The role of inertial cavitation in acoustic droplet vaporization

The vaporization of a superheated droplet emulsion into gas bubbles using ultrasound-termed acoustic droplet vaporization (ADV)-has potential therapeutic applications in embolotherapy and drug delivery. The optimization of ADV for therapeutic applications can be enhanced by understanding the physical mechanisms underlying ADV, which are currently not clearly elucidated. Acoustic cavitation is one possible mechanism. This paper investigates the relationship between the ADV and inertial cavitation (IC) thresholds by studying parameters that are known to influence the IC threshold. These parameters include bulk fluid properties such as gas saturation, viscosity, and surface tension. Additionally, the degree of superheat of the emulsion dispersed phase is also varied. In all cases the ADV threshold occurred at a lower rarefactional pressure than the IC threshold (p< 0.01) indicating that the phase-transition occurs before IC events. Both the ADV (p=0.7) and IC (p=0.4) thresholds were not statistically different for degassed and gas saturated conditions. The viscosity and surface tension are shown to influence both thresholds directly and inversely, respectively. Additionally, both thresholds display an inverse temperature dependence when the droplet is not superheated. Based on a choice of experimental parameters, it is possible to achieve ADV with or without IC.