Molecular dynamics simulations of cavitation bubble collapse and sonoluminescence

The dynamics of the medium within a collapsing and rebounding cavitation bubble is investigated by means of molecular dynamics (MD) simulations adopting a hard sphere model for the species inside the bubble. The dynamics of the surrounding liquid (water) is modelled using a Rayleigh-Plesset (RP)-type equation coupled to the bubble interior by the gas pressure at the wall obtained from the MD calculations. Water vapour and vapour chemistry are included in the RP-MD model as well as mass and energy transfer through the bubble wall. The calculations reveal the evolution of temperature, density and pressure within a bubble at conditions typical of single-bubble sonoluminescence and predict how the particle numbers and densities of different vapour dissociation and reaction products in the bubble develop in space and time. Among the parameters varied are the sound pressure amplitude of a sonoluminescence bubble in water, the noble gas mixture in the bubble and the accommodation coefficients for mass and energy exchange through the bubble wall. Simulation particle numbers up to 10 million are used; most calculations, however, are performed with one million particles to save computer run time. Validation of the MD code was done by comparing MD results with solutions obtained by continuum mechanics calculations for the Euler equations.