Chapter 7
Outlook
The studies presented in this thesis have brought us several steps further in the progress
of understanding the physics of acoustic resonances and acoustic streaming ows. The
numerical model, which we have developed and rened throughout the past three years, is
publicly available as supplemental material to Ref. 30 Appendix E, and it will hopefully
inspire others to make further progress along the path of the present studies.
The challenge of acoustophoretic focusing of sub-micrometer particles has been a main
topic throughout this thesis work. It has been addressed through studies of the basic
physics of the acoustic streaming ow and through engineering of a new single roll streaming
ow, that did not counteract the acoustophoretic focusing. The future progress in meeting
this challenge may follow two paths: (i) Continuing work on the single roll streaming ow
of the nearly-square channel to experimentally achieve better control of the streaming ow,
resulting in a more reliable focusing and focusing of even smaller particles. (ii) Theoretical
and experimental studies of the streaming ows and particle trajectories obtained when
switching between resonances with opposing streaming patterns, on a time-scale much
shorter than the build-up time of the acoustic streaming, thus ghting streaming with
streaming.
When studying the motion of suspended particles in the transient case, such as when
switching between resonances as described above, new challenges arise. Firstly, the force
on a particle due to the scattering of the now transient acoustic eld needs to be derived
analytically, since the usual time-averaged radiation force is valid only for the purely periodic
state. Secondly, the forces on the particles need to be integrated with a time step of
a fraction of the oscillation period, which makes the solution of particle trajectories over
several seconds a very demanding task using brute-force integration of the equations of
motion. Consequently, a new numerical method for prediction of acoustophoretic particle
trajectories in a transient acoustic eld needs to be developed.
The studies of this thesis all treat the two-dimensional uid domain of the channel cross
section, neglecting the vibrations of the surrounding solid material and assuming invariance
along the length of the channel. Modeling the vibrations in the solid, including the electromechanical
couplings in the piezo-electric transducer, and going from a two-dimensional
domain to a three-dimensional full chip model present a wide range of challenges in making
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