increasing the height-to-width ratio. In contrast, by replacing
water by a 50% glycerol-in-water mixture, we demonstrated how
to enhance the streaming effects. The former study may form a
good starting point for designing streaming-free devices for
handling of sub-micrometer particles, such as small cells,
bacteria, and viruses, and thus supporting concurrent experimental
efforts to suppress streaming, e.g., through averaging
over alternating actuation frequencies.43 The latter study is
pointing in the direction of developing devices with improved
mixing capabilities by enhancing streaming.44,45 We have thus
shown that our simulation tool has a great potential for enabling
improved design of acoustofluidic devices.
An important next step is to obtain a direct experimental
verification of our numerical simulation. As the relative
uncertainty of measured acoustophoretic particle velocities in
current experimental acoustofluidics is 5% or better,8 it is within
reach to obtain such an experimental verification. A problem is
of course that the streaming fields calculated in this work are in
the vertical plane, which is perpendicular to the usual horizontal
viewing plane, and thus specialized 3-dimensional visualization
techniques are required such as stereoscopic micro particle–
image velocimetry42,46 or astigmatism particle tracking velocimetry.
47 But even if such 3D-methods are complex to carry out,
it would be worth the effort given the great use of having a wellverified
numerical model of acoustophoretic particle motion.
Given a successful experimental verification, it would clearly be
valuable to extend the numerical model. One obvious step, which
is not conceptually difficult, but which would require significant
computational resources, would be to make a full 3D-model
taking the elastic properties of the chip surrounding the
microchannel into account. The relevance of such an extension
lies in the sensitivity of the acoustic streaming on the boundary
conditions. Only a full acousto-elastic theory would supply
realistic and accurate boundary conditions. Another class of
obvious model extensions deals with the modeling of the particle
suspension. A trivial extension would be to include gravity and
buoyancy, but more importantly and much more difficult would
be the inclusion of particle–particle and particle–wall interactions
that are neglected in the present work. These many-particle effects
include, e.g., the generation of streaming flow in the boundary
layer of each particle48 and not just the boundary layer of the wall.
After such an extension, our model could be used together with
high-precision experiments as a new and better research tool to
study and clarify the many yet unsolved problems with particle–
particle and particle–wall interactions in acoustofluidics.
The above-mentioned applications all demonstrate that our
numerical model is both timely and has a huge potential within
device design and studies of basic physical aspects of acoustophoresis.
This research was supported by the Danish Council for
Independent Research, Technology and Production Sciences,
Grants No. 274-09-0342 and No. 11-107021.
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