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The current model of planet formation lacks a good understanding of thegrowth of dust particles inside the protoplanetary disk beyond mm sizes. Inorder to investigate the low-velocity collisions between this type ofparticles, the NanoRocks experiment was flown on the International SpaceStation (ISS) between September 2014 and March 2016. We present the results ofthis experiment. We quantify the damping of energy in systems of multipleparticles in the 0.1 to 1 mm size range while they are in the bouncing regime,and study the formation of clusters through sticking collisions betweenparticles. We developed statistical methods for the analysis of the largequantity of collision data collected by the experiment. We measured the averagemotion of particles, the moment of clustering, and the cluster size formed. Inaddition, we ran simple numerical simulations in order to validate ourmeasurements. We computed the average coefficient of restitution (COR) ofcollisions and find values ranging from 0.55 for systems including a populationof fine grains to 0.94 for systems of denser particles. We also measured thesticking threshold velocities and find values around 1 cm/s, consistent withthe current dust collision models based on independently collected experimentaldata. Our findings have the following implications that can be useful for thesimulation of particles in PPDs and planetary rings: (1) The average COR ofcollisions between same-sized free-floating particles at low speeds (< 2 cm/s)is not dependent on the collision velocity; (2) The simplified approach ofusing a constant COR value will accurately reproduce the average behavior of aparticle system during collisional cooling; (3) At speeds below 5 mm/s, theinfluence of particle rotation becomes apparent on the collision behavior; (4)Current dust collision models predicting sticking thresholds are robust.

Multi-Particle Collisions in Microgravity: Coefficient of Restitution and Sticking Threshold for Systems of Mm-Sized Particles