Observation of bottom-up formation for charged grain aggregates related to pre-planetary evolution beyond the bouncing barrier

TL;DR

This study demonstrates that electrical charging can induce the formation of small grain aggregates in protoplanetary disks, surpassing the bouncing barrier and facilitating early planet formation through bottom-up growth.

Contribution

It provides experimental evidence that electrostatic charges enable grain aggregation at higher velocities, offering a new pathway beyond the bouncing barrier in planet formation.

Findings

Charged grains form irregular aggregates in microgravity experiments.

Electrostatic attraction allows sticking at velocities around 10 cm/s.

No bouncing collisions observed among hundreds of collisions.

Abstract

Context. Particles in protoplanetary disks go through a number of phases that are dominated by collisions. In each of these events, grains exchange electrical charge via triboelectric effects. This enhances the stability of particle aggregates. Aim. Dielectric grains are easily charged by collisions. Here, we investigate whether a charge is capable of inducing an aggregation of particles and we consider how collision properties, such as ticking velocities and collisional cross-sections, are altered. Methods. We explored aggregation in microgravity experiments based on the observation of the motion of submillimeter (submm) grains following many collisions. In the process, grains attract each other, collide, stick, and ultimately form small aggregates. Results. We observed a bottom-up formation of irregular aggregates from submm grains. While some of the observed trajectories during the approach of grains reflect the presence of a pure Coulomb potential, the motion is not always in agreement with pure Kepler motion. Higher-order potentials of multipole charge distributions stand as a plausible explanation for this behavior. An immediate consequence of charging is that the particles continue to stick to each other at velocities of $\sim 10 \, \rm cm/s,$ while surface forces of neutral grains are only expected to allow sticking below $\sim 1 \, \rm mm/s$. No bouncing collision was observed among hundreds of collisions in the given parameter range. Applied to early phases of planet formation, the forming aggregates are therefore the first steps in a new growth phase beyond the traditional bouncing barrier in planet formation.