Planetesimal Formation with Charged Grains

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In current models of planet formation, planets form from dust growing in different phases to ever larger bodies. In the size range between millimeter and meter, the growth mechanism changes from collisional hit-and-stick to a mechanism dominated by hydrodynamics within the gaseous protoplanetary disk, where particles are concentrated in dense clouds, which become gravitational instable, eventually. This transition phase is not well understood, is critical and forms the focus of this project. More specific, while dust grains easily stick together due to common surface forces, this no longer works for particles of millimeter size. They regularly bounce off each other under the conditions of protoplanetary disks. While bouncing is not constructive for further growth initially, it leads to triboelectric or collisional charge separation. Recent work showed that this has the potential for an efficient additional growth phase as charges builds up on grains and eventually bind particles within aggregates strongly. This might dominate the transitional evolution phase and in fact might be necessary to explain planet formation at all. The triboelectric or collisional charging of grains and the stability of forming clusters will be studied in this project. In laboratory experiments, clusters of charged grains will be stored in acoustic traps. Their charge state will be analyzed from net charges on grains to dipole moments and the mechanical strength will be measured. The strength will also be modeled numerically and compared to the experiments to estimate the effect of higher order multipoles or the influence of complex charge patterns on grains surfaces. These results will then be used to model the growth of a particle ensemble self-consistently to show how far this potentially crucial mechanism of charged particle evolution might lead in planet formation.