The interactions of charged, micron-sized grains are of fundamental importance in situations ranging from airborne pollutant coagulation to the clustering of dust in the interstellar medium. However, a full picture of how electrostatic interactions contribute to particle aggregation has remained elusive, mainly owing to the absence of direct, in-situ experiments.

We showed how this challenge can be tackled with an innovative laboratory experiment.  Using a freefalling stream of particles to create a low-gravity environment and tracking this stream with a high-speed video camera falling along with it, we observed how charged grains in their mutual electrostatic potentials can undergo attractive as well as repulsive trajectories similar to planetary orbits. We also observed, for the first time, how highly charged particles can attract and capture others to build up clusters particle by particle, and how this can lead to the formation of unique ‘granular molecules’ whose configurations resemble those of simple chemical molecules.

1. •Victor Lee, Scott R. Waitukaitis, Marc Z. Miskin, Heinrich M. Jaeger, “Direct Observation of Particle Interactions and Clustering in Charged Granular Streams”, Nature Physics, publ. online 7-13-15 link.
   

2. •click here for a movie (9.6Mb)
   

These results highlight the importance of polarization effects to provide efficient capturing and aggregation of dielectric grains via multiple collisions.  This can have implications for the very earliest stages of planet formation, which is believed to start via collisions among interstellar dust grains. Single head-on collisions typically do not dissipate enough energy to lead to sticking.  Electrostatic interactions have been speculated to help resolve this problem and promote aggregation, but prior to these results the growth of clusters by successive capture of individual particles via long-range electrostatic interactions had not been observed in detail. 

For more on this, see the News & Views article by Frank Spahn and Martin Seiß (University of Potsdam, Germany) here.