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Cracking with Electricity

Faults seem to give off a warning signal before they slip

Fenella Saunders

Troy Shinbrot is no stranger to research that defies standard beliefs. The Rutgers University biomedical engineer focuses on grains and powders, specifically how they mix and gain electric charges. A few years ago, this specialty led him to work on what’s called the Brazil nut effect: In a group of particles of different sizes (such as a container of mixed nuts), shaking makes the larger ones (the Brazil nuts) rise to the top. Common wisdom was that small particles could slip below larger ones, leaving the big ones on top with nowhere to go. Other researchers found that the grains would “convect,” rising in the center and sinking at the edges. Large particles would lift up with the whole bed and then get stranded on top, unable to fit into the narrow margins at the sides.

Shinbrot and his colleagues, however, found that large, lightweight particles would sink instead of rising, dubbed the reverse Brazil nut effect—but only when the grains were vibrated above a certain frequency that makes the bed “fluidize.” Then the mixture behaves like a liquid with typical buoyancy characteristics, so light objects rise while heavy ones sink.

The results were so counterintuitive that reviewers of the paper thought they were impossible. An editor of the journal Physical Review Letters tested it out for himself and confirmed Shinbrot’s findings, but was still nervous. “He called me up and wanted to make sure that I wasn’t playing a joke on him,” Shinbrot says. “He was a little anxious that there was something funny going on that would make him regret publishing the paper.”

Two years ago, Shinbrot again succeeded in convincing a journal that his unexpected results in another study were not spurious. In sandstorms, volcanic ash plumes and dust clouds in food, drug or coal processing, the grains spontaneously generate strong electrical charges and can sometime emit flashes or even explode. But the grains themselves are inert, so it’s hard to understand how they could charge. Shinbrot and his colleagues proposed a mechanism whereby particles are initially polarized by an external electric field. When they collide in a cloud, the contacting sides cancel their charges, leaving one particle with an overall negative charge and the other with a positive charge. Once they separate, the external field polarizes the grains again, adding one unit of charge to each particle with each collision. But Shinbrot’s results went so against accepted theories of electrostatic charging that for several years he considered the results to be unpublishable.

“Most physicists view these problems as solved; they figure the chapter is closed,” Shinbrot says. “They don’t look at the history and recognize that there are still many open subjects.”

Now Shinbrot has made a discovery that he freely admits is very strange and hard to understand, but he is pretty sure his results aren’t mistaken.

The problem started with powders destined for pharmaceuticals, which electrostatically charge during processing and stick to surfaces. “So there are problems where you want to mix two powders, and one of them might charge and one of them might not, or they might charge differently,” Shinbrot says. “That can cause them to separate, which is a severe concern when you want to have known amounts of drugs in each tablet.”

2012-09SciObsSaundersFA.jpgClick to Enlarge ImageBecause of his prior interest in dust storms, Shinbrot was aware that electrical discharges from moving powders were possible. He had also heard about visible flashes being reported at the time of earthquakes. “We had this instrumented tumbler, and these instruments for measuring charge,” he says. “To my knowledge nobody had put these two pieces of equipment together, and I just thought ‘well, I wonder what will happen.’”

And he was not expecting what did happen. “It seemed so utterly implausible,” Shinbrot recalls. He and his colleagues reported their results in the June 11 online edition of the Proceedings of the National Academy of Sciences of the U.S.A.

As the powder tumbled, before a large slab of it fractured off in what is called a slip event, the material emitted a detectable negative voltage. In other words, an electric warning signal occurred before the slip happened. Although it’s not a surprise that cracks and slips occur in a tumbling tub of powder, could such signals predict an earthquake, where the soil is also essentially a compacted powder? “We realized there were conceivable connections with geologic events,” says Shinbrot.

To firm up their results, Shinbrot and his colleagues used three different setups: the tumbler, a bed that simply tipped and one that slid from side to side (called a shear cell). In all cases, they tried to control for electrostatic charging in every way possible. “We increased the humidity, we used different materials, we used a static eliminator, we tried to measure in different regions,” he says. “We don’t know exactly how static charging would creep in, but I’m not going to rule it out completely.” However, he is pretty sure the electrical signal is coming from the crack itself, not from some kind of static discharge.

One piece of evidence is that the type of bed used seems to affect how far in advance the electrical signal preceded the slip event. The signal seems to be emitted when a precursor defect opens within the material. “If you just take a bucket of powder and tip it, you’ll get a precursor of a half second or so,” Shinbrot says. “If you put it in a tumbler, you might get that same precursor that doesn’t work its way from deep in the bed up to the surface and produce a visible effect for five seconds or so.”

2012-09SciObsSaundersFB.jpgClick to Enlarge ImageAdditionally, the location where the researchers placed the electric probe affected the amount of signal received. “It seems like cracks start at a particular location in the bed, and that’s consistent with our speculation that maybe the cracks are what are producing the voltage,” Shinbrot explains. “But why the cracks start there we have absolutely no clue.”

In the shear cell, which Shinbrot filled with ordinary flour, he and his colleagues could watch the electrical signal happen repeatedly as the cell slid from side to side and the same crack opened and closed. “But it still remains very strange that you can put a powder like flour into a container and get 200 volts out of it,” Shinbrot says.

Although Shinbrot cannot explain why cracks produce voltages, he theorizes that the mechanism is related to the dilation of grains before a slip event. “If you think of a stack of marbles, they’re all sort of interlocked because they’re all sitting against one another,” he explains. “If you try to move one, you actually have to lift it up over a little hump before it can flow.”

The effect, Shinbrot says, seems to be similar to other unusual behavior in everyday materials. It has been reported for some time that when transparent tape is peeled off of its roll, it emits light at the point of separation. Biting a wintergreen Lifesaver also produces a flash, with some of the energy large enough to produce x rays.

Shinbrot doesn’t yet know whether the electrical warning signal will be as clear if the grains are not all the same size. When the cracks are of a more jagged shape and less clearly defined, the signal is also affected. Both of these factors might impact the phenomenon’s usefulness for something like earthquake prediction. Shinbrot’s next move is to size up the test bed to a meter or two, as a first step in determining whether the effect might happen at all on a geologic scale. It’s possible that an increased area could decrease the stress on the grains and drop the signal—or the opposite could occur, and with more areas of contact to be broken the result could be magnified.

“If the effect grows with size, then we’ll try to collaborate with geophysicists and look at larger scale systems,” he says. “If it decreases with size, then we’ll just say ‘well this was an interesting trip,’ and we’ll go on with something else.”

Assuming there is a relationship between the group’s results and seismic events, Shinbrot hopes to coordinate the work with other indicators. For instance, earthquakes are known to emit acoustic signals, and Shinbrot plans to explore any possible connection between them and the electrical discharges.

But he faces some unusual challenges in this area, for which his history of unconventional studies may have prepared him: “If you look up ‘earthquake lightning,’ you’ll find an equal number of websites that talk about it having something to do with UFOs or government conspiracies or whatnot, as authentic scientific research. We don’t want to be tarred by the same brush. But it made this topic a very interesting one to study. In the past 10 years there have been some serious scientific studies, and I think there is hope that in the next 10 years this may get on a firmer scientific footing.”

It seems likely that no matter what, Shinbrot will persevere in finding the exceptions in physics that show that the field is still full of surprises. As he says, “That’s what makes it fun.”

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