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 squeal of rubber on a hard floor occurs when tiny areas of slip between the sole of the shoe and the floor move at supersonic speeds — and in some experiments, the process involved miniature, lightning-like sparks. In addition, the findings may lead to an improved understanding of earthquakes and aid in the design of gripping surfaces.
Scientists have long explained squeaks from shoes, bicycle brakes and tires in terms of stick-slip friction, a stop-and-go cycle in which surfaces repeatedly catch and then break free. That model works well for many hard against hard systemsas door hinges.
But soft materials such as rubber behave differently when sliding over 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 UK and the French National Center for Scientific Research. They used high-speed optical imaging and synchronized sound to see soft rubber move quickly along smooth glass.
But what they saw was not smooth. Instead, the movements gathered into opening pulses, which swept over the rubber in starts and stops.
“Basically, these findings challenge the long-held assumption that soft material friction can be fully captured by simplified, one-dimensional ‘stick-slip’ models,” first author of the study. Adel Djelloulia postdoctoral fellow at Harvard, told LiveScience in an email.
Little lightning bolts everywhere
The findings reveal more about the physics of friction. In classic stick-slip friction, the entire contact surface alternates between sticking and sliding. In this study, however, the movement was more localized, as only small areas opened and slid, and then progressed, while other regions remained 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, these sparks, or electrical discharges, appeared to trigger the slip pulses. The sparks weren’t the main source of the squeaking sound, but they did show how electrical energy could build up in the system as 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 limited the pulses and caused them to repeat at regular intervals.
In effect, the ridges acted as guides, channeling the pulses into a repeating cycle. This locked the sound to a specific frequency, or tone. The team found that this squeak frequency was mainly dependent 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,” Djellouli said. “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 funnest part was the relief in the lab when we finally finished recording after three days of constant, loud squeaking. Our colleagues were very happy to finally get some peace!”
What sneakers can 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 parts of a fault suddenly break and slide at very high speeds.
“Soft friction is usually considered slow, but 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,” co-author of the study. Shmuel Rubinsteina professor of physics at the Hebrew University of Jerusalem and a visiting professor at SEAS, said i a statement.
Beyond shedding light on the physics of earthquakes, the work could help engineers design surfaces that switch between smooth and grippy states as needed.
“Adjusting frictional behavior on the fly has been a long-standing engineering dream,” Katia Bertoldia professor of applied mechanics at Harvard, said i the statement. “This new insight into how surface geometry controls sliding pulses paves the way for tunable friction metamaterials that can transition from low-friction to high-grip states on demand.”
Djellouli, A., Albertini, G., Wilt, J., Tournat, V., Weitz, D., Rubinstein, S., & Bertoldi, K. (2026). Creaking at soft-stiff frictional interfaces. Nature. https://doi.org/10.1038/s41586-026-10132-3
