  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.
