bubbles appears out of the bulk, forming a trail of bubbles, or a “streamer”.
Bubbles travel along directional paths from the invisible source in the liquid towards
junctions with other streamers, which causes a dendritic form. The invisible sources
of bubbles remain active over many acoustic cycles. Probably, invisible sub-micron
bubbles, stabilized or from degassing processes in the liquid, migrate to the apparent
bubble source and merge to visible size, feeding the stream of bubbles.
Further sources of small bubbles are larger ones: driven by the acoustic field, they
can undergo surface instabilities. By pinch-off or even by apparent “explosions” [9]
one or many daughter bubbles can be produced. The generating bubble may be
freely floating in the liquid, or be attached to an immersed object or a wall. If fixed,