The effect of a wave with a varying traveling component on the bubble activity as well as the physical
force generated by microbubbles on a surface has been studied. The acoustic emission from a collection
of bubbles is measured in a 928 kHz sound field. Particle removal tests on a surface, which actually measures
the applied physical force by the bubbles on that surface, indicate a very strong dependence on the
angle of incidence. In other words, when the traveling wave component is maximized, the average physical
force applied by microbubbles reaches a maximum. Almost complete particle removal for 78 nm silica
particles was obtained for a traveling wave, while particle removal efficiency was reduced to only a
few percent when a standing wave was applied. This increase in particle removal for a traveling wave
is probably caused by a decrease in bubble trapping at nodes and antinodes in a standing wave field.
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