Influence of acoustofluidic parameters on velocity streaming of sonicated medical microbubbles

Ultrasonic theranostics using echogenic bubbles allow real-time traceability and site-specific, on demand medicine delivery. Ultrasonic manipulation of microbubbles is studied through illustrating the effect of acoustofluidic properties on flow and interaction of bubble populations. Significant displacement and clustering of medical microbubbles are observed for excitations resulting in acoustic pressures as low as 6 kPa. Microbubble displacements are found to be negligible at 4 kPa. The measured velocities show good agreement with analytical calculations at pressure levels below the threshold of non-linear bubble behaviour. Higher pressures caused faster bubble movement and higher frequencies resulted in faster cluster formation. An extreme effect is also reported where the secondary radiation force overcomes the primary radiation force, which results in bubbles moving towards the acoustic source