Oscillations, Waves, and Interactions - GWDG

168 W. Lauterborn et al.
R [µ m]
100
80
60
40
20
0
0 5 10 15 20 25 −1
0 5 10 15 20 25
t [µ s]
Figure 31. Radius vs. time curve of a laser-generated bubble in an acoustic field of frequency
44 kHz and amplitude 159 kPa. The seeding phase of the bubble is ϕs = 184 ◦ . The
MD simulation was run with 64000 particles. The simulation corresponds to the measurement
presented in Fig. 25(a).
Of course, the MD method is not restricted to the study of single, sonoluminescing
bubbles. The method has also been applied to simulate the interior dynamics of
laser-generated bubbles. A particular challenge, and a field for further exploration,
is the definition of correct initial conditions to describe an expanding, hot bubble
immediately after laser breakdown. In the calculation presented in Fig. 31 for a
laser bubble in a sound field, initial conditions were chosen that are believed to be
reasonable and consistent with experimental observations of the laser plasma and
emerging bubble: the initial radius of 1.5 µm corresponds to the extension of the
laser plasma, and the initial temperature was taken as T0 =3000 K with one third of
the water molecules being dissociated. The results on this figure correspond to the
experimentally investigated bubbles shown in Fig. 25(a). Even a casual inspection
with the naked eye shows that the simulation captures the essential features of the
radial dynamics observed in the experiment.
6 Conclusion
The single bubble in a liquid is a conceptually simple two-phase system that nevertheless
is difficult to tackle in reality. A multitude of physical and chemical effects occur
simultaneously and interact while the bubble undergoes possibly extreme changes in
its thermodynamic state, in particular at collapse. Instabilities of position and shape
can make it difficult to observe single bubbles experimentally. As we hope to have
shown experiments have profited, in this respect, from the method of optic cavitation
and the discovery of stable single bubble trapping.
In real-world situations and applications, such as ultrasonic cleaning and chemical
processing, bubbles usually appear in clouds and experience strong interaction with
each other and with their environment, as clearly demonstrated by a host of selforganization
effects [37]. In these settings the bubbles usually have a limited lifetime
and undergo changes of size and composition. Nevertheless, they can often be viewed
3
2
1
0
P A,eff [bar]