6.4. QUANTITATIVE COMPARISON OF ACOUSTOPHORETIC PARTICLE MOTION55
tion force dominated motion to streaming-induced drag force dominated motion should be
interpreted. Furthermore, the dependence of the critical particle size on the uid medium
and frequency, which is often not illuminated due to the main focus on the particle size,
was emphasize by visualizations of particle motion in a high viscosity medium and for a
higher-order resonance.
The transition from streaming velocities to predicted particle trajectories converted
complex theoretical calculations into something that was easy to relate to. This strengthened
the intuition of particle motion in acoustophoresis systems and was one of the most
important steps in reaching out to an experimentally driven community.
6.4 Quantitative comparison of acoustophoretic particle mo-
tion
The ultimate validation of both analytical and numerical models is a direct quantitative
comparison, with no free parameters, to experimental measurements. This was achieved
in Ref. 11 Appendix B. The streaming studied throughout this thesis is in the vertical
cross section of a microchannel, which makes it challenging to experimentally measure the
particle motion in this plane, as it is parallel to the optical axis. This can however be
achieved using defocussing techniques, such as Astigmatism Particle Tracking Velocimetry
(APTV), which derives the position of the particle along the optical axis by its defocused
image. This enabled measurements of streaming dominated particles in the vertical channel
cross section. In order to avoid a free scaling parameter in the theoretical prediction
of the streaming velocity eld, the motion of large radiation force dominated particles was
measured at the same position in the channel, from which the acoustic energy density
in the channel could be derived. With the indirectly measured acoustic energy density,
there was no free parameters left in the theoretical prediction of the motion of the small
streaming-dominated particles. This enabled a quantitative comparison between theory
and experiment of the two-dimensional acoustic streaming velocity eld, which had not
been done before. With reasonable statistics on the experimental measurements, the measurements
and predictions agreed with dierences around 20%, which was a low deviation
given state-of-the-art in the eld. The reconstructed three-dimensional trajectories and
two-dimensional comparison are shown in Fig. 6.3. This was a very satisfying validation
of both the analytical and numerical models, the culmination of all our eorts, and presumably
that which have received the least attention by others.
6.5 Focusing of sub-micrometer particles
The interest in acoustic streaming within the acoustouidic community is mainly due
to the fact that it is an obstruction when trying to focus small particles by use of the
acoustic radiation force. One example of such a particle is the E. coli bacteria, which is
about 1 m in size and responsible for fresh water contamination. It is present in such
small concentrations that an up-concentration step is necessary in order to detect it, and
acoustophoresis is a candidate for this task. To up-concentrate such small particles with