Action of microbubbles when insonified: experimental evidence

Primary and secondary Bjerknes forces result from pressure gradients in the incident and scattered ultrasonic fields. These forces and their dependence on experimental parameters are described. Both primary and secondary Bjerknes forces are shown to have a significant effect on the flow of microbubbles through a small vessel during insonation. The primary Bjerknes force produces displacement of microspheres across a 100 micron vessel radius for a small transmitted acoustic power. The secondary Bjerknes force produces a reversible attraction and aggregation of microspheres with a significant attraction over a distance of approximately 100 microns. The magnitude of the secondary Bjerknes force is proportional to the inverse of the squared separation distance, and thus two aggregates accelerate as they approach one another