Molecular dynamics simulation of the response of a gas to a spherical piston: implications for sonoluminescence

Sonoluminescence is the phenomena of light emission from a collapsing gas bubble in a liquid. Theoretical explanations of this extreme energy focusing are controversial and difficult to validate experimentally. We propose to use molecular dynamics simulations of the collapsing gas bubble to clarify the energy focusing mechanism, and determine physical parameters that restrict theories of the light emitting mechanism. In this paper, we model the interior of a collapsing noble gas bubble as a hard sphere gas driven by a spherical piston boundary moving according to the Rayleigh-Plesset equation. We also include a simplified treatment of ionization effects in the gas at high temperatures. The effects of water vapor are neglected in the model. By using fast, tree-based algorithms, we can exactly follow the dynamics of 10(6) particle systems during the collapse. Our preliminary model shows strong energy focusing within the bubble, including the formation of shocks, strong ionization, and temperatures in the range of 50 000-500 000 K. Our calculations show that the gas-liquid boundary interaction has a strong effect on the internal gas dynamics, and that the gas passes through states where the mean free path is greater than the characteristic distance over which the temperature varies. We also estimate the duration of the light pulse from our model, which predicts that it scales linearly with the ambient bubble radius. As the number of particles in a physical sonoluminescing bubble is within the foreseeable capability of molecular dynamics simulations, we also propose that fine scale sonoluminescence experiments can be viewed as excellent test problems for advancing the art of molecular dynamics.