A new experiment has uncovered the mechanism responsible for the screeching sound made by peeling sticky tape. Using a combination of ultrafast imaging and synchronized acoustic recordings, Sigurdur Thoroddsen and colleagues at King Abdullah University of Science and Technology have shown that the noise is produced by a rapid train of tiny shockwaves, released through a specialized form of stick–slip motion. The research is published in Physical Review E.

If you’ve ever used sticky tape, you’ll probably be all too familiar with the harsh sound it makes as it peels away from a surface. Yet despite decades of experimental scrutiny, physicists have yet to fully explain the origins of this intriguing acoustic effect.

Previous studies established that peeling proceeds via a “stick–slip” mechanism—a jerky motion characterized by brief, rapid accelerations interrupted by sudden stops. Similar dynamics underpin phenomena ranging from earthquakes to the squeak of basketball shoes on a polished wooden court. However, the fine details of how this process unfolds in peeling tape turned out to be more complex than they first appeared.

Basketball shoes on a gym floor, bicycle brakes in need of a tune-up, or the squeal of tires are everyday examples of squeaking sounds. Such sounds have long been attributed to stick-slip friction, or 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-on-rigid interfaces, like shoes on a floor.

To shed light on this little-understood physical process, researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS), in collaboration with the University of Nottingham and the French National Center for Scientific Research, have used high-speed imaging to investigate the dynamics of soft solids sliding rapidly on rigid substrates.

In a study published in Nature, the team led by first author Adel Djellouli, a postdoctoral fellow in the lab of Katia Bertoldi, the William and Ami Kuan Danoff Professor of Applied Mechanics at SEAS, reports that squeaking emerges from a previously unseen mechanism.

In crowds, most people are strangers to you, and everyone else for that matter. However, until now, the effect of stranger-to-stranger interactions on the choices people make in crowds has not been properly examined. Ziqi Wang and Federico Toschi from the TU/e Department of Applied Physics and Science Education, along with Alessandro Gabbana at the University of Ferrara in Italy, explored how strangers influence people’s choices in crowds at Eindhoven Centraal railway station. The research is published in the journal Proceedings of the National Academy of Sciences.

“Using a collection of special overhead sensors, we gathered data on how pedestrians move over a three-year period, from March 2021 to March 2024,” says Toschi. “This amounted to about 30,000,000 pedestrian trajectories and included people getting off trains and those waiting on the platform. We collaborated with ProRail on this project, as we have done in previous studies on how pedestrians move in Eindhoven Centraal station.”

Toschi has been studying pedestrian dynamics for some time and was jointly awarded the 2021 Ig Nobel Prize for physics for work on how pedestrians keep a certain distance from each other in crowds.