In the realm of everyday experiences, the peculiar and often irritating squeak produced when a basketball shoe scrapes across a gym floor, bicycle brakes are over-applied, or tires skid sharply on pavement has stumped scientists for decades. Until now, the prevailing explanation for such sounds has been rooted in the stick-slip friction phenomenon—a well-understood cycle where surfaces alternately grip and slip past each other. This explanation, however, largely applies to contacts between hard, rigid surfaces, leaving the auditory mysteries of soft-on-rigid interfaces, such as rubber soles on polished wood or glass, insufficiently explained. A groundbreaking investigation led by researchers at Harvard University, in collaboration with experts from the University of Nottingham and the French National Center for Scientific Research, has now illuminated a novel mechanical process responsible for these ubiquitous squeaks, revolutionizing our understanding of frictional sound generation.
At the heart of this research lies the application of cutting-edge experimental techniques, including high-speed imaging capable of capturing events at nearly one million frames per second, coupled with precise audio synchronization. By observing the intimate contact interface between soft rubber and rigid glass, the scientists discovered that frictional sliding does not happen as a smooth or uniform event. Instead, motion is highly localized and propagates as dynamic, wave-like disturbances they identified as supersonic opening slip pulses. These pulses rapidly detach and reattach the surfaces in a repetitive cycle, traveling at speeds close to or exceeding the speed of sound within the material interface. This startling discovery fundamentally alters the way we conceive of friction at soft-rigid boundaries.
Unlike the chaotic irregularities of stick-slip friction traditionally thought to create frictional noise, the squeaking sounds emerging from these opening slip pulses are remarkably regular and quantifiable. The frequency of the squeak matches the repetition rate of these pulses, which propagate along the interface with clockwork precision. This periodicity is not random but is governed by the geometric configuration of the surfaces involved. In experiments where the sliding occurred between flat rubber blocks and glass, the pulses’ behavior was complex, generating broadband noise reminiscent of rushing or swooshing. However, introducing tread-like thin ridges or patterns on the rubber surface dramatically transformed the pulse dynamics, confining their movement into spatially regular cycles and producing clear, tonal squeaks.
This innovative revelation demonstrated that surface geometry functions as a waveguide for slip pulses, trapping and stabilizing their propagation. The engineered ridges cause the pulsations to lock into a characteristic frequency dictated by the dimensions of the rubber block, especially its height. This precise relationship between physical size and sound frequency is so reliable that the researchers astoundingly designed blocks capable of playing a recognizable tune—the iconic Star Wars theme—merely by manipulating their geometric parameters and sliding them by hand. Such control over friction-driven acoustics suggests new avenues for engineering materials with on-demand tunable frictional properties, offering exciting possibilities in soft robotics, prosthetics, and material sciences.
Further deepening the intrigue, the team observed an ephemeral luminous phenomenon accompanying the slip pulses—tiny flashes of triboelectric discharge, akin to miniature lightning bolts. These electrical discharges, generated by the friction between the rubber and glass, appear to play a role in triggering or enhancing the slip pulses. This coupling of mechanical motion and electrostatic phenomena provides a novel insight into the complex physics of friction, where electrical and mechanical fields profoundly interact at material interfaces. The implications extend towards smarter material design, where electrical feedback can be exploited to modulate frictional performance dynamically.
The physical mechanisms behind these opening slip pulses intriguingly parallel processes seen in geophysics, particularly earthquake mechanics, where rupture fronts propagate along tectonic faults at supersonic speeds. In a remarkable convergence of scales and disciplines, the researchers suggest that the fast slip pulses in soft frictional interfaces are governed by similar underlying dynamics as earthquakes, despite vast differences in size and materials. This cross-disciplinary insight not only enhances our understanding of seismic events but also deepens fundamental knowledge of frictional phenomena across materials and scales.
Challenging traditional one-dimensional friction models, the findings emphasize the importance of interface dimensionality and geometry. The research shows how even minute surface features in two or three dimensions can drastically alter the dynamics of sliding contacts. This nuance reveals that frictional behavior is far richer and more complex than previously believed. For engineers and physicists, this means that simplified models may be insufficient for predicting or controlling friction in many practical applications, including tires, footwear, brakes, and biomechanical systems.
The broader implications of this study extend into the design of metamaterials—materials engineered to have properties not found in nature. By harnessing the insights gleaned about geometric confinement and slip pulse dynamics, future materials can be designed to switch frictional states rapidly and controllably. Such materials could transition from nearly frictionless sliding to powerful grip by modulating their surface patterns or structural dimensions. This “tunable friction” could revolutionize industries spanning transportation, manufacturing, sports equipment, and beyond.
The study was carried out through a remarkable international collaboration among premier institutions: Harvard University, CNRS at Université du Mans, Hebrew University of Jerusalem, and the University of Nottingham. Supported by multiple funding bodies, including the U.S. National Science Foundation, Simons Foundation, BASF, and the Swiss National Science Foundation, this research laid foundational groundwork for advancing tribology—the science of friction—into the realm of dynamic, geometry-governed phenomena.
As this emerging field develops, the fascinating convergence of tribology with nonlinear physics, materials science, electrical phenomena, and geophysics promises to rewrite textbooks and redefine technological potentials. The humble and oft-maligned squeak of a sneaker on a gym floor now stands as a testament to the complexity and beauty hidden in everyday materials interactions and heralds an exciting frontier of research on friction that transcends traditional boundaries.
This discovery reveals that the next generation of frictional materials and devices might not only be quieter or grippier but smart enough to respond to their environments dynamically and with unprecedented precision. The bridge formed between soft material friction and earthquake physics has illuminated how essential surface geometry and dynamic pulsations are to frictional sound generation. What started as curiosity about an annoying squeak has blossomed into a transformative journey at the cutting edge of applied mechanics and material science.
Subject of Research: Not applicable
Article Title: Squeaking at soft–rigid frictional interfaces
News Publication Date: 25-Feb-2026
Web References:
https://dx.doi.org/10.1038/s41586-026-10132-3
Image Credits: Adel Djellouli / Bertoldi lab at Harvard
Keywords
Friction, Material properties, Materials, Metamaterials, Physics, Applied physics, Mathematical physics, Mechanics
