Jetting of acoustic cavitation bubbles during microdroplet vaporization*

Acoustic droplet vaporization (ADV) is a phase-change process where metastable micron- and sub-micron-sized droplets are converted to microbubbles upon application of high-intensity, high-frequency ultrasound. ADV can be leveraged in several biomedical applications such as gas embolotherapy, ablation techniques such as histotripsy and thermal ablation, and targeted drug delivery. By performing ultra-high-speed video-microscopy, we characterize the dynamics of the initially nucleated vapor bubbles whose asymmetric collapse can lead to the formation of high-speed jets. Jetting is found to occur due to three distinct origins, namely - (i) the pressure gradient arising from the acoustic field in the droplet; (ii) the interaction of the bubble with the droplet interface; and (iii) the interaction between vapor bubble pairs nucleated at different phases of acoustic excitation. We combine experimental results with numerical simulations and theoretical modeling to determine the dependence of jetting on parameters related to the bubble dynamics, acoustic driving, and spatiotemporal coordinates. Doing so, we aim at predicting the direction and intensity of bubble jetting and comment on its possible contribution to sonoporation in biomedical applications.