Oscillations, Waves, and Interactions - GWDG

160 W. Lauterborn et al.
Figure 24. Experimentally measured times of first collapse of the optically generated
bubble (circles) and numerically fitted curve obtained with the Gilmore model. The fit
yields the phase calibration and the acoustic amplitude that cannot be measured by a
hydrophone in situ without significantly disturbing the sound field.
a large maximum radius is attained later and later in the acoustic cycle (Fig. 23).
This leads to a discontinuity in collapse times vs. seeding phase as shown in Fig. 24.
Experimental values are plotted along with numerical data obtained with the Gilmore
model for parameters that are fitted to yield minimum deviation. By comparing
experimental data with numerical results from a proven model the bubble can be
used as a pressure sensor in a self-consistent way: the fit yields the zero of the
seeding phase and the amplitude of the acoustic pressure at the bubble position.
The jump in collapse time corresponds to the point where the sound field is able to
reverse the bubble motion before the first, immediate collapse occurs. This method
of pressure determination has the advantage that no hydrophone, which invariably
tends to disturb the sound field, has to be immersed in the liquid, and that it can
also be used with a closed cuvette.
Femtosecond laser pulses have been used for bubble generation in these experiments
to reduce the amount of non-condensable gas that is created in the breakdown
process. Femtosecond pulses have a low breakdown threshold and thus a small plasma
volume, so the gas production is minimized. Less gas means a more violent collapse as
the bubble can shrink to a smaller volume before the pressure in the bubble medium
counteracts the inertially dominated liquid inflow. In this way a high energy concentration
and collapse strength may be achieved, that can be assessed by measuring
bubble luminescence and shock wave emission as its indicators, and their dependence
on acoustic driving pressure, for example.
Figure 25 presents an investigation of bubble luminescence for optically generated
bubbles in an ultrasonic field. Light emission of the bubbles in dependence on their
seeding phase ϕs was measured for bubbles of the type shown in Fig. 22(right). While,