Failed Growth at the Bouncing Barrier in Planetesimal Formation

TL;DR

This study experimentally investigates dust aggregate collisions, revealing that growth is temporary and aggregates tend to bounce apart, supporting the idea of a bouncing barrier in planetesimal formation.

Contribution

It provides detailed experimental evidence that aggregate collisions lead to temporary clustering and eventual detachment, reinforcing the bouncing barrier concept.

Findings

Aggregates stick with 20% probability during collisions.

Clusters are fragile and detach after subsequent collisions.

Growth is temporary, supporting the bouncing barrier hypothesis.

Abstract

In laboratory experiments, we studied collisions of ensembles of compact (filling factor 0.33) millimeter dust aggregates composed of micrometer quartz grains. We used cylindrical aggregates, triangular aggregates, square aggregates, and rectangular aggregates. Ensembles of equal size aggregates as well as ensembles with embedded larger aggregates were studied. The typical collision velocities are 10-20 mm $\rm s^{-1}$. High spatial and temporal resolution imaging unambiguously shows that individual collisions lead to sticking with a high probability of 20 percent. This leads to connected clusters of aggregates. The contact areas between two aggregates increase with collision velocity. However, this cluster growth is only temporary, as subsequent collisions of aggregates and clusters eventually lead to the detachment of all aggregates from a cluster. The contacts are very fragile as aggregates cannot be compressed further or fragment under our experimental conditions to enhance the contact stability. Therefore, the evolution of the ensemble always leads back to a distribution of individual aggregates of initial size. This supports and extends earlier experiments showing that a bouncing barrier in planetesimal formation would be robust against shape and size variations.