Planetesimal formation: On the evolution of super strong charge spots from colliding grains

We conducted experiments with ensembles of colliding sub-millimeter basalt particles under prolonged microgravity conditions on a suborbital flight.  The collisions were induced by agitating the experiment chamber. During these collisions, the particles charge, as was measured by applying an electric field. In addition to the beads, the windows also charge ever stronger over time. Beads first start to stick to the windows weakly. Agitation of the experiment chamber removes them again. However, over time grain trajectories become more influenced by spots on the windows and more grains remain sticking at moderate agitation. Eventually, grains can no longer be removed even by the strongest agitation. By evaluating the different kinds of trajectories and motions from oscillating to perfect sticking, we estimate the growth of charge spots involved in this specific dynamic situation. We speculate that collisions of grains of different composition in protoplanetary disks should also create strong charge spots, which can act as strongly stabilizing element in preplanetary growth.