Theoretical criterion for acoustic aggregation.

Theory was given previously for the radiation force between two spherical particles immersed in a fluid and subjected to a sound field. From this theory an expression has been obtained for the effective energy of interaction. A minimum energy, defining a "binding energy" Wmin, occurs when the particles are in contact and aligned with their line of centers perpendicular to the oscillatory motion in the local sound field. When Wmin is large enough, dimers are formed which combine to build larger aggregates. A criterion for aggregation was obtained, similar to one arrived at by Schwan for aggregation produced by alternating electric fields, by setting Wmin equal to the thermal energy kT. Calculations suggest that aggregation of blood cells may occur during applications of medical ultrasound under some conditions. Experiments are needed to test possibilities.