Particle column formation in a stationary ultrasonic field

Abstract

Acoustic radiation-force theory for noninteracting particles predicts that when a particle suspension is exposed to a stationary ultrasonic field, the particles may become concentrated at half-wavelength intervals. In a plane stationary field, these particle concentrations would have the shape of uniform planar sheets. Gould and Coakley have observed that particles within these sheets often redistribute to form striated columns in the direction of the ultrasonic beam. In this paper, it is shown that particle columns can result from radial nonuniformity of the ultrasonic beam. Theory is presented that describes the equilibrium distribution of particles when subjected to the lowest axially symmetric acoustic mode of a cylindrical waveguide having a pressure-release wall. Particles with higher density and lower compressibility than that of the host medium will form a column along the axis of a wide waveguide but will migrate to the wall of a narrow waveguide. The direction of radial movement for lower density particles is a function of compressibility but not of waveguide diameter. Results of experiments carried out using aqueous polystyrene suspensions in tubes having thin walls and driven at 3 MHz with a transducer designed to excite primarily the lowest axially symmetric mode were consistent with theory.