Initial growth and coalescence of acoustically vaporized perfluorocarbon microdroplets

Acoustic droplet vaporization (ADV) is a technique whereby liquid droplets are vaporized into gas bubbles using ultrasound. This process and the resulting bubbles have been proposed for embolization, drug delivery, aberration correction, and bubble-enhanced high intensity focused ultrasound. To increase the efficacy of these applications, high-speed photography was used to study the initial phase-transition process. One-hundred and seven albumin-stabilized dodecafluoropentane droplets with diameters ranging from 3 to 20 mum were vaporized in a 100 mum inner-diameter polyethylene tube. Sixteen optical full-frame images and an optical streak image were obtained to record the vaporization, using a water immersion microscope (12 pixels per micron resolution). Framing rates were up to 13 MHz and streak speeds were up to 64 lines per microsecond. First, the impact of two- versus thirteen-cycle vaporization pulses was analyzed. It was found that with a two-cycle vaporization pulse only a portion of the droplet phase transitioned, whereas with a thirteen-cycle vaporization pulse the entire droplet phase transitioned. Using thirteen-cycle vaporization pulses, it was then observed that the bubbles all grew to approximately the same diameter within the first 2 mus. It was additionally observed that as neighboring bubbles grew in the first 15 mus, they could coalesce.