Microparticle concentration in short path length ultrasonic resonators: Roles of radiation pressure and acoustic streaming

Abstract

Acoustic streaming in ultrasonic (1.4–3.0 MHz) circular and rectangular resonators of path length approximately one-half or one quarter wavelength (λ) has been characterized by particle image velocimetry (PIV) using fluorescent 1 μm diam latex markers. Particles of all diameters examined (1, 24, 80 μm) moved into pressure node planes within 4 s of initiation of sonication. The larger particles then moved within that plane to one or more preferred positions. 1 μm particles in a λ/2 cylindrical resonator with a single nodal concentration region for larger particles were convected by Rayleigh-type streaming from the center of the node plane to its edge. In contrast, particles concentrated at many loci in two planes of a second cylindrical and a rectangular chamber. Small scale wall-associated Rayleigh-type vortices occurred in a λ/4 chamber. More unexpectedly, wall-independent bulk suspension vortices, with circulation planes parallel to the transducer radiating surface, were recorded in both resonators. Tracer particles experienced radial forces that drove them towards or away from the center of the vortices to be concentrated at its center or entrained in a vortex perimeter ring. These different outcomes are discussed in terms of lateral radiation force distribution in the node planes.