1.3. HISTORIC REVIEW 3
Figure 1.2: Forces on particles in a standing acoustic wave. Fdrag is the drag force (dark green arrows)
on the particle induced by the generated acoustic streaming ow (light green arrows), and Frad is the
acoustic radiation force (dark red arrows) induced by the scattering of the acoustic wave on the particle.
The light red arrows indicate the directionality of the radiation force, working to focus the particles at the
node of the standing pressure wave (magenta lines). In the left half of the channel examples of a large
particle are shown, and in the right side of the channel examples of a small particle are shown. The two
arrows on each particle indicate the relative magnitude of the two forces on that particle. The length of
the arrows are scaled dierently on the large and the small particles.