The Sound of Settled Science

A Shocking New Understanding of Static Electricity

A new study has found that the age-old understanding of this everyday phenomenon—one item becoming positively charged while the other becomes uniformly negative—is incorrect.

By Douglas Main

When you rub your hair with a balloon, your hair sticks to it. However, the common explanation behind this elementary school science demonstration may not be correct. A new study proposes a different story that goes against the common wisdom on static electricity that has prevailed for centuries. 
The traditional explanation for the balloon experiment goes like this: Friction causes the balloon and hair to transfer electrons, leaving each item with a uniform opposite charge. One is entirely negative and one positive, and they are then attracted to each other via static electricity. But Northwestern University researcher Bartosz Grzybowski led a study that appeared in Science last week that found things are not so black-and-white. His team's close examination of statically charged objects shows that both contain pockets of negative and positive charges. It is only the net total charge of each object that leads to their attraction. Furthermore, he found, static electricity is not caused solely by a migration of electrons or ions from one item to the other. In fact, Grzybowski says, static electricity may arise from a significant transfer of materials such as surface molecules. 
Grzybowski admits it's bizarre to find a huge surprise in a topic that has been studied since Greek polymath Thales of Miletus first rubbed amber on wool in 600 B.C., and found it could then attract light objects like feathers. Leading lights such as Nikola Tesla and Michael Faraday have studied the phenomenon, but they too reached the same conclusion. "One assumption common to all these models is that one material was positively charged, and one negatively charged," Grzybowski says. "This is actually not true." 
Perhaps we shouldn't be too surprised: Static electricity is a weird phenomenon to begin with, arising from contact between two insulators—materials that don't conduct electricity, but can create it when rubbed together. To test it in the lab, Grzybowski and colleagues used not balloons, but materials like the common polymers PDMS and Teflon. He pressed samples of insulators together before separating them (rubbing them could create more electrification but would make results harder to analyze). He then used Kelvin probe microscopy to measure molecular charges in the material. With this technique, a scientist runs a tiny probe over the microscopic hills and valleys of surfaces, and the probe vibrates differently over differently charged regions, creating a map of the charges. That's how Grzybowski saw that each material had a random patchwork of positive and negative charges, and neither was uniformly charged. In addition, his tests showed that PDMS and Teflon exchange silicon and fluorine atoms upon contact, a more significant transfer of material than ever previously shown. 
Case Western Reserve University chemical engineer Daniel Lacks says this new understanding is both fascinating and surprisingly practical. For instance, photocopying depends on precisely delivering charges to ink particles so they end up in the right place on the paper. But Lacks recalls several examples of powders becoming unexpectedly charged and exploding during manufacturing, something engineers could hopefully avoid with better knowledge of static electricity. That knowledge could also lead to better industrial coatings, which would help people like the manufacturer of polyethylene that Lacks advises. During the creation of polyethylene, sometimes the particles get unexpectedly charged and stick to the side of the reactor vessel. "Then you have to shut it down and clear out the chunks with chainsaws and blowtorches," he says. 
Grzybowski's new study also provides new puzzles for scientists to investigate. While the new study overturns some older beliefs about static electricity, it doesn't fully explain how the phenomenon works. "It's a great day when you come to the office and somebody shows you that your beliefs are wrong," UCLA physicist Seth Putterman says. 
Putterman says one thing that remains unexplained after this new study—and surprises him—is that the geometry of the charge pattern (that map of the different charges) doesn't change significantly as the two statically charged object move together and the charge decreases. To him, this implies that ions that move around easily on an object's surface are not causing static electricity. If they were, they should change the charging pattern that Grzybowski's team saw on the surface, he says. "To me this means you have extra electrons trapped deep inside the material causing the [static electricity], and they can't go walking around the surface as would ions," he says. That's because the electrons are bound up inside the material. 
Whatever the explanation proves to be, Harvard University chemist Logan McCarty says it's incredible something so common as static electricity remains such a mystery. "It's certainly more complicated than we have naively believed for many years."