The behavior of a single isolated bubble in a sound field and even
the interaction of a single bubble with a surface are very well understood.
A lot of experimental and theoretical work has resulted in an
in-depth understanding of bubble oscillations and even the asymmetric
bubble collapse has been elucidated. Very often, Rayleigh–
Plesset type of equations are able to reproduce the main bubble
effects [1,2]. The behavior of bubble clouds is being studied, but a
thorough understanding of multi-bubble systems is still lacking
[3–6]. This multi-bubble behavior is important since most applications
rely on the global effect of a multitude of bubbles and very
often, the interaction of a bubble cloud with a surface needs to be
controlled. The applications range from the production of nano-particles
[7] over the treatment of cancer cells [8] towards the cleaning
of complicated 3 dimensional structures [9] or even fragile nanometer
sized structures [10]. Most of the high end applications require a
precise control over the number of bubbles, bubble size distributions,
bubble–bubble and bubble-surface interactions. All these
factors will influence the efficiency of the targeted application. A
precise control is even more important when the allowed physical
forces exerted on a surface are limited to a small process window
[11]. An excellent control over the physical forces exerted by the
bubbles on the surface is therefore extremely important. A multitude
of physical phenomena, which include Schlichting or boundary
layer streaming [12,13], microstreaming [14], water hammer force
due to bubble collapse [15] and even the possible formation of shock
waves [16], are responsible for the physical forces acting on a surface.
All these effects depend on the sound field and as a result, a
precise control of the sound field will be necessary to reach the level
of control which is necessary for the high end applications. However,
sound reflections are often not well controlled in real life applications.
Thiemann et al. [17] investigated a focussed standing wave
with a share of a traveling wave away from the transducer. It was
observed that bigger bubbles were trapped in nodes parallel to the
transducer, while smaller bubbles (so called streamers) showed a
fast movement away from the transducer.
Here, we will show that acoustic reflections will have a large
influence on the physical force exerted by microbubbles on a surface.
The use of anechoic materials together with careful positioning
of the treated object, allows to maximize the traveling
component of the sound field. It is shown that this can dramatically
improve the particle removal process efficiency.
is probably caused by a decrease in bubble trapping at nodes and antinodes in a standing wave field