Oscillations, Waves, and Interactions - GWDG

The single bubble – a hot microlaboratory 167
calculations. The white line marks the bubble wall. The center temperature reaches
about 16000 K upon collapse where the bubble attains a minimum value of about
0.8 µm. This is in the range of values reported for sonoluminescing bubbles derived
from optical spectra of the light emitted. It is seen that the temperature distribution
is very inhomogeneous. The density in the bubble turns out to come close to or even
exceed that of liquid water.
Figure 29(bottom) presents an example for the capability of the MD method to investigate
bubble chemistry. For the same bubble as before the density of OH radicals
is plotted in dependence on space and time. The hydroxyl radical is an important
chemical species in sonochemistry that can dissolve in the water and mediate further
reactions there. The OH distribution closely matches the temperature field. Obviously
it is also produced in an outgoing compression wave heading toward the bubble
wall. As the temperature drops the OH is quickly consumed again by chemical reactions.
Furthermore, in this spherically symmetric collapse the OH concentration
remains small in the cold gas-vapour layer near the bubble wall. This means that
OH uptake by the liquid should be small as well. It is difficult to accurately predict
the amount of OH going into the liquid because the uptake coefficients are not (or
not well) known.
The production of OH is closely correlated with the distribution of water vapour
in the bubble. It has been shown that in the late collapse phase the water vapour
cannot diffuse out of the bubble any more and remains trapped [32,36]. Also, in
the presence of non-condensable gas, e. g. noble gases, demixing occurs so that the
lighter and hotter molecules accumulate at the center of the bubble. MD simulations
confirm this effect and also yield measurable effects in bubble chemistry caused by
the redistribution of water vapour.
When fewer particles are used in a molecular dynamics calculation the bubble
dynamics can be followed over several oscillation cycles with acceptable computation
time. Even with as few as 50000–100000 particles the radial dynamics obtained
closely matches results obtained with conventional ODE bubble models (Fig. 30).
The advantage is that the MD calculation also provides information on the vapour
content and the evolution of chemical species in the bubble over this long time.
r[µm]
50
40
30
20
10
0
0 20 40 60 80
t[µs]
Figure 30. Radius vs. time
curve for an SBSL bubble
with the same parameters as
in Fig. 29. The MD simulation
was run with 110000
particles.