Colliding dust grains charge each other up

Particles in a cloud can electrify themselves

ALL CHARGED UP Ash particles generated lightning storms when Mount Rinjani erupted in Indonesia in 1995. Oliver Spalt/Wikimedia Commons

It’s the ultimate love-at-first-sight story: In the middle of the desert, hundreds of miles from anything else, lonely sand grains meet up in a crowd and decide to electrify each other. Sparks fly. 
Physicists have long puzzled over why sand grains and other small particles can build up electrical charges as they collide with one another, sometimes to the point of discharging lightning in dust storms or plumes of volcanic ash. Now, a paper appearing online April 11 in Nature Physics suggests that particles transfer electrical charge vertically during a smashup, such that positive charges move downward and negative charges move up in the cloud.
The findings could help combat a wide variety of practical problems, such as the adhesion of charged dust to solar panels on a Mars rover or the generation of dangerous electrical discharges that sometimes occur when a helicopter takes off in the desert. Dust clouds can create problems in grain silos, where charge sometimes builds up and leads to explosions, and in the pharmaceutical industry, where particles of ground-up drugs can become charged and not mix properly, says Hans Herrmann, a materials researcher at ETH Zurich.
Herrmann says he became interested in the problem after watching lightning in swirling sands over dune fields at night. “Normally when particles collide, they neutralize,” he says. “How could it be that charges increase?”
Working with ETH colleague Thomas Pähtz and Troy Shinbrot of Rutgers University’s campus in Piscataway, New Jersey, Herrmann developed a model to explain how the charging happened. Before colliding, the grains have an overall neutral charge but are polarized by a background electric field, with a negative charge towards the top of the grain and a positive charge towards the bottom, relative to the ground. Upon colliding, the particles neutralize each other at the point of contact, but when they separate again they became further polarized, with additional charges building up on the grains’ edges.
“Every time there’s a collision you end up pumping charge from the top to the bottom,” says Shinbrot. The researchers ran computer simulations and then a series of experiments with glass beads to confirm the theory. 
Daniel Lacks, a materials physicist at Case Western Reserve University in Cleveland, says the new study could be identifying one of several mechanisms at work in particle clouds. In earlier work, Lacks showed that electrical charging depends on the size of particles in question, with smaller particles tending to charge negatively and larger particles tending to charge positively. 
“The bottom line is that something is needed to break the symmetry when two particles of identical composition collide, in order for one particle to charge negatively and the other to charge positively,” he says. For particles of different sizes, he says his mechanism might be in play; for identically sized particles, the new model might explain it.
Some challenges remain, such as explaining where the background electric field that charges the particles came from. But Herrmann says the work is philosophically satisfying, in answering a long-held question, and may yet have practical applications.  

Alexandra Witze is a contributing correspondent for Science News. Based in Boulder, Colo., Witze specializes in earth, planetary and astronomical sciences.

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