Oscillations, Waves, and Interactions - GWDG

Normalized Energy
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The single bubble – a hot microlaboratory 157
0 2 4 6 8 10
γ
Figure 20. Luminescence
of laser-induced bubbles collapsing
asymmetrically near
a solid wall. Dependence of
the light energy normalized
to the light emitted from
a spherical bubble on the
stand-off parameter γ.
due to pressure gradients in the liquid must vanish not to induce bubble translation.
Also, streaming of the liquid must be avoided for the same reason. The amplitude
of the sound field is constrained to a small interval where the bubble is in a stable
diffusive equilibrium, in accord with the liquid’s gas content. The bubble collects a
certain amount of noble gas and has a rest radius of a few micrometres, typically.
Thus by means of stable bubble trapping a limited range of the bubble’s parameter
space is available for exploration. The advantage of the method is that the bubble
can be scrutinized easily, e. g. by high-speed photography, because it stays at a fixed
location and can oscillate clock-like for many hours.
By means of optic cavitation steady and transient bubble dynamics in a sound
field can be explored in a much larger parameter space than available with bubble
trapping, while the advantage of being able to image the bubble dynamics at least
for a limited amount of time is preserved. Furthermore, bubble translation and
the acoustic and hydrodynamic forces on a bubble, e. g. drag forces, as well as the
interaction of two or more bubbles can be studied in a well-defined setting. Thus the
combination of optic generation and acoustic driving represents an ideal laboratory
to study many aspects of single bubble behaviour and also to control bubble motion
by carefully chosen parameter changes – an aspect that up to now has not been
pursued extensively in experiments.
Fig. 21 shows the basic experimental setup for optic cavitation in a sound field.
As described previously the bubble is generated by focussing a (nanosecond to femtosecond)
laser pulse in the liquid. Simultaneously an ultrasonic field is applied in
the cuvette at one of its main resonance frequencies, preferably the (1,1,1) mode, to
achieve high sound pressures. An essential experimental feat is that the laser pulse
can be applied at a well defined phase of the acoustic field which yields reproducible
initial conditions for the ensuing bubble dynamics. For that purpose electronic devices
were built that synchronize the laser operation with the acoustic field.
One issue of central importance in bubble physics is the question of how large a
degree of energy focussing can be achieved in the violent collapse of a cavitation bub-