Impact-Induced Energy Transfer and Dissipation in Granular Clusters under Microgravity Conditions

The impact-induced energy transfer and dissipation in granular targets without any confining walls are studied by microgravity experiments. A solid projectile impacts into a granular target at low impact speed ($0.045 \leq v_p \leq 1.6$~m~s$^{-1}$) in a laboratory drop tower. Granular clusters consisting of soft or hard particles are used as targets. Porous dust agglomerates and glass beads are used for soft and hard particles, respectively. The expansion of the granular target cluster is recorded by a high-speed camera. Using the experimental data, we find that (i)~a simple energy scaling can explain the energy transfer in both, soft- and hard-particles granular targets, (ii)~the kinetic impact energy is isotropically transferred to the target from the impact point, and (iii)~the transferred kinetic energy is $2$~-~$7$\% of the projectile's initial kinetic energy. The dissipative-diffusion model of energy transfer can quantitatively explain these behaviors.