Static electricity study finally explains why rubbing balloon on hair makes it stick

CLEVELAND, Ohio — When we rub a balloon on our hair and it sticks, we explain to our amused and amazed children that there is static electricity. But we’re often met with the quite popular kid response: “Why?”

Now new research offers an answer to the age-old question.

It is the force behind fun science experiments with balloons and the reason toner sticks to the photocopier paper. But static electricity, or triboelectric charging, is much bigger than that. It probably helped form planets from space dust and assisted in the beginnings of life on earth.

Changes in microstructure, such as this void and fibrils created by straining a polymer sheet, appear to control how the material charges through friction. (Photo credit Case Western Reserve University)

On the other hand, static electricity can be a destructive force when it sets off explosive charges in dust. Think of coal dust in mines or sugar and flour dust in food-processing plants.

Researchers at Case Western Reserve University wanted to learn more about triboelectric charging so they might improve safety and perhaps harness the energy for productive uses. Their new study pointed to tiny holes and cracks in the microstructure of materials as the starting point for what becomes an electrical charge.

“Electrostatic charging can be seen everywhere, but we noticed some cases where materials appeared to charge more–like a balloon rubbed on your head, or packing peanuts sticking to your arm when you reach into a package,” explains Dan Lacks, one of the study’s lead authors.

“Our idea was that a strain on the materials was causing a higher propensity for the materials to become charged,” he adds. “After blowing polystyrene to create the expanded polystyrene that comprises the peanut, the material maintains this distinct charging behavior indefinitely.”

Scientists were already aware that rubbing two different materials together creates a charge. For this study, they wanted to determine how strain impacts charging. They used two types of polymer (polytetrafluoroethlyne or PTFE) film, one strained and one unstrained.

“Triboelectric charging experiments are generally known for their–as some would say–charmingly inconsistent results,” says Andrew Wang, who coauthored the study. “What was surprising to me, initially, was the consistency of the unstrained versus strained charging results.”

Researchers found a similar pattern of charge transfer in one direction, as though the materials were of completely different chemical combinations.

After rubbing, unstrained films carried negative charges while strained films carried positive charges. Although the findings did not occur all of the time, they were considered significant. The authors found that charges were more random when one strained film was rubbed against another unstrained film.

Although some changes in microstructure could be seen with the naked eye, scientists needed to use X-ray diffraction and Raman spectroscopy to see what was happening at the atomic level. The only difference was seen in the strained film, where holes and fractures — tiny voids in the material — were caused by the stretching process. These changed the microstructure.

Scientists tested these concepts on a computer, using molecular simulations of strained materials. They could see how the voids started but could find fewer other changes. They say this confirms that it is changes in the microstructure of the materials that is causing the overall charge transfer.

“We think the void regions and the fibrils we see around them when we strain the polymer have different bonding and thus charge differently,” says Lacks.

Although the experiment focused on one material, strain may affect all materials, concludes Mohan Sankaran, another lead author.  “The strain we put on the PTFE was large because we were looking for big effects,” he says. “All materials may have a little strain from processing.”

The researchers are now looking at granular materials and other polymers, such as polystyrene peanuts and plastic bags. They want to figure out the science behind static electricity and then find a way to control the charging changes that occur.

And they say their goal is twofold: to help prevent the destructive aspects of triboelectric charging, such as explosions, and to find ways to use it for helpful things, such as car paints and spray tans that stick better and reduce product waste.

They might even take their studies off the ground for manned and unmanned space missions to study the moon, Mars and asteroid dust.

The full study is published in the journal Physical Review Materials.