New data zap views of static electricity

Charges build up due to exchange of material, study suggests

A balloon rubbed against the head can be both a hair-raising and a hair-tearing experience, a new study suggests. Clumps of balloon and hair invisible to the naked eye may break off each object during contact and stick to the other.

The existence of this exchange could challenge traditional theories about how static electricity builds up, a process known as contact electrification.

“The basic assumptions people have made about contact electrification are wrong,” says Bartosz Grzybowski, a physical chemist at Northwestern University in Evanston, Ill. He and his colleagues describe their new take on static electricity online June 23 in Science.

It’s long been known that some insulators — materials that don’t conduct electricity — build up charge when rubbed together. One object is usually assumed to pick up positive charges uniformly distributed across its surface, while the other picks up negative charges. Where these charges come from isn’t known for sure, though some experiments point to the movement of charged particles such as electrons or ions.

Working within this framework, many scientists have rubbed together different insulators and ranked them from those with the greatest tendency to turn positive, such as wool, to those that tend to go negative, such as Teflon. Materials farther apart on this “triboelectric series” are thought to be more able to charge each other, setting up the kind of situation that can end with a shock.

Last year, though, Grzybowski’s team showed that identical pieces of polymer can charge each other when touched. Now they’ve use a technique called Kelvin force microscopy to take a closer look at various polymers brought into contact. The surface of each object, they discovered, was not simply positive or negative — but coated by an intricate quilt of randomly distributed positive and negative patches. These patches, tens of nanometers across, averaged to provide a small overall charge.

Nanoscale differences in how two pieces of polymer stick together and rip apart may explain these patches. Using spectroscopic techniques that reveal the identities of surface atoms, the researchers showed that tiny blobs of material tend to break off one polymer and stick to the other — perhaps tearing bonds in way that leaves charged chemical groups dangling.

“It’s not the inherent properties of two polymers that matter,” says Grzybowski. “It’s because of the material transfer that you develop charge.”

But other scientists haven’t ruled out the idea that electrons or perhaps ions are also involved in static charge.

The new study “conclusively shows that clumps are being transferred,” says Daniel Lacks, a chemical engineer at Case Western Reserve University in Cleveland. “But we don’t know yet whether that’s the thing that dominates the charging. It could work in combination with other things.”

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