Oscillations, Waves, and Interactions - GWDG

162 W. Lauterborn et al.
Figure 26. Dynamics and luminescence
of bubbles generated at
the seeding phase ϕs =331 ◦ . The
measurements are presented as in
Fig. 25. Here, the bubbles are shot
in the positive-pressure phase of
the sound field and collapse immediately.
Upon the second, strong
collapse light is emitted.
Nearly perfectly round bubbles can be obtained when laser-breakdown is initiated
in the positive sound pressure phase, so that the generated bubble remains initially
small and is finally expanded in the subsequent half cycle. The laser energy is largely
dissipated, and so are the initial pressure and flow anisotropies induced by the elongated
shape of the laser plasma.
This method for bubble inception has been used to study bubble luminescence as
a function of acoustic driving pressure. One result is shown in Fig. 27. It presents
the luminescence pulse energy (in terms of photomultiplier voltage) vs. acoustic
pressure. Measurements were restricted to the first oscillation cycle after bubble
generation because, at large driving pressures, the bubbles tend to disintegrate after
the collapse and are thus not well defined anymore. The figure shows that with
increasing pressure the probability to observe luminescence for a certain laser shot
tends toward 100%, and that the average luminescence pulse energy increases as well.
It can be concluded that, in fact, by increasing the driving amplitude the bubble
collapse becomes more energetic. Above pa ≈ 4 bar, which is near the cavitation
threshold in the cuvette, the light yield does not increase anymore. The reason for
this behaviour remains to be clarified. It may be caused by a shape instability of the
bubble, and by the fact that for a fixed frequency of the driving field the bubble can
only be expanded to a certain maximum size within one half-cycle of the oscillation.
To observe a larger number of oscillation cycles of laser-generated bubbles in a
sound field the acoustic amplitude must be chosen in a way not to excite surface
oscillations. This is the case for sound levels around 1.2 bar as used in SBSL experiments.
In fact, if the gas content of the liquid is also adjusted correctly, the transition
of a laser-generated bubble towards a single stably oscillating, light-emitting bubble
can be studied. Figure 28 presents but one example of measurements performed on
this phenomenon. Here, the light emission is recorded by means of a sensitive, gatable
ICCD camera. The camera was opened for 2.28 ms (corresponding to 100 oscillation