Particle gathering and microstreaming near ultrasonically activated gas-filled micropores

Nonthermal bioeffects of ultrasound can be induced by the physical mechanisms of radiation force and acoustic microstreaming. In this study, microscopical observations of the influence of these mechanisms on small particles near ultrasonically activated gas-filled micropores in thin plastic sheets were compared to approximate theoretical treatments of the particle behavior. The microstreaming flow was localized within a toroidal eddy around an active micropore, as indicated by isopycnic 1-micron polystyrene spheres. The size of the flow pattern seemed to decrease slowly with increasing frequency in the range 0.7-3.5 MHz. Flow was radially inward along the surface, rather than outward as expected from the theory. All the particles tested, which included particles with density less than, equal to, or more than the medium, seemed to accumulate at the micropores. From the theory for the radiation force, the particles less dense than the medium were expected to be repulsed from the micropores. Therefore, the theories for these phenomena, at least to the level of approximation available, may not be completely adequate for the detailed analysis of these phenomena.