Effect of shell on the stability, radial dynamics, and acoustic characteristics of phase-shift droplets

Shell-stabilized, phase-shift droplets of a perfluorocarbon liquid have been investigated for a wide range of applications. The shell plays a key role in maintaining droplet stability and impacting the acoustic responses of generated bubbles via acoustic droplet vaporization (ADV). While shell behavior under ultrasound exposure is well-studied with contrast microbubbles, it is not fully explored for phase-shift droplets during and post ADV. By integrating ultra-high-speed microscopy and time-lapse confocal imaging, we investigated the radial dynamics during ADV and the growth rate post-ADV for monodisperse, micron-sized perfluorohexane droplets with three different shells: lipid, polymer, and albumin. Droplet size distribution was not significantly different for each formulation for up to three weeks. The expansion ratio during ADV was significantly lower (∼26%) for lipid-coated droplets compared to the other shells under similar acoustic parameters (2.5 MHz and 5 MPa). The growth rate post-ADV, recorded up to 2 min, showed an initial fast rate constant followed by a slow rate constant for all ADV-bubbles, which were significantly different among the droplets, highlighting the effect of shell. We will also discuss the acoustic characteristics of droplets with varying shell formulation. The findings will help optimize shell composition, benefiting both diagnostic and therapeutic applications of ADV.