Microcavitation dynamics of vaporized perfluorocarbon nanodroplets captured with an acoustical camera

Perfluorocarbon nanodroplets are on-demand cavitation nuclei for numerous biomedical applications. These nanodroplets remain in a metastable state until an ultrasound pulse of sufficient negative pressure initiates conversion from a liquid droplet into a gaseous micro- or nano-bubble in a process termed acoustic droplet vaporization (ADV). Extensive research has gone into understanding their interaction with ultrasound at varying acoustic and ambient conditions. Recent studies have detected intra- and post-excitation collapse emissions indicative of inertial cavitation. This study aims to further understand the post-ADV dynamics of perfluorocarbon nanodroplets with an acoustical camera technique. Nanodroplets were activated with a low-frequency ultrasound pulse (5 MHz) while being insonified with a high-frequency (35 MHz) probing wave. The side scattered emissions from the probing wave were captured with another matched high-frequency transducer. The amplitude modulation of these emissions is proportional to the radial strain; thus, a relative radius-time curve can be extracted from the scattered signal after filtering and enveloping. Results show that this technique can capture the radial growth and collapse curves of these newly formed bubbles and aligns well with intra- and post-excitation emissions indicative of inertial cavitation. This approach could be useful to understand post-ADV dynamics of various droplet formulations.