High-frequency acoustic droplet vaporization is initiated by resonance

TL;DR

This paper demonstrates that acoustic resonance, caused by sound speed mismatch, significantly enhances vaporization of low-boiling droplets, with experimental evidence showing size-dependent nucleation driven by pressure amplification.

Contribution

It reveals a novel resonance mechanism governing droplet vaporization, linking size, frequency, and pressure amplification, supported by high-speed imaging and theoretical analysis.

Findings

Resonance occurs at a specific size-frequency relationship (kR~0.65).

Pressure amplification inside droplets increases nucleation rates by orders of magnitude.

Vaporization is triggered only when droplets match the resonance size.

Abstract

Vaporization of low-boiling point droplets has numerous applications in combustion, process engineering and in recent years, in clinical medicine. However, the physical mechanisms governing the phase conversion are only partly explained. Here, we show that an acoustic resonance can arise from the large speed of sound mismatch between a perfluorocarbon microdroplet and its surroundings. The fundamental resonance mode obeys a unique relationship $kR$~$\sim$ 0.65 between droplet size and driving frequency that leads to a 3-fold pressure amplification inside the droplet. Classical nucleation theory shows that this pressure amplification increases the nucleation rate by several orders of magnitude. These findings are confirmed by high-speed imaging performed at a timescale of ten nanoseconds. The optical recordings demonstrate that droplets exposed to intense acoustic waves generated by inter-digital-transducers nucleate only if they match the theoretical resonance size.