Researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences teamed with scientists from the University of Nottingham and the French National Center for Scientific Research to uncover the physical mechanism that produces the squeak of sneakers and similar sounds at soft-on-rigid interfaces, as this article from Harvard University reports. Using high-speed optical imaging capturing up to 1,000,000 frames per second, synchronized sound measurements, and a sliding rig inspired by early friction experiments, the team observed what happens when soft rubber slides quickly across a hard surface, such as glass. The familiar squeak does not come from simple stick-slip friction, as traditionally thought, but instead from what the researchers call “opening slip pulses.” These are rapid, localized detachment fronts that propagate along the interface at near-supersonic speeds and repeatedly detach and reattach rubber from the rigid substrate. The repeating pulses set the frequency we hear as a squeak.
The study found that the visible structure of the rubber surface plays an important role. Flat rubber surfaces produce irregular pulses and broadband noise. But when the surface includes thin ridges similar to tread patterns, the detachment fronts become confined to a regular, periodic cycle. These geometric waveguides force the pulses to repeat at predictable intervals, producing a distinct pitch determined by system dimensions such as block height. In laboratory demonstrations, blocks with designed heights even produced recognizable musical notes.
Unexpectedly, experiments also revealed that tiny electrical discharges, akin to miniature lightning, sometimes accompany the formation of slip pulses. Beyond explaining an everyday sound, the research has broader implications: the dynamics of these slip fronts resemble rupture fronts in geological faults, offering a surprising connection between sneaker squeaks and earthquake physics. The work challenges conventional friction models for soft materials and suggests pathways to engineer surfaces with tunable friction and acoustics.