5. Conclusion
It can be concluded that the physical forces generated by bubbles
in a sound field on a surface are maximized when the standing
wave component is minimized. The sound emission spectra do not
show a clear trend, which indicates that bubble activity in the bulk
of the fluid is not influenced to a large extend. Nevertheless, particle
removal increased considerably around the transmission angle
of the Si wafer. It is likely that an increase in the number of bubbles
at the wafer surface results in an improved cleaning. The resonant
bubbles cannot be trapped in nodes and antinodes when a traveling
wave is applied, and as a result, those bubbles can be transported
to the wafer surface by primary Bjerkness forces. The
resonant fast moving streamer bubbles are probably responsible
for the improved cleaning and have been observed before for a
230 kHz sound field [17]. This is an important finding for applications
that rely on microbubble-surface interactions.
Acknowledgements
The authors thank Geert Doumen for his technical support and
Entegris and ProSys Inc. for their hardware support.