The first direct observations of individual interactions between charged, sub-millimetre grains have been reported by researchers in the US. The experiments were done to mimic conditions present when planets are first forming, and reveal that particles attract and repel each other through electrostatic forces. The particles also combine, often via multiple collisions, to form clusters with molecule-like configurations.
Understanding how fine particles interact is fundamental to a variety of situations – including the accretion of interstellar dust during planetary formation, the clustering of biomolecules in industrial processes and the coagulation of hazardous airborne pollutants.
Long-range electrostatic interactions are believed to play an important role in the interaction of tiny particles, sometimes causing the particles to accumulate in larger lumps. The particles themselves can be chemically neutral but can gain large positive or negative charges through friction during collision events. However, exactly how electrostatic forces affect the aggregation process is poorly understood because experiments must be done in the absence of gravity.
Tracking shot
Now, Victor Lee and colleagues at the University of Chicago have developed a new experimental set-up that minimizes the effect of gravity by observing the particles in free fall within a 3 m-tall vacuum chamber. Zirconium-dioxide–silicate grains – each with diameters on the scale of a few tenths of a millimetre – were allowed to fall through the chamber in a dilute stream. Next to the vacuum chamber, a high-speed camera was allowed to fall alongside the grains, guided by two low-friction rails. By recording the behaviour of particles through a window running the length of the chamber, Lee and colleagues were able to study particle interactions for up to 0.2 seconds in a low-gravity environment – before the camera’s descent was gently arrested by foam pads.
In a separate test, the researchers determined the net charge on individual grains by applying a strong electric field across the falling stream and measuring the resulting acceleration of the grains.
The observations revealed significant long-range attractive and repulsive electrostatic interactions between the charged particles, with some particles travelling in Keplerian orbits relative to each other. Grains were also seen to aggregate through a series of bouncing collision events. This allows for the development of particle clusters from collisions at higher relative velocities than would be expected with simple, head-on collisions. This, the researchers say, is relevant to the accumulation of dust in planetary formation.
We witness a delicate dance involving electrostatic-induced orbits
Victor Lee, University of Chicago
“By removing air drag and gravity, we witness a delicate dance involving electrostatic-induced orbits, cluster aggregation and annihilation events, and even the formation of ‘molecules’ of oppositely charged grains,” says Lee. “Our results reveal the essential ingredients for making such dust clump together, perhaps explaining why the ground beneath our feet is there in the first place.”
Meticulous and “gutsy” study
Troy Shinbrot of Rutgers University in the US – who was not involved in this study – commends the research for confirming, on an individual-particle basis, how identical grains can acquire strong relative charges, and revealing in meticulous detail the complex interactions between fine particles. “The work is both technically impressive and involved a certain ‘gutsiness’ by the researchers, who fearlessly dropped a $20,000 high-speed camera thousands of times to track [the] falling particles,” he says.
“Proof that long-range electrostatic interactions lead to Keplerian orbits between these small grains is exciting to read,” agrees Jürgen Blum of the Technische Universität Braunschweig in Germany. Blum is sceptical, however, about the suggested applicability of the study to the processes of planetary formation. “The number of elementary charges per grain is huge [in this experiment] and much, much larger than ever possible in a planet-forming environment, due to the discharging ability of the partly ionized gas,” he notes. Blum also points out that the smaller grain sizes found in a protoplanetary disc would also result in a lower ratio of Coulomb-to-Van-der-Waals forces in the interactions between grains.
Lee and colleagues have made a video showing interactions between a number of particles during free fall.
The research is described in Nature Physics.
