Oscillations, Waves, and Interactions - GWDG

172 R. Mettin
(a) (b)
Figure 1. Bubbles in tap water below a sonotrode tip (5 mm diameter, 20 kHz) with
background illumination: exposure time in (a) 1 ms, in (b) 1 µs.
sides rather established usage in ultrasonic cleaning and ultrasonically assisted chemistry,
relatively new and even exotic applications gain attention. These range from
nanoparticle production and sewage sludge activation via biomedical applications
like contrast imaging and cancer treatment up to the prospect of table-top nuclear
fusion reactions. In parallel with the increased interest, research methods and technical
equipment have improved likewise in recent years. This is true in particular for
high-speed imaging systems and computer power. Consequently, research activity is
very active and growing, and new insights are obtained rapidly.
The aim of this article is to focus on the destiny of individual bubbles in acoustic
cavitation. The acoustic field causes a permanent driving of the once created cavity,
and this leads to peculiar effects that prolongate and complicate the life of a bubble.
One important microscopic aspect is the bubble collapse [7]. On a meso- and
macroscopic scale, it is important that the bubbles often translate quite fast due to
acoustic forces. 1 Additionally, they can shrink or grow by gas diffusion processes,
merge with other bubbles, or split into smaller ones, and these processes are also
strongly influenced by the acoustic field.
In typical set-ups of cavitation applications, an acoustic field is created by an ultrasonic
transducer. Here, one can roughly distinguish between localized sources,
like the tip of an ultrasonic horn, and plane emitters of larger size (compared to
the acoustic wavelength). In any case, cavitation manifests itself to the unaided observer
as “clouds” or “bubbly zones” of various shapes and extensions. Of course, the
cavitating region consists of many individual, often well separated and surprisingly
spherical bubbles. This is even true at rather high bubble densities, as is demon-
1 This might be seen in contrast to hydrodynamic cavitation, where the liquid ruptures
under a hydrodynamic flow condition which induces negative pressures. Then, cavities are
typically attached to a body surface, or are convected by the flow to normal pressure zones,
and disappear. Therefore, the life of a hydrodynamic cavitation bubble is usually quite
different, and its location is determined by the body and/or the liquid flow.