  Ultrasound-driven oscillating micro-bubbles have been used as active
actuators in microfluidic devices to perform manifold tasks such as mixing,
sorting and manipulation of microparticles. A common configuration consists on
side-bubbles, created by trapping air pockets in blind channels perpendicular
to the main channel direction. This configuration consists of acoustically
excited bubbles with a semi-cylindrical shape that generate significant
streaming flow. Due to the geometry of the channels, such flows have been
generally considered as quasi two-dimensional. Similar assumptions are often
made in many other microfluidic systems based on \emph{flat} micro-channels.
However, in this paper we show that microparticle trajectories actually present
a much richer behavior, with particularly strong out-of-plane dynamics in
regions close to the microbubble interface. Using Astigmatism Particle Tracking
Velocimetry, we reveal that the apparent planar streamlines are actually
projections of a \emph{streamsurface} with a pseudo-toroidal shape. We
therefore show that acoustic streaming cannot generally be assumed as a
two-dimensional phenomenon in confined systems. The results have crucial
consequences for most of the applications involving acoustic streaming as
particle trapping, sorting and mixing.
