Flow generated around particle clusters in a rotating ultrasonic waveguide

A chamber cavity, which has a square cross section and pressure-release walls, is used to produce a well-defined, 160-kHz standing ultrasonic field. A suspension of latex microspheres in aqueous metrizamide fills the chamber. The chamber rotates about a horizontal axis producing the centripetal force necessary to contain the buoyant spheres in the axial region. At low particle concentrations, clusters of microspheres form at half-wavelength intervals near the axial positions of acoustic pressure amplitude (p0) minima, as expected because of rotational and acoustic radiation forces. At higher concentrations, additional particle distributions are often seen that suggest the presence of flow. When high concentrations of larger particles are used, small clusters also form at axial positions of p0 maxima. Theory for acoustic streaming in a rotating fluid predicts flow speeds that are too small to account for the observed flow. Reasonable agreement with observations is obtained using a theory for flow generated by the buoyant gravitational force acting on the clusters.