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.

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Up to now the numerical calculations for the converging shock or compression wave model of sonoluminescence have been based primarily on continuum methods. Here an alternative approach by molecular dynamics simulation is shown to be feasible with today’s computer memory and speed. Both approaches are compared. Results of the simulations are presented for the scaling behavior with the number of particles, for different boundary conditions (spherical and ellipsoidal bubbles) and for one and more species in the bubble. The dependence of the light intensity and pulse duration on the sphericity of the bubble is discussed. [Work supported by the Graduiertenkolleg Stroemungsinstabilitäten und Turbulenz.]

Bubbles in a standing sound field can be trapped at a pressure antinode and driven to strongly nonlinear oscillations with fast collapse, whereby shock waves and also faint, short light pulses are emitted. The physical and chemical processes in the interior of the bubbles associated with these phenomena are still not completely understood and also defy spatially resolved experimental investigation. Numerical modeling primarily relies on continuum methods by solving partial differential equations. Here an alternative approach is presented. The processes within collapsing sonoluminescing bubbles are investigated by molecular dynamics (MD) simulations with several million particles.

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