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 ADV and inertial cavitation (IC) thresholds (measured as peak rarefactional pressures) by studying parameters that are known to influence the IC threshold. These parameters include bulk fluid properties such as gas saturation, temperature, viscosity, and surface tension; droplet parameters such as degree of superheat, surfactant type, and size; and acoustic properties such as pulse repetition frequency and pulse width. In all cases the ADV threshold occurred at a lower rarefactional pressure than the IC threshold, indicating that the phase transition occurs before IC events. The viscosity and temperature of the bulk fluid are shown to influence both thresholds directly and inversely, respectively. An inverse trend is observed between threshold and diameter for droplets in the 1 to 2.5 µm range. Based on a choice of experimental parameters, it is possible to achieve ADV with or without IC.

I. Introduction

Emulsions containing micron- and nanometer-sized perfluorocarbon (PFC) droplets are being studied in diagnostic and therapeutic applications of ultrasound (US). For example, the use of PFC nanodroplets has been explored in US molecular imaging and the targeted delivery of therapeutic agents [1]–[4]. Another research area is acoustic droplet vaporization (ADV), where a superheated micron-sized liquid droplet—stabilized by a surfactant shell—is phase transitioned into a gas bubble using US. The superheated liquid is a straight-chain PFC, which belongs to a class of compounds that are inert, hydrophobic, lipophobic, and biocompatible [5]. The therapeutic potentials of ADV were described by Apfel [6] and Kripfgans et al. [7] in applications such as embolotherapy and drug delivery. Additional applications of ADV include its use in phase aberration correction [8], [9].