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Highlights • A numerical method is presented for the acoustic vaporization threshold of microdroplets by improving the Rayleigh-Plesset equation to properly treat the supercritical state occurring at rapid bubble collapse. • The van der Waals equation of state is employed to more accurately consider the supercritical state instead of the ideal gas equation. • The effective ADV threshold is observed to increase as the acoustic frequency increases and the droplet radius decreases, as experimentally observed in the previous studies. • The present numerical predictions of ADV threshold comparable to the experimental data., A numerical model is presented for the acoustic vaporization threshold of a dodecafluoropentane (or perfluoropentane) microdroplet. The model is based on the Rayleigh-Plesset equation and is improved by properly treating the supercritical state that occurs when a bubble collapses rapidly and by employing the van der Waals equation of state to consider the supercritical state. The present computations demonstrate that the microdroplet vaporization behavior depends intricately on bubble compressibility, liquid inertia and phase-change heat transfer under acoustic excitation conditions. We present acoustic pressure-frequency diagrams for bubble growth regimes and the ADV threshold conditions. The effects of acoustic parameters, fluid properties and the droplet radius on the ADV threshold are investigated.

In acoustic droplet vaporization (ADV), a cavitated bubble grows and collapses depending on the pressure amplitude of the acoustic pulse. During the bubble collapse, the surrounding liquid is compressed to high pressure, and liquid compressibility can have a significant impact on bubble behavior and ADV threshold. In this work, a one-dimensional numerical model considering liquid compressibility is presented for ADV of a volatile microdroplet, extending our previous Rayleigh-Plesset based model [Ultrason. Chem. 71 (2021) 105361]. The numerical results for bubble motion and liquid energy change in ADV show that the liquid compressibility highly inhibits bubble growth during bubble collapse and rebound, especially under high acoustic frequency conditions. The liquid compressibility effect on the ADV threshold is quantified with varying acoustic frequencies and amplitudes.

Acoustically-triggered droplet vaporization (ADV) in a tube is numerically studied by extending a level-set method to determine the bubble-droplet interface as well as the droplet-ambient liquid interface and to include droplet evaporation and bubble compressibility effects. The Rayleigh-Plesset equation for ADV in a tube is improved by correctly evaluating the transition from spherical to tube flows. The computations of ADV show that the liquid pressure variation along the tube caused by droplet vaporization and bubble condensation is an important factor determining bubble survival and growth. The influence of tube radius on bubble growth in ADV is quantified in various acoustic amplitude and frequency conditions. Computations are also performed to investigate bubble-bubble interaction in acoustic vaporization of two droplets.