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Large scale molecular dynamics (MD) simulations are performed to study and tomodel the ejecta production from the dynamic fragmentation of shock-loadedmetals under melt conditions. A generic 3D crystal in contact with vacuumcontaining about 10^8 atoms and with a sinusoidal free surface roughness isshock loaded so as to undergo a solid-liquid phase change on shock. Thereflection of the shock wave at the interface metal/vacuum gives rise to theejection of 2D jets/sheets of atoms (Richtmyer-Meshkov instabilities in thecontinuum limit) which develop and break up, forming ejecta (fragments) ofdifferent volumes (or mass). The fragmentation process is investigated byanalyzing the evolution of the resulting volume distribution of the ejecta as afunction of time. Two metals are studied (Cu and Sn) and the amplitude of theroughness is varied. The simulations show that the associated distributionsexhibit a generic behavior with the sum of two distinct terms of varyingweight, following the expansion rate of the jets: in the small size limit, thedistribution obeys a power law dependence with an exponent equal to 1.15 plusor minus 0.08 and in the large size limit, it obeys an exponential form. Thesetwo components are interpreted, with the help of additional simple simulations,as the signature of two different basic mechanisms of fragmentation. The powerlaw dependence results from the fragmentation of a 2D network of ligamentsarranged following a fractal (scale free) geometry and generated when thesheets of liquid metal expand and tear. The exponential distribution resultsfrom a 1D Poisson fragmentation process of the largest ligaments previouslygenerated. Unlike the power law distribution, it is governed by acharacteristic length scale which may be provided by energy balance principle.

Power law and exponential ejecta size distributions from the dynamic fragmentation of shock-loaded Cu and Sn metals under melt conditions