Scientists unlock vital clue to strange quirk of static electricity

Static electricity—specifically the triboelectric effect, aka contact electrification—is ubiquitous in our daily lives, found in such things as a balloon rubbed against one’s hair or styrofoam packing peanuts sticking to a cat’s fur (as well as human skin, glass tabletops, and just about anywhere you don’t want packing peanuts to be). The most basic physics is well understood, but long-standing mysteries remain, most notably how different materials exchange positive and negative charges—sometimes ordering themselves into a predictable series, but sometimes appearing completely random.

Now scientists at the Institute of Science and Technology Austria (ISTA) have identified a critical factor explaining that inherent unpredictability: It’s the contact history of given materials that controls how they exchange charges in contact electrification. They described their findings in a new paper published in the journal Nature.

Johan Carl Wilcke published the first so-called “triboelectric series” in 1757 to describe the tendency of different materials to self-order based on how they develop a positive or negative charge. A material toward the bottom of the list, like hair, will acquire a more negative charge when it comes into contact with a material near the top of the list, like a rubber balloon.

The issue with all these lists is that they are inconsistent and unpredictable—sometimes the same scientists don’t get the same ordering results twice when repeating experiments—largely because there are so many confounding factors that can come into play. “Understanding how insulating materials exchanged charge seemed like a total mess for a very long time,” said co-author Scott Waitukaitis of ISTA. “The experiments are wildly unpredictable and can sometimes seem completely random.”

A cellulose material’s charge sign, for instance, can depend on whether its curvature is concave or convex. Two materials can exchange charge from positive (A) to negative (B), but that exchange can reverse over time, with B being positive and A being negative. And then there are “triangles”: Sometimes one material (A) gains a positive charge when rubbed up against another material (B), but B will gain a positive charge when rubbed against a third material (C), and C, in turn, will gain positive charge when in contact with A. Even identical materials can sometimes exchange charge upon contact.