3D Structure and Stability of Particle Clusters close to the Bouncing Barrier &#8211; New Experiments
Authors: Wenders Nico , Joeris Kolja , Teiser Jens 
Published: July 2024

3D Structure and Stability of Particle Clusters close to the Bouncing Barrier &#8211; New Experiments&#160;Collisions of particle clusters in the protoplanetary disk are a key process in the matter of planet formation and therefore formation of solar systems. [1] It is generally accepted that the early phases of planet formation are governed by such collisions.&#160; Collisions can result in different outcomes, such as growth, fragmentation, or restructuring of the cluster. Disk models cannot resolve the detailed physical processes involved, so laboratory experiment help to understand this phase of planet formation. Growth by collision is stalled once a critical aggregate size (Stokes number &#8776; 1) is reached, as particles bounce off each other instead of sticking or transferring mass. This critical size range is referred to as the bouncing barrier.Recent experiments showed that collisional charging is a key process to allow growth beyond the growth barrier. [2] Collisions among particles lead to charge separation resulting in a broad charge distribution within the ensemble, changing the collision dynamics and the resulting structure of growing agglomerates.Which collision outcome occurs depends on numerous properties of the participating agglomerates. The structure of these clusters is an important aspect in determining how stable the cluster is. In experiments with only one camera perspective much of the structural information gets lost. Due to the projection of a three-dimensional object on the two-dimensional plane depth information is not easily accessible. Using stereo vision, one can calculate the position of particles a cluster in three-dimensional space and further get a more precise description of the clusters before and after the collision to get an in depth understanding of cluster stability. We present novel experiments on the stability of growing clusters and their three-dimensional structure.To analyze the stability of clusters the experiments are executed in low gravity at the Gravitower Bremen, providing around 2 s of microgravity. Additionally, the experiments are performed under vacuum to exclude the influence of gas drag. The main part of the experiment is a test cell (free volume of 5 cm x 5 cm x 5 cm) with a smaller particle compartment at the bottom. The test cell is agitated in vertical direction, inducing particle-particle collisions while the experiment is still on ground, leading to the formation of clusters of charged particles (either basalt or glass particles).In microgravity, clusters and single particles are ejected into the free volume by shaking the chamber. Inside the chamber the cluster interacts with the granular gas, which results in abrasion of the cluster surface due to ongoing collisions with small particles. When the cluster hits the wall, the cluster can shrink even further or may be fragmented in smaller clusters and single particles. Therefore, these experiments enhance our understanding of the cluster stability while interacting with a granular gas and when colliding with more solid objects. The experiments are observed with a high temporal resolution and stereo vision. The stereo vision is obtained with a pair of mirrors generating two different images onto different sections of the sensor. Together with an exact calibration of the optical system, this enables a detailed 3D reconstruction of the agglomerates. Therefore, these experiments help to correlate the 3D structure of clusters with the collisional dynamics.&#160;[1] Wurm, Gerhard, and Jens Teiser. "Understanding planet formation using microgravity experiments." Nature Reviews Physics 3.6 (2021): 405-421.[2] Steinpilz, Tobias, et al. "Electrical charging overcomes the bouncing barrier in planet formation." Nature Physics 16.2 (2020): 225-229