Why petting your cat leads to static electricity

• Link to: Northwestern Now Story

• Static electricity was first observed in 600 B.C., but researchers have struggled to explain how rubbing causes it
• In 2019, Northwestern researchers discovered nanosized surface deformations at play
• The same researchers now say different electrical charges build up on the front and back parts of a sliding object, creating a current

EVANSTON, Ill. — Anyone who has ever pet a cat or shuffled their feet across the carpet knows that rubbing objects together generates static electricity. But an explanation for this phenomenon has eluded researchers for more than two millennia.

Now, Northwestern University scientists have finally uncovered the mechanics at play.

When an object slides, the front and back parts of that object experience different forces, researchers found. This difference in forces causes different electrical charges to build up on the front and back parts of the object. And the difference in electrical charges creates a current, leading to a light zap.

The study was published yesterday (Sept. 17) in the journal Nano Letters.

“For the first time, we are able to explain a mystery that nobody could before: why rubbing matters,” said Northwestern’s Laurence Marks, who led the study. “People have tried, but they could not explain experimental results without making assumptions that were not justified or justifiable. We now can, and the answer is surprisingly simple. Just having different deformations — and therefore different charges — at the front and back of something sliding leads to current.”

An expert in surface structures, Marks is a professor emeritus of materials science and engineering at Northwestern’s McCormick School of Engineering. Karl Olson, a Ph.D. student in Marks’ research group, is the paper’s first author.

Greek philosopher Thales of Miletus first reported friction-induced static electricity in 600 B.C. After rubbing amber with fur, he noticed the fur attracted dust.

“Since then, it has become clear that rubbing induces static charging in all insulators — not just fur,” Marks said. “However, this is more or less where the scientific consensus ended.”

Marks and his team started to unravel the mystery in 2019. In a study published in Physical Review Letters, they reported that rubbing two materials together bends tiny protrusions on the surfaces of those materials. Those bent, deformed protrusions give rise to voltages, the researchers found.

“In 2019, we had the seed of what was going on. However, like all seeds, it needed time to grow,” Marks said. “Now, it has blossomed. We developed a new model that calculates electrical current. The values for the current for a range of different cases were in good agreement with experimental results.”

A concept called “elastic shear” lies at the heart of the new model. Elastic shear can occur when a material resists a sliding force. If a person pushes a plate across a table, the plate will resist sliding. As soon as the person stops pushing it, the plate stops moving. This added friction — caused by the resistance to sliding — causes electrical charges to move.

“Sliding and shear are intimately connected,” Marks said.

Although static electricity can cause funny mishaps, like hair standing on end after going down a playground slide, it also can lead to serious problems. For example, sparks from static electricity cause industrial fires and even explosions. It also can hinder consistent dosing for powdered pharmaceuticals. With a better understanding of the mechanisms at play, researchers potentially could introduce new solutions to these issues.

“Static electricity affects life in both simple and profound ways,” Marks said. “Charging grains with static electricity has a major influence on how coffee beans are ground and taste. The Earth would probably not be a planet without a key step in the clumping of particles that form planets, which occurs because of the static electricity generated by colliding grains. It’s amazing how much of our lives are touched by static electricity and how much of the universe depends on it.”

The study, “What puts the ‘tribo’ in triboelectricity,” was supported by Northwestern Engineering.