Groundbreaking Physics Discovery Unravels the Mystery Behind Squeaking Sneakers

United States - Ekhbary News Agency

Groundbreaking Physics Discovery Unravels the Mystery Behind Squeaking Sneakers

The distinctive squeak of sneakers on a basketball court is an instantly recognizable sound, an almost inevitable part of the game traditionally attributed to the physics of friction. Yet, despite the ubiquity of similar high-pitched noises—from screeching tires to aging bike brakes and windshield wipers—surprisingly little in-depth research has explored the detailed dynamics of squeaky surfaces. Now, a pioneering international study has yielded pivotal insights, shedding new light on this everyday phenomenon and revealing unexpected scientific frontiers.

Published recently in the prestigious journal Nature, the study challenges long-held theories about stick-slip friction, the process believed to generate these sounds through regular cycles of two objects adhering and moving between one another. While it was understood that this phenomenon played a role, the explanation was not comprehensive enough to encompass every influencing factor. This gap in understanding prompted an international research team, including scientists from Harvard University and the University of Nottingham, to undertake a detailed examination of the complex physical relationships underlying the squeak.

The project began with a straightforward question: "Why do basketball shoes squeak?" stated Adel Djellouli, a study co-author and materials scientist at Harvard's School of Engineering and Applied Sciences (SEAS). To answer this, Djellouli's team not only leveraged advanced technology but also drew inspiration from none other than Leonardo da Vinci, the 15th-century polymath famous for devising an angled contraption to aid his experiments exploring friction physics. Building upon over 500 years of friction research, the team utilized internal reflection imaging alongside cameras capable of recording at one million frames per second to document the shifting contact points between rubber sneaker soles and a glass surface. Simultaneously, delicate tools precisely measured the audio produced during each minute squeak.

The results were startling, contradicting established theories regarding stick-slip events. Instead of occurring randomly, squeaking sound frequencies were found to be determined by the repetition rate of propagating pulses. This repetition speed, in turn, is dictated by the rubber sneaker's stiffness and thickness. Further experiments using flat-sided rubber blocks on glass also revealed far more complex and irregular noise pulses, resembling broader, swishing sounds, thereby proving that surface geometry is a major factor in how friction generates squeaks. "We were surprised that tiny surface features could so strongly reorganize frictional motion," added study co-author Gabriele Albertini, a materials scientist at the University of Nottingham. "These results challenge the assumption that friction can be fully captured by simplified one-dimensional models."

Djellouli, Albertini, and their colleagues eventually developed such a profound understanding of these relationships that they could arrange rubber blocks at various heights and manually play Darth Vader's theme song from Star Wars. Coincidentally, the team uncovered another fascinating frictional consequence reminiscent of a galaxy far, far away: occasionally, slip pulses created triboelectric discharges—essentially, tiny instances of 'force lightning.'

Beyond engineering quieter sneakers, these novel findings are poised to significantly advance some of the world's most sophisticated engineering materials. "Tuning frictional behavior on the fly has been a long-standing engineering dream," explained Katia Bertoldi, a SEAS materials scientist and co-author. "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." This capability could revolutionize fields requiring precise control over surface interactions, from robotics to advanced manufacturing.

The ramifications of this research also extend to much larger, geological phenomena. The very same physics observed in these slip pulses are mirrored during earthquakes, where tectonic faults produce high-speed ruptures that can sometimes propagate faster than the speed of sound. "These results bridge two fields that are traditionally disconnected: the tribology of soft materials and the dynamics of earthquakes," stated physicist Shmuel Rubinstein. "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." This discovery represents a significant leap in our understanding of the physical world, promising wide-ranging practical applications from everyday products to immense geological events.

Ekhbary News Agency