Free Collisions in a Microgravity Many-Particle Experiment. IV. - Three-Dimensional Analysis of Collision Properties

TL;DR

This study presents a three-dimensional analysis of dust particle collisions in microgravity, revealing rare sticking events that could influence planet formation models, and compares 2D and 3D collision data accuracy.

Contribution

It provides the first detailed 3D collision analysis of dust aggregates in microgravity, highlighting the importance of 3D data for accurate collision parameter assessment.

Findings

Four out of 52 collisions resulted in sticking.

Some sticking occurred at velocities previously predicted to cause bouncing.

3D analysis shows 2D projections are generally reliable for velocity but not for impact parameters.

Abstract

The bouncing barrier, a parameter combination at which dust particles in the protoplanetary disk always rebound in mutual collisions, is one of the crucial steps of planet formation. In the past years, several experiments have been performed to determine the mass and velocity regimes at which perfect bouncing does occur and those where there is a chance of the aggregates sticking together. We conducted a microgravity experiment, which allows us to investigate free collisions of millimeter-sized SiO2 dust aggregates at the relevant velocities. We analyzed 52 collisions in detail with velocities of 3.4 mm/s to 6.2 cm/s and found four of them leading to sticking, while the other aggregates rebounded. Three out of the four sticking collisions occurred at velocities where previously only bouncing had been predicted. Although the probability for sticking is low, this opens a new possibility for growth beyond millimeter sizes. Our setup allowed us to obtain the complete three-dimensional collision information. Since most previous experiments were interpreted based on two-dimensional information, we compare our three-dimensional values with those obtained if only one projection had been available. We find that the error of a two-dimensional analysis of the collision velocity is very small. The distribution of the coefficient of restitution in the two-dimensional view is representative of the real case, but for any given collision its value can be far off. Impact parameters always have to be analyzed three-dimensionally, because the two-dimensional values are not meaningful in any way.