The familiar squeak of athletic shoes on basketball courts isn’t simply the sound of friction, according to recent research published today in the journal Nature. Scientists have discovered that the sound originates from microscopic areas of slipping between the shoe sole and the floor moving at supersonic speeds—and, in some instances, generating miniature electrical sparks.
Researchers at Harvard University’s John A. Paulson School of Engineering and Applied Sciences, collaborating with experts from the University of Nottingham in the U.K. And the French National Center for Scientific Research, used high-speed optical imaging and synchronized audio recordings to analyze the movement of soft rubber against smooth glass. They observed that motion doesn’t occur as a uniform slide, but rather in fast, wrinkle-like fronts termed “opening slip pulses” that repeatedly detach and reattach.
“Fundamentally, these findings challenge the long-held assumption that soft-material friction can be fully captured by simplified, one-dimensional ‘stick-slip’ models,” explained Adel Djellouli, a postdoctoral fellow at Harvard and the study’s first author, in an email to Live Science. The traditional “stick-slip” model adequately explains friction in hard-on-hard systems, such as door hinges, but proves insufficient for understanding the behavior of softer materials like rubber.
The research revealed that the localized slipping generates vibrations that are perceived as squeaks. In some experiments, the team observed tiny flashes of light—described as miniature “lightning” sparks—during the friction process. While not the primary cause of the squeaking sound, these electrical discharges indicated that energy builds up within the system as the rubber moves.
Interestingly, the study found that the shape of the rubber significantly influences the pitch of the squeak. Flat rubber blocks produced irregular slip pulses, resulting in a broad “whoosh” sound. However, adding thin ridges to the rubber confined the pulses, creating a repeating cycle and a distinct tone. The frequency of the squeak was directly related to the height of the ridges.
The researchers even harnessed this principle to play the iconic “Imperial March” theme from “Star Wars” using blocks of varying heights. “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 told Live Science. “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.”
The implications of this research extend beyond improving shoe design and understanding everyday noises. The slip pulses observed in the experiments share similarities with rupture fronts in earthquakes, where sections of a fault suddenly break and slide at high speeds. “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,” said study co-author Shmuel Rubinstein, a professor of physics at the Hebrew University of Jerusalem and a visiting professor at Harvard, in a statement.
Katia Bertoldi, a professor of applied mechanics at Harvard, suggested the findings could too lead to the development of surfaces with tunable frictional properties. “Tuning frictional behavior on the fly has been a long-standing engineering dream,” she said in a statement. “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.”
