Ultrafast dynamics of the acoustic vaporization of phase-change microdroplets

Abstract

Superheated emulsion droplets are a promising tool for localized drug delivery. The physical mechanisms underlying the ultrasound-triggered vaporization of phase-change emulsions are largely unexplored. Here we study the acoustic vaporization of individual micron-sized perfluoropentane droplets at a nanoseconds timescale. The nucleation and growth of the vapor bubbles was imaged at frame rates up to 20 Mfps. The droplet vaporization dynamics was observed to have three distinct regimes: (1) prior to nucleation, a regime of droplet deformation and oscillatory translations; (2) a rapid growth of a vapor bubble enhanced by ultrasound-driven rectified heat transfer; and (3) a final phase characterized by a relatively slow expansion that is fully dominated by heat transfer. A method to measure the moment of inception of the nucleation event with respect to the phase of the ultrasound wave is proposed. A simple physical model captures quantitatively all of the features of the subsequent vapor bubble growth. In addition, we study the role of gas through a model for a vapor-gas bubble, including thermal diffusion and gas diffusion inside the liquid and we find good agreement with the experimental data. We underline the fundamental role of gas diffusion to prevent total recondensation of the bubble at collapse.