Oscillations, Waves, and Interactions - GWDG

156 W. Lauterborn et al.
Pulse width (ns)
10
8
6
4
2
0
0.0 0.5 1.0 1.5
Maximum bubble radius (mm)
Figure 19. Pulse width of the light emission upon collapse of a laser-induced bubble versus
the maximum attained radius.
Figure 19 shows the luminescence pulse width of laser-generated, freely collapsing
bubbles in dependence on the maximum bubble radius from where the collapse
starts. The pulse width increases linearly with bubble size and is quite long for
millimetre-sized bubbles (in the nanosecond range, and thus accessible to high-speed
photodetectors) as compared to the small trapped bubbles collapsing from maximum
sizes of about 100 µm. The little box near the origin indicates the pulse widths
measured for small trapped bubbles [16].
Current theories of single-bubble sonoluminescence attribute the light emission to
a strong heating of the bubble medium by converging compression or shock waves
launched within the bubble shortly before the collapse is stalled [17]. The energy
concentration by this mechanism should therefore depend critically on the degree of
sphericity imparted to these waves. Experimental evidence for this relation is given
by Fig. 20 which shows how the light diminishes with increasing perturbation of
the bubble collapse [26]. The asphericity acquired during collapse is controlled by a
plane solid surface whose distance to the breakdown site is altered. A bubble far
from the boundary collapses spherically (large γ), a bubble nearer to the boundary
collapses the more aspherically the nearer it is to the boundary. An astounding result
was found, namely that the light emission ceases at the quite large value of γ = 4.
This means that the bubble must retain a sufficiently round shape upon collapse in
order to produce luminescence light. Further experimental results corroborating this
hypothesis are reported in the next section.
4 Laser-induced bubbles in a sound field
In order to keep a bubble stably oscillating in an acoustic bubble trap special conditions
have to be met. The bubble’s free radial motion may not be disturbed, so it
has to be kept well away from boundaries and other bubbles. The net average force