New research has revealed exactly how sounds like squeaking trainers, bike brakes in need of a tune-up or chalk on a blackboard occur and how they could be stopped.
These sounds were long attributed to stick-slip friction: a cycle of intermittent sticking and sliding between surfaces. While this framework explains many rigid-on-rigid systems such as door hinges, it does not fully capture the physics of soft-rigid interfaces.
Researchers from the University of Nottingham's Faculty of Engineering have contributed to an international study led by Harvard's John A. Paulson School of Engineering and Applied Sciences (SEAS), in collaboration with CNRS (France), investigating the dynamics of soft solids sliding rapidly on rigid substrates. In a study published in Nature, the team reports that squeaking emerges from a previously unseen mechanism: supersonic opening pulses. The rupture dynamics of these pulses share key features with fracture fronts observed in tectonic faults, offering a surprising new model for studying earthquake mechanics.
Using high-speed optical imaging and synchronised audio measurements, the researchers directly visualised the contact interface between soft rubber and rigid glass. They discovered that sliding does not happen uniformly. Instead, motion localises into supersonic opening slip pulses: rapid wrinkle-like detachment fronts that propagate along the interface at speeds comparable to shear wave velocities in the material.
The researchers slid either a basketball shoe or rubber sample against a transparent glass plate fitted with LEDs along its edges; where the rubber touches the glass, the contact region lights up, tracking how contact changes in space and time. The contact patterns were filmed with a high-speed camera (up to 100,000 frames per second) while recording the sound with a microphone, using either a researcher wearing the shoe (to mimic real play) or a rubber block mounted on a guided rail system that precisely controlled the normal load and the sliding force.
They discovered that the audible squeak is not produced by random stick-slip events. Rather, the squeaking sound frequency is set by the repetition rate of these propagating pulses.
"This project started with a simple question: why do basketball shoes squeak?" said Dr Djellouli, lead author from Harvard University. "We combined total internal reflection imaging with cameras capturing up to one million frames per second to visualize the evolving contact between rubber and glass. To drive sliding, we adapted a configuration conceptually similar to Leonardo da Vinci's friction experiments from the 15th century."
The experiments further revealed that geometry plays a decisive role in sound generation. When rubber blocks with flat contacting surfaces were slid along glass, the pulses were complex and irregular, producing broadband noise. Introducing thin ridges dramatically altered the dynamics: the pulses became confined and periodic.
We were surprised that tiny surface features could so strongly reorganize frictional motion. These results challenge the assumption that friction can be fully captured by simplified one-dimensional models and highlight the critical role of interface dimensionality.
In a surprising twist, the high-speed images revealed another phenomenon accompanying the squeak: lightning. The study showed that under high normal load, the slip pulses are triggered by triboelectric discharges, miniature lightning bolts caused by the friction of the rubber, highlighting just how violent the "unzipping" event truly is.
This geometric confinement forces the pulse repetition rate to lock into a characteristic frequency determined by the system dimensions. The researchers observed a robust scaling relationship in which the squeak frequency depends primarily on the block height, a relationship so precise that the researchers were able to design blocks of varying heights to play the Star Wars theme song by hand.
Beyond the sound, the study found that these opening slip pulses significantly impact the overall frictional resistance.
"Tuning frictional behavior on the fly has been a long-standing engineering dream," said Prof. Bertoldi. " 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."
The implications extend far beyond squeaky shoes. The physics governing these slip pulses, specifically how two surfaces move relative to each other, mirror earthquake dynamics, where ruptures and slip pulses propagate along tectonic faults at extremely high speeds, approaching and sometimes exceeding the speed of sound.
"These results bridge two fields that are traditionally disconnected: the tribology of soft materials and the dynamics of earthquakes," said Prof. Rubinstein one of the co-authors of the study. "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."
