If you’ve ever wiggled a balloon against your hair, you know that rubbing together two different materials can generate static electricity. But rubbing bits of the same material can create static, too. Now, researchers have shot down a decades-old idea of how that same-stuff static comes about.
The same-material phenomenon produces important real-world effects, such as generating lightning in volcanic eruptions, gumming up the processing of powders in manufacturing, and causing explosions in grain elevators. Physicists thought they understood what was going on, but suddenly "the dominant theory appears to be dead," says Troy Shinbrot, an applied physicist at Rutgers University, New Brunswick, in New Jersey who was not involved in the new work.
The balloon trick involves rubbing two different insulating materials—your hair and the latex of the balloon—that hold on to electric charges with different strengths. So more positive electric charge builds up on one and more negative charge collects on the other. Like charges repel one another, which explains why your hair then stands on end. You might think that rubbing pieces of the same material wouldn't create static, as they hold charges with the same strength. But charging can happen if the pieces are of different sizes. In 1986, John Lowell and William Truscott of the University of Manchester Institute of Science and Technology in the United Kingdom explained how that might work.
They imagined rubbing a small insulating sphere across a plane of the same material, which stands in for a large object. They assumed that the two surfaces were speckled with energetic electrons trapped at random spots, presumably because they had been kicked out of their usual lower energy niches within the material. When surfaces touch, the negatively charged electrons can jump from their high-energy perches on one surface to low-energy states on the other.
If equal numbers of electrons hopped in both directions, nothing much would change. But that's where the size difference comes in. As Lowell and Truscott explained it, only one point of the sphere touches the plane, and it has just a few electrons to give and a larger number of empty states with which to absorb them. In contrast, a larger streak of the plane comes in contact with the sphere, so it has plenty of electrons to give. So more electrons hop from plane to sphere than vice versa, leaving the sphere negatively charged and the plane positively charged and creating the static. Other researchers showed how the theory could apply to grains of two different sizes.
Unfortunately, the theory doesn't work, report Heinrich Jaeger, a physicist at the University of Chicago in Illinois, and colleagues. They mixed grains of insulating zirconium dioxide-silicate with diameters of 251 micrometers and 326 micrometers and dropped them through a horizontal electric field, which pushed positively charged particles one way and negatively charged particles the other. They tracked tens of thousands of particles—by dropping an $85,000 high-speed camera alongside them. (See video above.) Sure enough, the smaller ones tended to be charged negatively and the larger ones positively, each accumulating 2 million charges on average.
Then the researchers probed whether those charges could come from electrons already trapped on the grains' surfaces. They gently heated fresh grains to liberate the trapped electrons and let them "relax" back into less energetic states. As an electron undergoes such a transition, it emits a photon. So by counting photons, the researchers could tally the trapped electrons. "It's pretty amazing to me that they count every electron on a particle," Shinbrot says.
The tally showed that the beads start out with far too few trapped electrons to explain the static buildup, Jaeger says. In fact, even if the researchers try to make trapped electrons boil up to the surface by exposing the grains to light, the density of trapped electrons remains less than 1/100,000 of what would be needed to explain the effect, the researchers report in a paper in press at Physical Review Letters.
"They show pretty convincingly that the idea of the transfer of these trapped electrons is not valid," says Daniel Lacks, a chemical engineer at Case Western Reserve University in Cleveland, Ohio, who applied the Lowell and Truscott theory to granular materials.
If the grains aren't swapping electrons, then where do the charges come from? They could come from hydroxide ions in a layer of water a molecule thick that inevitably coats the grains, Jaeger speculates. Or the charging could involve the transfer of the zirconium material from grain to grain, notes Keith Forward, a chemical engineer at California State Polytechnic University, Pomona.
Determining which scenario is right may be tough. Repeating the experiment under conditions that would eliminate any water would be very difficult, Jaeger says. Forward suggests that it might be easier to try to detect the presence of hydroxide ions using chemistry. Solving this little mystery could help materials scientists and engineers control the effect, which could be a boon to drug manufacturers and other industries.
(Video credit: Scott Waitukaitis and Heinrich Jaeger, the University of Chicago)