Trapping of solid particles by cavitation-induced acoustic streaming

Cavitation clouds can be generated by short, high-amplitude, focused ultrasound pulses such as those used in histotripsy. This cavitation activity can be used to noninvasively eliminate occlusive blood clots. It has also been observed in-vitro that free flowing thromboemboli in the blood are attracted to and eroded by the cavitation cloud. Such phenomenon may be useful to prevent embolism during cardiovascular interventions. This study investigated the mechanism of solid particle trapping by the presence of a cavitation cloud in a vessel. A vessel phantom was constructed using gelatin gel and a peristaltic pump provided flow through the vessel lumen. Spherical agarose beads were used to mimic clot particles in the experiment. A 1-MHz focused transducer was used to generate a cavitation cloud in the vessel lumen. High-speed photography and particle image velocimetry were used to measure the fluid flow in the phantom generated by acoustic streaming and the interaction of the flow with agarose particles. Acoustic streaming velocities as high as 1 m/s were generated at the focus in the vessel when cavitation was observed. A high-velocity fluid jet was generated through the focus, along the direction of propagation, with fluid vortices generated to either side of the focus. Particles were generally trapped near the edge of the cavitation cloud in the vortex. Estimation of the fluidic pressure field indicated a negative pressure gradient towards the area around the focus. These results suggest that the streaming creates a region of low pressure near the focus, which effectively traps the particle and provides a counter-force to the drag caused by the blood flow sweeping the particle downstream.