Effects of vapor-liquid phase transitions on sound-wave propagation: A molecular dynamics study

Y Asano and H Watanabe and H Noguchi, PHYSICAL REVIEW FLUIDS, 7, 064302 (2022).

DOI: 10.1103/PhysRevFluids.7.064302

To understand ultrasonic cavitation, it is imperative to analyze the effects of the vaporliquid phase transitions on sound-wave 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 sound waves using the molecular dynamics (MD) simulations of Lennard-Jones fluids. In the first-order liquid-to-vapor 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 sound waves. 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 sound-wave 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 sound waves. To the best of our knowledge, this is the first direct observation of the interaction using MD simulations.

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