We now know why shoes squeak, and it involves miniature lightning bolts

The ubiquitous squeak of sneakers on a basketball court may be caused by more than just friction, a new study suggests.

Researchers have found that the sharp chirp of rubber on a hard floor happens when tiny areas of slipping between the shoe's sole and the floor move at supersonic speeds — and, in some experiments, the process involved miniature, lightning-like sparks. What's more, the findings could lead to an improved understanding of earthquakes and aid in the design of grippy surfaces.

The new study, published Feb. 25 in the journal Nature, shows that soft rubber does not slide the way many people imagine. Instead of the whole sole sticking and then slipping at once, motion bunches into fast, wrinkle-like fronts called "opening slip pulses" that detach and reattach the rubber across the contact zone. Those repeating pulses generate the vibrations that our ears hear as squeaks.

Scientists have long explained squeaks from shoes, bicycle brakes and tires using stick-slip friction, a stop-and-go cycle in which surfaces repeatedly catch and then break free. That model works well for many hard-on-hard systems, like door hinges.

But soft materials like rubber behave differently when they slide across rigid surfaces.

To understand the physics of this process, researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) teamed up with experts from the University of Nottingham in the U.K. and the French National Center for Scientific Research. They used high-speed optical imaging and synchronized audio to watch soft rubber move quickly along smooth glass.

But what they saw was not smooth sliding. Instead, motion bunched up into opening slip pulses, sweeping across the rubber in starts and stops.

"Fundamentally, these findings challenge the long-held assumption that soft-material friction can be fully captured by simplified, one-dimensional ‘stick-slip’ models," first study author Adel Djellouli, a postdoctoral fellow at Harvard, told Live Science in an email.

Tiny lightning everywhere

The findings reveal more about the physics of friction. In classic stick-slip friction, the whole contact surface alternates between sticking and slipping. In this study, however, the motion was more localized, as only small regions opened and slipped, and then moved on, while other regions stayed in full contact.

For some experiments, the team also saw tiny flashes caused by the friction, which they described as miniature "lightning" sparks. In some tests, those sparks, or electrical discharges, appeared to trigger the slip pulses. The sparks were not the main source of the squeaking noise, but they showed how electrical energy could build up in the system when the rubber moved.

The researchers also found that the rubber's shape, more than its movement, was the main determinant of the squeak's pitch.

When flat rubber blocks slid across the glass, the slip pulses were irregular, producing a broad "whoosh" rather than a clean squeak. But when the researchers added thin ridges to the rubber, the ridges confined the pulses and made them repeat at regular intervals.

In effect, the ridges acted like guides, channeling the pulses into a repeating cycle. This locked the sound into a specific frequency, or tone. The team found that this squeak frequency depended mainly on the height of the rubber ridges.

In fact, the pattern was so reliable that the team designed blocks of different heights and used them to play the Imperial March theme from "Star Wars" by hand.

"When it came time to actually play the Star Wars theme song, we had to rehearse for three solid days to get the video right," said Djellouli. "None of us are exactly trained in making music with squeaky rubber blocks, so getting the timing and technique down took a lot of practice. I think the funniest part was the relief in the lab when we finally finished the recording after three days of constant, high-pitched squeaking. Our colleagues were very happy to finally have some quiet again!"

What sneakers may have in common with earthquakes

The findings have implications beyond shoe design. The slip pulses in the experiments share key features with rupture fronts in earthquakes, where sections of a fault suddenly break and slide at very high speeds.

"Soft friction is usually considered slow, yet we show that the squeak of a sneaker can propagate as fast as, or even faster than, the rupture of a geological fault, and that their physics is strikingly similar," study co-author Shmuel Rubinstein, a professor of physics at the Hebrew University of Jerusalem and a visiting professor at SEAS, said in a statement.

Beyond shedding light on the physics of earthquakes, the work could help engineers design surfaces that switch between slippery and grippy states on demand.

"Tuning frictional behavior on the fly has been a long-standing engineering dream," Katia Bertoldi, a professor of applied mechanics at Harvard, said in the statement. "This new insight into how surface geometry governs slip pulses paves the way for tunable frictional metamaterials that can transition from low-friction to high-grip states on demand."