Growing Super Large Sub-Decimeter Pebbles in Protoplanetary Disks: An Erosion Limit for Electrostatic Clustering from Suborbital Experiments

We conducted experiments with ensembles of colliding sub-millimeter basalt particles under prolonged microgravity conditions on a suborbital flight. In these experiments, the sample motion was excited at different levels. Initial shaking of the particles simulates a classical phase of bouncing in early planet formation. During these collisions, the particles charge, as was measured by applying an electric field. Further moderate shaking then leads to the formation of compact clusters of the charged beads, up to several centimeters in size. These clusters grow by individual impacts up to a velocity threshold of about a meter per second. Beyond that velocity, clusters are eroded. We therefore find a shift in barriers from a bouncing particle at millimeters per second to an eroding cluster at meters per second, going along with a shift in particle size from sub-millimeter up to several centimeters. This allows growth well into a size regime that might make clusters prone to hydrodynamic instabilities and subsequent planetesimal formation.