Squishy materials reveal new physics of static electricity 

Rub a balloon on your hair and the balloon typically picks up a negative electric charge, while your hair goes positive. But a new study shows that the charge an object picks up can depend on its history. The number of times an object had previously touched another determined whether the object became negatively or positively charged when touched again, researchers report in the Feb. 20 Nature.

The work could be a step toward understanding the effects behind the phenomenon of static electricity, in which electric charge accumulates on materials after they are rubbed or touched together. Although static electricity is a daily phenomenon, scientists still don’t understand how the charge transfer works. The phenomenon is important for everything from lightning storms to pollination. But, “we are just absolutely clueless, like mega-clueless, as to what’s actually happening,” says physicist Scott Waitukaitis of the Institute of Science and Technology Austria, or ISTA, in Klosterneuburg.

Scientists don’t know what is being transferred from one material to another when objects touch. It could be electrons, electrically charged atoms called ions or small bits of material. Even reproducibility is a struggle: The same experiment can give a different result on different days or in different laboratories. That has made it difficult to draw clear conclusions. 

So Waitukaitis and colleagues simplified things. They studied electric charge in experiments with a single material, a squishy polymer called polydimethylsiloxane, or PDMS. They touched different squares of the material together, measuring the charge transferred. (The squishiness is helpful for ensuring that the two objects make good contact with one another in the experiments.)

At first, the samples seemed to exchange charge randomly. But eventually, the researchers discovered a pattern. A sample that had been touched to other samples many times would charge negative when touched to a fresh one.

The researchers also found that the samples formed what’s known as a triboelectric series. That’s an ordering based on which material in a pair takes a negative charge, and which a positive charge, when touched. For example, a ballon usually goes negative when it touches your hair. But a balloon touched to Teflon would typically get a positive charge. A triboelectric series usually involves different types of materials, but the different chunks of PDMS formed their own series, too. Contact history mattered there, as well. The triboelectric series formed after the samples had many previous contacts. 

The researchers examined the PDMS samples in detail to determine what was causing the effect. They found that the samples that had been touched repeatedly were smoother on very small distance scales of about 10 nanometers. 

What that means for the mysteries of static electricity isn’t yet clear. But the result illuminates the source of some of the confusion. “It helps [us] understand the previous irreproducibility, in that you have these materials that you think are all the same but there’s going to be subtle differences in the nanostructure,” says chemical engineer Daniel Lacks of Case Western Reserve University in Cleveland. “That, I believe, is a key result.”

The discovery was “a mixture between accidental and sheer stubbornness on my part,” says physicist Juan Carlos Sobarzo, also of ISTA, who performed the experiments. When the experiments didn’t work as expected, he tried them again, day after day, until they did. That led the researchers to realize that the repetition itself was key to getting a triboelectric series, in that the samples had to have been touched many times. “If I hadn’t followed my gut, we could’ve missed the importance of contact history.”

Sobarzo, it seems, had just the right touch.