friction

We’re all familiar with the high-pitched squeak of basketball shoes on the court during games, or tires squealing on pavement. Scientists conducted several experiments and discovered that the geometry of the sneakers’ tread patterns determines the squeak’s frequency, enabling the team to make rubber blocks set to specific frequencies and slide them across glass surfaces to play 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.” 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 paper published in the journal Nature.

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 that deals with the dynamics of interacting surfaces in relative motion. Da Vinci’s notebooks depict how he pulled rows of blocks using weights and pulleys, an approach that is still used in frictional studies today, as well as examining the friction produced in screw threads, wheels, and axles. The authors of this latest paper used an experimental setup similar to da Vinci’s.

The squeaking of sneakers on a gym floor is usually attributed to friction, specifically a stick-slip variety that involves cycles of sticking and sliding between two surfaces. But that model is best suited for interfaces involving two rigid objects, such as squeaking door hinges. Sneaker soles sliding across a gym floor involves one hard object (the floor) and one soft one (the sneaker sole). 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 glass plate, simultaneously capturing sound and visual imagery of what was happening between the sole and the glass (i.e., the frictional interface). They identified opening pulses traveling in the sliding direction non-uniformly, resulting in temporary local supersonic separations between the shoe soles and the glass plate. Those audible squeaks aren’t random; the frequency is determined by the repetition rate of the generated pulses.