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Static cling? It's not what you think

Northwestern University's Bartosz Grzybowski explains the mechanism behind contact electrification.

For millennia, scientists have puzzled over the reason why rubbing two insulators together can produce static cling — and you may be shocked to hear that the standard explanation is wrong.

Static electricity, also known as contact electrification, is "one of the oldest areas of scientific study," researchers from Northwestern University observe in their paper on the subject, published online today by the journal Science. Questions about the phenomenon's cause date back to around 600 B.C., when Thales of Miletus conducted experiments with amber charging against wool.

The traditional view was that electrons were transferred from the surface of one material to another — for example, from a plastic balloon to the strands of hair on a child's head. That would cause one material to carry a slight positive charge while the other material carried a slight negative charge. Because opposites attract, the hair would be drawn toward the balloon, resulting in that cute "bad hair day" look.

To test that explanation, the Northwestern team took an ultra-close look at the static-charged surfaces of plastic material as well as silicon and aluminum, using Kelvin force microscopy. What they found was different from what they expected. The surfaces were actually "mosaics" of electrically charged nanoscale regions, alternating between positive and negative charges. When the surfaces were rubbed together, tiny patches were transferred from one surface to the other.

"It's not just transfer of electrons when two pieces of material come together," principal study author Bartosz Grzybowski, a chemistry professor at Northwestern, told Science in a video clip. "It's about transfer of material that then mediates the buildup of charge."

When those nano-bits of material are torn away from the surfaces as a result of the rubbing, that breaks chemical bonds and leads to changes in the net electric charge of each material. So when you rub a plastic balloon on a child's head, tiny flecks of that balloon are actually being rubbed onto the little one's locks of hair.

"A picture that emerges is that contact electrification is a complex process involving a combination of, at least, bond cleavage, chemical changes and material transfer occurring within distinct patches of nanoscopic dimensions," the researchers write. "The exact relationship between these effects — and possibly also those due to the presence of surface water and local electric fields — remains unclear but prompts several intriguing questions for future research."

Grzybowski and his colleagues point out that contact electrification isn't just a parlor trick: Through the ages, the phenomenon has sparked technologies ranging from photocopying and laser printing to do-it-yourself biodiesel and spray painting. Grzybowski said his research group was already trying to apply what they've learned to come up with better ways to apply coatings to surfaces. So it's nice to know that even after 2,600 years of study, our view of contact electrification isn't ... heh, heh ... static.

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In addition to Grzybowski, the authors of "The Mosaic of Surface Charge in Contact Electrification" include H.T. Baytekin, A.Z. Patashinski, M. Branicki, B. Baytekin and S. Soh. For more about the research, check out this report from the Nobel Intent blog.

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