Multi-particle collisions in microgravity: Coefficient of restitution and sticking threshold for systems of mm-sized particles

Abstract

Context. The current model of planet formation lacks a good understanding of the growth of dust particles inside the protoplanetary disk beyond mm sizes. A similar collisional regime exists in dense planetary rings. In order to investigate the low-velocity collisions between this type of particles, the NanoRocks experiment was flown on the International Space Station (ISS) between September 2014 and March 2016. We present the results of this experiment. Aims. The objectives of our data analysis are the quantification of the damping of energy in systems of multiple particles in the 0.1 to 1 mm size range while they are in the bouncing regime, and the study of the formation of clusters through sticking collisions between particles. Methods. We developed statistical methods for the analysis of the large quantity of collision data collected by the experiment. We measured the average motion of particles, the moment of clustering, and the cluster size formed. In addition, we ran simple numerical simulations in order to validate our measurements. Results. We computed the average coefficient of restitution (COR) of collisions and find values ranging from 0.55 for systems including a population of fine grains to 0.94 for systems of denser particles. We also measured the sticking threshold velocities and find values around 1 cm/s, consistent with the current dust collision models based on independently collected experimental data. Conclusions. Our findings have the following implications that can be useful for the simulation of particles in PPDs and planetary rings: (1) The average COR of collisions between same-sized free-floating particles at low speeds (< 2 cm/s) is not dependent on the collision velocity; (2) The simplified approach of using a constant COR value will accurately reproduce the average behavior of a particle system during collisional cooling; (3) At speeds below 5 mm/s, the influence of particle rotation becomes apparent on the collision behavior; (4) Current dust collision models predicting sticking thresholds are robust.

Near-zero cohesion and loose packing of Bennu’s near subsurface revealed by spacecraft contact

“…The bulk shape of Bennu, including some long-running ridges ( 4 ), suggests the existence of some cohesion or strength to provide structural shape, but with relatively low values of cohesion less than ~1 Pa ( 4 , 23 ). These values are consistent with expectations derived from numerical modeling for loosely packed cohesive grains of 100 μm and larger ( 24 , 25 ) but is less than some experimental measures with fine grains ( 26 , 27 ) and some inferences of 3 to 300 Pa for other rubble-pile asteroids ( 14 , 24 , 28 ). Bennu, unlike other small asteroids such as Eros and Itokawa ( 29 ), has no extensive ponded deposits of fine particles ( 5 ), and spectral and thermal studies have indicated minimal coverage by dust at the surface ( 17 , 30 ).…”

Predictions for the Dynamical States of the Didymos System before and after the Planned DART Impact

“…If this is the case, Didymos's surface and structure should possess proper conditions that can resist disturbance by the falling of DART-driven ejecta to avoid complete collapse. The key issue, however, is that granular behaviors and slow impact processes in microgravity are likely different from those observed on a terrestrial surface (Garcia et al 2015;Murdoch et al 2017;Brisset et al 2019;Sánchez & Scheeres 2021). Therefore, such disturbing processes could act differently on Didymos, which needs further quantitative assessment.…”

Size Dependence of the Bouncing Barrier in Protoplanetary Dust Growth

“…Laboratory experiments have reported that low-velocity collisions of dust aggregates result in bouncing rather than sticking under certain conditions (e.g., Langkowski et al 2008;Beitz et al 2011;Jankowski et al 2012;Schräpler et al 2012Schräpler et al , 2022Weidling et al 2012;Kothe et al 2013;Kelling et al 2014;Brisset et al 2016Brisset et al , 2017Brisset et al , 2019. The bouncing behavior has also been reported in earlier numerical simulations using soft-sphere discrete element methods (e.g., Wada et al 2011;Schräpler et al 2012;Seizinger & Kley 2013;Osinsky & Brilliantov 2022).…”