A Boston Celtics game-inspired friction test finally pinned down the sneaker squeak

As he watched the Boston Celtics play from the stands of TD Garden, one noise kept catching Adel Djellouli’s ear.

“This squeaking sound when players are sliding on the floor is omnipresent,” he said. “It’s always there, right?”

That distinctive squeak of basketball shoes against hardwood has become an integral part of the game’s auditory experience. For players and fans alike, the sound serves as a constant backdrop to the action – a sonic signature of quick cuts, defensive slides, and sudden changes in direction.

Djellouli, a materials scientist at Harvard University, transformed his casual observation into a full-fledged scientific inquiry. What exactly causes that iconic sound? The question led to groundbreaking research that was published Wednesday in the prestigious journal Nature.

To solve the mystery, Djellouli and his research team devised a straightforward yet effective experiment. They repeatedly slid a sneaker against a smooth glass plate while recording the sound with a microphone. Simultaneously, they captured the interaction using high-speed cameras to observe what happened at the interface between shoe and surface.

The findings revealed a fascinating physical phenomenon. As athletes move across the court, tiny sections of their shoe soles momentarily lose contact with the floor before regaining it – thousands of times per second. This rapid sequence of detachment and reattachment creates microscopic ripples or wrinkles that travel through the rubber sole.

“That squeaking is basically your shoe rippling, or creating wrinkles that travel super fast. They repeat at a high frequency, and this is why you get that squeaky noise,” Djellouli explained.

The team’s experiments demonstrated that the distinctive tread patterns on basketball shoes play a crucial role in generating the characteristic sound. When researchers tested featureless blocks of flat rubber against glass, they observed chaotic, disorganized ripples but heard no squeaks. This suggests that the ridge-like designs on athletic shoe soles organize these micro-movements into patterns that produce the clear, high-pitched sound basketball fans recognize worldwide.

While previous studies have examined similar friction-based phenomena, this research stands apart by focusing on much higher speeds and establishing a direct connection between these rapid pulses and the resulting acoustic output.

The implications extend far beyond basketball courts. As physicist Bart Weber noted in an editorial accompanying the research, “Friction is one of the oldest and most intricate problems in physics,” yet despite its everyday presence, it remains “difficult to predict and control.”

A deeper understanding of friction mechanics could inform multiple fields of study. Geologists might apply these insights to better comprehend how tectonic plates slide and interact during earthquakes. Engineers could develop methods to reduce friction and wear in machinery, potentially saving energy and extending equipment lifespan.

For everyday consumers, the research might eventually lead to practical applications – like designing shoes that don’t squeak in inappropriate settings such as quiet office hallways or libraries. While the current study doesn’t offer immediate solutions to unwanted squeaking (though internet remedies like rubbing soap or dryer sheets on soles abound), it provides foundational knowledge that could inform future footwear design.

One particularly intriguing experiment revealed that altering the rubber thickness affected the pitch of the squeak. This discovery suggests the possibility of engineering shoes that produce sounds at frequencies beyond human hearing range.

“We can now start designing for it,” said Weber, who is affiliated with the Advanced Research Center for Nanolithography and the University of Amsterdam. “We can start making interfaces that either do it if we want to hear this sound, or don’t do it if we don’t want to hear it.”

For basketball purists, however, the familiar squeak might remain an essential element of the game’s atmosphere – a sound as fundamental to the sport as the swish of the net or the bounce of the ball.

The research serves as a reminder that even the most familiar aspects of our daily experience can contain unexplored scientific mysteries waiting to be unraveled.