Molecular dynamics simulation study of ultrasound induced cavitation

Cavitation, though widely studied, has aspects that remain poorly understood, especially at the microlevel. This study employs molecular dynamics simulations using LAMMPS to investigate cavitation processes under sinusoidal ultrasonic forces, examining the effects of varying ultrasound pressures and frequencies. Voronoi tessellation was utilized to analyze cavity volume and cavity volume distributions. Our study reveals a two-stage cavitation process characterized by an initial cavity formation phase followed by a rapid growth phase. Higher ultrasound pressures significantly increase cavity volumes and promote transient cavitation, whereas higher frequencies suppress system expansion, leading to steady-state cavitation with smaller cavity sizes. Interestingly, despite variations in the largest molecular volume across different conditions, the cavitation time remains consistently at one-quarter of the ultrasound period. This invariance highlights that the amplitude of ultrasound pressure primarily dictates bubble size, thereby influencing the localized hotspots and micro-jet impacts generated during compression and collapse. Furthermore, as the ultrasound frequency increases, the cavitation time extends notably. At frequencies as high as 20 GHz, the bubble radius does not reach a critical intersection point, represented by 30 % of the maximum bubble radius, due to the high-frequency oscillations. These oscillations hinder full bubble compression after expansion, maintaining higher gas-phase content and suppressing significant cavitation effects. These findings underscore the complex interplay between pressure and frequency in determining cavitation dynamics and their associated microstructural impacts.