Self-Regulation of Amplifying Sticking Forces During Collisional Growth of Preplanetary Pebbles

Initial phases of planet formation are ruled by sticking between dust particles and aggregates thereof. Simple van der Waals forces are too small to allow growth into a size regime where hydrodynamic instabilities might take over. However, tribocharging during the bouncing phase can increase the stickiness. Based on the experiments presented here, we propose that these electrostatic sticking forces might be self-regulating. That is, bouncing collisions might amplify the charges to the point where particles stick together and no further bouncing and charging occurs. This way, bouncing phases might be turned into growth phases, just adapting to the need for a given parameter set (grain size, collision velocity).

This concept is suggested by our experimental investigation of charged particles under prolonged microgravity conditions. We placed an ensemble of monodisperse sub-millimeter basalt spheres in a collision chamber on a suborbital flight. The experimental procedure included agitating the chamber and hence the basalt spheres. Collisions generated charges on the surfaces of the involved particles via tribocharging. In addition to the beads, the glass windows of the chamber charged too sporadically and over time in certain spots. At the beginning, a gentle shake was enough to remove the particles from the charge spots. Over the course of the experiment, successive collisions led to an increase of charges on the charge spots. Particles became harder to remove until they finally couldn’t be removed at all, even at the maximum vibration amplitude. We estimated the charge evolution on the spots by evaluating the motion of grains interacting with the spots. This includes trajectories of unbound grains, the oscillations of captured particles and, finally, the analysis of rigid-like sticking. These localized charge spots could enhance preplanetary growth of agglomerates during bouncing. As collisional charging stops if particles stick together, the charging and electrostatic forces might be self-regulating to the point where they are just large enough for the given parameters to promote growth. Supposedly, this also works during fragmentation phases of pebbles, which would shift the fragmentation barrier and allow the growth of still larger pebbles.