Physicists have directly observed, for the first time, how highly charged dust-sized particles attract and capture others to build up clusters particle by particle. This process can lead to the formation of “granular molecules” whose configurations resemble those of simple chemical molecules.

These interactions are fundamentally important in situations ranging from airborne pollutant coagulation to the clustering of dust in interstellar space. Nevertheless, a full picture of how electrostatic interactions contribute to particle aggregation has remained elusive, mainly owing to the absence of direct, in-situ experiments.

In a recent paper published in the journal Nature Physics, a research team at the University of Chicago has shown how to experimentally resolve this problem. Spearheading the project was Victor Lee, a graduate student in physics, working with co-authors Scott Waitukaitis, PhD’13; Marc Miskin, PhD’14; and Heinrich Jaeger, the William J. Friedman and Alicia Townsend Professor in Physics.

Using a free-falling stream of particles to create a low-gravity environment and tracking the stream with a high-speed video camera falling along with it, the team observed how charged grains in their mutual electrostatic interactions can undergo attractive as well as repulsive trajectories similar to planetary orbits. The team’s results highlight the importance of polarization effects in promoting the capture and aggregation of 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,” Jaeger said. “Single head-on collisions typically do not dissipate enough energy to lead to sticking.”

Scientists have long speculated that electrostatic interactions could help colliding particles stick together instead of flying apart. But the Chicago team has now observed in detail, for the first time, cluster growth by successive capture of individual particles via long-range electrostatic interactions.

In related work, a team led by UChicago’s Karl Freed, the Henry G. Gale Distinguished Service Professor Emeritus of Chemistry, and Juan de Pablo, the Liew Family Professor of Molecular Engineering, has just completed calculations that can explain some of the “granular molecule” configurations that Lee and his co-authors see in their experiments.

“One thing their paper makes clear is that the effects we were able to track directly with the granular material have wide-ranging importance for much smaller particles, including colloids, nanoparticles and molecules,” Jaeger said.

Moreover, their theory has applications to widespread areas of biophysics, materials science, ionic solutions, the operation of batteries, the thermodynamics of ionic solutions, and the description of ion pairing, electrochemical processes, the folding and binding of proteins and nucleic acids, the directed assembly of DNA coated nanoparticles into arrays, and more.