Effects of gas-liquid phase transitions on soundwave propagation: A molecular dynamics study

TL;DR

This study uses molecular dynamics simulations to analyze how gas-liquid phase transitions affect soundwave propagation, revealing different behaviors in first-order and continuous transitions and identifying critical anomalies.

Contribution

It provides the first direct MD simulation observations of bubble interactions with soundwaves and distinguishes effects of different phase transition types on acoustic properties.

Findings

Waveforms change discontinuously in first-order transitions.

Waveforms vary continuously near the critical point.

Attenuation parameters diverge in the continuous transition region.

Abstract

To understand ultrasonic cavitation, it is imperative to analyze the effects of the gas-liquid phase transitions on soundwave propagation. Since current methods based on fluid dynamics offer limited information, it is imperative to carry out further research on this phenomenon. In this study, we investigated the effects of cavitation and near-critical fluid on soundwaves using the molecular dynamics (MD) simulations of Lennard-Jones fluids. In the first-order liquid-to-gas transition region (far from the critical point), the waveform does not continuously change with the temperature and source oscillation amplitude owing to the discontinuous change in the density due to the phase transition. Meanwhile, in the continuous transition region (crossing near the critical point), the waveform continuously varies with temperature regardless of the amplitudes because phase separation is not involved in this region. The density fluctuations increase as the amplitude increases; however, it does not affect the waveform. Thus, we clarified that the first-order and continuous transitions have different impacts on soundwaves. Moreover, we determined the acoustic characteristics, such as attenuation and nonlinear parameters, by comparing the results of the numerical solution of Burgers' equation and MD simulation. Burgers' equation clearly describes the soundwave phenomenon until phase separation or bubble formation occurs. In the continuous transition region, the attenuation parameters tend to diverge, reflecting a critical anomaly trend. We observed the bubbles move forward with the oscillation of their radii owing to their interaction with the soundwaves. This is the first direct observation of the interaction using MD simulations.