Ejecta from liquid gallium during planar impact experiments

When a solid or liquid metal surface is subject to shock loading, ejecta may be produced by two separate phenomena: growth of surface asperities from Richtmyer-Meshkov instability (RMI); and cavitation (micro-spall) at the surface from shock release if the material is liquid or shock-melts. In the present study we shock-loaded liquid gallium to modest pressures (4.5 GPa). The liquid gallium was sealed in a steel capsule and impacted with an explosively-drive steel flyer plate. Thickness of the gallium sample was varied to change the shock loading from a square pulse (supported) to an unsupported triangular pulse. Two surface conditions were also considered: a clean liquid surface, and an oxidized surface. Gallium free surface and ejecta cloud velocities were recorded using photonic Doppler velocimetry. Ejecta mass flux was measured using piezoelectric pins. Ejecta mass flux versus ejecta cloud velocity was extracted from integration of the pin voltage response assuming inelastic collision. The oxidized samples generated high-speed ejecta (uejecta/usurface > 1.5), indicative of RMI-driven jetting of asperities. The samples with liquid surfaces generated only low-speed ejecta (uejecta/usurface < 1.5), indicative of only micro-spall. For the oxidized samples, the unsupported shock case generated significantly more, higher-speed, ejecta compared to the supported shock case. In contrast, there was little influence of shock support on the mass flux profile for the clean liquid surface cases.