Keplerian Orbits in Freely Falling Charged Grains

We report high-speed tracking of freely falling charged grains in the co-moving frame. Elliptical and hyperbolic Keplerian orbits due to electrostatic interaction between oppositely charged grains are observed directly. By using Nelder-Mead optimization method, we are able to reconstruct the Keplerian trajectories in three-dimensional space. We also find that electrostatic force can lead to aggregation in a granular stream.