  Sound can move particles. A good example of this phenomenon is the Chladni
plate, in which an acoustic wave is induced in a metallic plate and particles
migrate to the nodes of the acoustic wave. For several years, acoustophoresis
has been used to manipulate microparticles in microscopic scales. In this fluid
dynamics video, submitted to the 30th Annual Gallery of Fluid Motion, we show
the basic mechanism of the technique and a simple way of visualize it. Since
acoustophoretic phenomena is essentially a three-dimensional effect, we employ
a simple technique to visualize the particles in 3D. The technique is called
Astigmatism Particle Tracking Velocimetry and it consists in the use of
cylindrical lenses to induce a deformation in the particle shape, which will be
then correlated with its distance from the observer. With this method we are
able to dive with the particles and observe in detail particle motion that
would otherwise be missed. The technique not only permits visualization but
also precise quantitative measurements that can be compared with theory and
simulations.
