  Acoustically driven air pockets trapped in artificial crevices on a sur- face
can emit bubbles which organize in (interacting) bubble clusters. With
increasing driving power Fernandez Rivas et al. [Angew. Chem. Int. Ed., 2010]
observed three different behaviors: clusters close to the very pits out of
which they had been created, clusters pointing toward each other, and merging
clusters. The latter behavior is highly undesired for technological purposes as
it is associated with a reduction of the radical production and an enhancement
of the erosion of the reactor walls. The dependence on the control parameters
such as the distance of the pits and the conditions for cluster-merging are
examined. The underlying mechanism, governed by the secondary Bjerknes forces,
turns out to be strongly influenced by the nonlinearity of the bubble
oscillations and not directly by the number of nucleated bubbles. The Bjerknes
forces are found to dampen the bubble oscillations, thus reducing the radical
production. Therefore, the increased number of bubbles at high power could be
the key to understand the experimental observation that, after a certain power
threshold, any further increase of the driving does not improve the
sonochemical efficiency.
