The physics of squeaking sneakers

Source: Ars Technica

Background

We’re all familiar with the high‑pitched squeak of basketball shoes on a court or tires squealing on pavement. Recent experiments have shown that the geometry of a sneaker’s tread pattern determines the squeak’s frequency. By designing rubber blocks tuned to specific frequencies and sliding them across glass, researchers were able to play recognizable tunes such as Star Wars’ “Imperial March.”

“Tuning frictional behavior on the fly has been a long‑standing engineering dream,” said co‑author Katia Bertoldi of Harvard University. “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 dynamics revealed by these results are also similar to those of tectonic faults, offering a new model for the mechanics of earthquakes, according to their paper in Nature.

Historical Context

Leonardo da Vinci is usually credited with conducting the first systematic study of friction in the late 15th century, a subfield now known as tribology—the study of interacting surfaces in relative motion. Da Vinci’s notebooks show experiments with rows of blocks pulled by weights and pulleys, investigations of friction in screw threads, wheels, and axles—methods that still inform modern friction studies. The authors of the latest paper used an experimental setup reminiscent of da Vinci’s approach.

Experiments

The research team slid commercial basketball shoes (Nike CU3503‑100) across a smooth, dry glass plate while simultaneously recording sound and high‑speed visual imagery of the frictional interface. They observed opening pulses traveling in the sliding direction in a non‑uniform manner, creating temporary local supersonic separations between the shoe soles and the glass. These separations generate audible squeaks whose frequency is not random; it is set by the repetition rate of the generated pulses.

Findings

• Tread geometry controls squeak frequency. By adjusting the pattern of the sole’s tread, the team could predict and tune the acoustic output.
• Slip‑pulse dynamics observed in soft‑on‑rigid interfaces differ from classic stick‑slip models that apply to two rigid bodies (e.g., squeaking door hinges).
• The same pulse mechanisms appear in tectonic fault slip, suggesting that sneaker‑squeak experiments can serve as a laboratory analog for earthquake mechanics.

These insights open the door to tunable frictional metamaterials—engineered surfaces that can switch between low‑friction and high‑grip states on demand, with potential applications ranging from sports equipment to seismic research.