Abstract
This thesis presents studies of boundary-driven acoustic streaming in microuidic channels,
which is a steady ow of the uid initiated by the interactions of an oscillating acoustic
standing wave and the rigid walls of the microchannel. The studies present analysis of the
acoustic resonance, the acoustic streaming ow, and the forces on suspended microparticles.
The work is motivated by the application of particle focusing by acoustic radiation forces in
medical, environmental and food sciences. Here acoustic streaming is most often unwanted,
because it limits the focusability of particles smaller than a given critical size. One of the
main goals of this thesis work has been to overcome this limitation.
The main text of this thesis serves to give an introduction to the theory and numerical
models applied in the ve journal papers supplied in the Appendixes, which constitute this
thesis work.
Based on rst- and second-order perturbation theory, assuming small acoustic amplitudes,
we derived the time-dependent governing equations under adiabatic conditions.
The adiabatic rst- and second-order equations are solved analytically for the acoustic eld
between two orthogonally oscillating plates. Furthermore, under general thermodynamic
conditions, we derive the time-dependent rst- and second-order equations for the conservation
of mass, momentum, and energy. The coupling from uid equations to particle
motion is achieved through the expressions for the streaming-induced drag force and the
acoustic radiation force acting on particles suspended in the uid. Lastly, the numerical
method is discussed, with emphasis on how proper numerical convergence is ensured.
Three numerical studies are presented, in which the acoustic resonance and the acoustic
streaming ow are investigated, both in the transient regime and in the purely periodic
state. The solutions for the periodic acoustic resonance and the steady streaming ow
are used to simulate the motion of suspended particle in a Lagrangian description, which
mimics experimental particle tracking velocimetry.
In the forth study, the numerical model is used to engineer a single roll streaming ow,
which does not counteract the focusing by the acoustic radiation force, contrary to the
usual quadrupolar streaming ow. The single roll streaming ow is observed experimentally
in a nearly-square channel, and acoustophoretic focusing of E. coli bacteria and 0.6 µm
particles is achieved. These particles are considerably smaller than the critical particle size
of approximately 2 µm for the usual half-wavelength resonance in a rectangular channel.
The fth study presents a quantitative comparison of analytical, numerical, and experimental
results for the streaming-induced drag force dominated motion of particles
suspended in a water-lled microchannel supporting a transverse half-wavelength resoiii