By Geoff Brumfiel
Scientists have explained how lightning can occur even in the driest deserts. A new theory describes how neutral dust can gain an electrical life of its own.
For centuries, researchers have known that clouds of neutral particles can sometimes gain a net charge. This can cause even the driest sand to generate lightning, and sugar refineries and coal-processing plants can experience unexpected explosions. Most researchers have ascribed such events to static build-up, but Troy Shinbrot, a physicist at Rutgers University in Piscataway, N.J., was unconvinced. Under normal conditions, sand and dust don't conduct electricity, he says, so how could they generate fields strong enough to spark massive lightening bolts? "These materials are insulators under very dry conditions, so where are the charges coming from?" he asks.
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Charge of the balloon brigade
Shinbrot sat up at night for months thinking about it, and eventually he developed a theory. He began by visualizing the sand particles as party balloons. In an electric field, he thought, the balloons would polarize: In other words, each balloon would develop a positive and negative hemisphere.
He then thought about what would happen if a negative hemisphere from one balloon touched the positive hemisphere of another. The touching hemispheres would neutralize, but each balloon's other hemisphere would not because they are in an independent electric field.
When the balloons parted ways, they would repolarize in the electrical field around them. But, as the balloons repolarize, the hemispheres that never came into contact with each other would gain an extra unit of charge. In this way, the balloons could gain very, very high charges, even though they were initially neutral.
By Shinbrot's own admission, the idea of neutral particles charging through the act of neutralizing "just didn't seem right". But when he began modeling his theoretical sand particles, he found that the idea held up. Moreover, the models predicted optimal densities of particles where the effect should be most pronounced.
Shinbrot and his team tested their models with an experiment. The team placed colored glass beads into a jar and placed it under a 30-kilovolt electrical field. They then puffed air through the jar and watched what happened. Sure enough, at non-optimal densities, only a few beads became charged (see video), but at the optimal density predicted by the model, many beads took flight (see video). "This crazy idea seemed to work," he says. The team's work was published online on April 11 in Nature Physics.
A farewell to chainsaws
"I think their model makes sense," says Daniel Lacks, a chemical engineer at Case Western Reserve University in Cleveland, Ohio. It may seem like esoteric research, but it will probably end up coming in handy, he adds.
Small particles are commonplace in industrial processes, and charging is a perennial problem. For example, particles used in the production of polyethylene--the world's most common plastic, used in plastic shopping bags--often gain a static charge and cling to the walls of reaction chambers, clogging up the equipment. At the moment, the solution is decidedly low-tech. "They have to go into reactors with chainsaws and blowtorches", to cut away the accumulated mass of particles, says Lacks. The new model could help improve production techniques.
But the mystery isn't entirely solved. For one thing, the team still hasn't explained the origin of the external electrical field needed to kick off the charging process. Shinbrot thinks it may be possible for moving clouds of dust or sand to self-generate fields. But Lacks suspects that unknown mechanisms may also be contributing. "Everything seems right," says Lacks, "but I don't think this is the final story."
