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We performed micro-gravity collision experiments in our laboratory drop-towerusing 5-cm-sized dust agglomerates with volume filling factors of 0.3 and 0.4,respectively. This work is an extension of our previous experiments reported inBeitz et al. (2011) to aggregates of more than one order of magnitude highermasses. The dust aggregates consisted of micrometer-sized silica particles andwere macroscopically homogeneous. We measured the coefficient of restitutionfor collision velocities ranging from 1 cm/s to 0.5 m/s, and determined thefragmentation velocity. For low velocities, the coefficient of restitutiondecreases with increasing impact velocity, in contrast to findings by Beitz etal. (2011). At higher velocities, the value of the coefficient of restitutionbecomes constant, before the aggregates break at the onset of fragmentation. Weinterpret the qualitative change in the coefficient of restitution as thetransition from a solid-body-dominated to a granular-medium-dominated behavior.We complement our experiments by molecular dynamics simulations of porousaggregates and obtain a reasonable match to the experimental data. We discussthe importance of our experiments for protoplanetary disks, debris disks, andplanetary rings. The work is an extensional study to previous work of our groupand gives a new insight in the velocity dependency of the coefficient ofrestitution due to improved measurements, better statistics and a theoreticalapproach.

The physics of protoplanetesimal dust agglomerates. VII The low-velocity collision behavior of large dust agglomerates