Time dependent boundary conditions for large scale atomistic simulations of shocked surface instabilities*

Shock induced surface instabilities such as Richtmyer-Meshkov instabilities at perturbed metal vacuum/gas interfaces result in metal material ejecta. For strong shock waves the material ejected is initially in the form of fluid sheets when the surface perturbation is two-dimensional. These sheets ultimately break up to form a distribution of droplets. Large-scale non-equilibrium molecular dynamics simulations of such instabilities allow the investigation of the dynamics of the breakup process but are limited in length and time scales by rarefaction wave reflections at the boundaries ultimately leading to spall that may affect the instability growth. A time-dependent boundary condition based on the self-similar character of the release wave is presented that mitigates boundary reflections and reduces unwanted tensile waves behind the perturbed interface zone. This boundary condition can be used to increase the wavelength and time scale for breakup simulations. We discuss the details of this method and results of NEMD simulations of a shocked Cu interface with a single mode perturbation characterized by a wavelength of λ= 13 nm and a wavenumber amplitude product kh₀ = 1/2.