The physics of squeaking sneakers
We all know the high-pitched squeak of basketball shoes on the court during games, or the screech of tires on the sidewalk. The scientists conducted several experiments and found that the geometry of the sneaker treads determines the frequency of the squeak, allowing the team to make rubber blocks tuned to specific frequencies and slide them across glass surfaces to play “Imperial March” from Star Wars.
“Tuning friction behavior on the fly has been a long-held engineering dream,” said co-author Katia Bertoldi of Harvard University. “This new understanding of how surface geometry governs sliding impulses paves the way for tunable friction metamaterials that can transition from low-friction to high-adhesion states on demand.” In addition, the dynamics revealed by these results are similar to those of tectonic faults and thus give scientists a new model for the mechanics of earthquakes, according to their new article published in the journal Nature.
Leonardo da Vinci is generally credited with conducting the first systematic study of friction in the late 15th century, a subfield now known as tribology that deals with the dynamics of interacting surfaces in relative motion. Da Vinci’s notebooks describe how he pulled rows of blocks using weights and pulleys, an approach that is still used today in friction studies, and also examines the friction produced in screw threads, wheels, and axles. The authors of the latter article used an experimental setup similar to that of Leonardo da Vinci.
The squeaking of sneakers on a gym floor is usually attributed to friction, particularly a variety of stick-slip that involves cycles of sticking and sliding between two surfaces. But this model is better suited to interfaces involving two rigid objects, such as squeaky door hinges. Sneaker soles sliding on a gym floor involve a hard object (the floor) and a soft object (the sole of the sneaker). Bertholdi et al. wanted a more complete understanding of the dynamics of soft-on-rigid interfaces.
First, the team slid commercial basketball shoes (the Nike CU3503-100) across a smooth, dry plate of glass, simultaneously capturing audio and visual images of what was happening between the sole and the glass (i.e., the friction interface). They identified opening pulses moving non-uniformly in the direction of sliding, resulting in temporary local supersonic separations between the shoe soles and the glass plate. These audible squeaks are not random; the frequency is determined by the repetition rate of the generated pulses.
