Lowering of acoustic droplet vaporization threshold via aggregation

Acoustically sensitive emulsion nanodroplets composed of perfluorocarbon have shown great potential for advanced medical diagnosis and therapy but are limited by the required high acoustic droplet vaporization (ADV) threshold for clinical applications. This study investigates the use of an ultrasonic standing wave to lower the ADV threshold while maintaining the generated bubble size in the required size range, ensuring the generation of inertial cavitation and corresponding physical effects. The results showed that disperse nanodroplets were manipulated to form micron-sized aggregations, and the required ADV threshold was significantly lowered, while a similar size range of the microbubbles generated by disperse nanodroplets was maintained. The threshold could be further regulated by adjusting the aggregation size via controlling the concentration of the disperse nanodroplets. Furthermore, the internal pressures in the aggregations with different sizes were calculated to determine their ADV thresholds theoretically, which were shown to be in good agreement with the experimental results.