Oscillations, Waves, and Interactions - GWDG

(a) 1 mm
(b)
Acoustic cavitation 175
Figure 3. Nucleation from the bulk liquid into streamers in a 20 kHz sound field (two
frames from a high-speed recording, exposure time 1.5 µs). Several streamer sources on
frame (a) change into a single one on frame (b) within the interframe time of 30 ms.
it can form a spatially stable source of a continuous bubble stream. To control such
attached bubble sources can be one way of a cavitation structure control [10].
Examples of microbubble generation are shown later in Figs. 7 and 9. The process
of repetitive ejection seems to be a common cavitation bubble source if objects or
walls are contributing, and for a complete description, gas diffusion into such source
bubbles has to be taken into account. The attached bubbles might even be very
small, as recent investigations on “nano-bubbles” suggest [11].
3 Bubble dynamics
Once a bubble is created in the liquid, it will show variations of size and possibly
shape as a reaction to the sound field. On a longer time scale, translational and
diffusional processes can take place. From the point of view of a single, individual
bubble, these issues are captured by the term bubble dynamics. A very useful model
for investigation of bubble dynamics is a spherical gas and vapour bubble. Here it
is usually assumed that the gas inside the bubble is non-condensable (and thus its
mass is conserved, at least on a fast time scale), and the vapour is instantaneously
evaporating or condensing at the bubble walls, leading always to the equilibrium
vapour pressure inside. 3 The presence of a non-condensable gas (in many cases air)
can be questioned, but keeping in mind the nucleation mechanism from stabilized
micro (or nano) gas bubbles, this assumption usually makes sense. 4 Indeed, an
3 Refinements of this model include gas diffusion and vapour evaporation and condensation
dynamics, but they lead in many parameter regimes in water to rather small corrections.
They are important for a good description of the collapse, which is not central here, and
thus they are neglected in the following.
4 Of course, also pure vapour cavitation might be encountered in some situations, like
in liquid helium [12]. However, here we are dealing mainly with water-air systems under
normal conditions.