Direct observation of particle interactions and clustering in charged granular streams

Clustering of fine particles is of crucial importance in settings ranging from the early stages of planet formation(1-3) to the coagulation of industrial powders and airborne pollutants(4-7). Models of such clustering typically focus on inelastic deformation and cohesion(1,4,6,8). However, even in charge-neutral particle systems comprising grains of the same dielectric material, tribocharging can generate large amounts of net positive or negative charge on individual particles, resulting in long-range electrostatic forces(9-11). The effects of such forces on cluster formation are not well understood and have so far not been studied in situ. Here we report the first observations of individual collide-and-capture events between charged submillimetre particles, including Kepler-like orbits. Charged particles can become trapped in their mutual electrostatic energy well and aggregate via multiple bounces. This enables the initiation of clustering at relative velocities much larger than the upper limit for sticking after a head-on collision, a long-standing issue knownfrompre-planetary dust aggregation(1,12). Moreover, Coulomb interactions together with dielectric polarization are found to stabilize characteristic molecule-like configurations, providing newinsights for the modelling of clustering dynamics in a wide range of microscopic dielectric systems, such as charged polarizable ions, biomolecules and colloids(13-16).