A basketball shoe sole sliding on a wooden floor, a finger on a glass pane, a bicycle brake...: many everyday phenomena produce a squeaking sound, caused by friction at the interface between a soft material and a rigid material.
The mechanism at the origin of this squeak was not fully understood, and it is to study it that researchers have carried out several experimental studies that allow us to understand how friction between the two materials generates the squeaking noise.
Illustration image Pixabay
A first study focused on a simple example: commercial basketball shoes sliding on a smooth glass plate at a speed of approximately 1 m/s (about 3.3 ft/s). To observe the phenomenon, an acoustic device records the emitted sound, while the interface between the sole and the glass is visualized using an optical system and ultra-fast imaging.
The study revealed the existence of opening impulses, that is, temporary separations of the interface, which propagate in the sliding direction at approximately 80 m/s (about 260 ft/s), much faster than the sliding speed, with a frequency close to 5000 Hz. Now, this same frequency appears simultaneously in the spectrum of the recorded sound, in the repetition rate of the impulses. This clearly links the squeaking sound to the dynamics of the interface opening impulses. Similar behavior is observed when rubbing a finger on glass.
To go further in the study of this mechanism, the team conducted another series of experiments with silicone blocks of well-defined dimensions (40 × 40 × 20 mm), which they slid on smooth rigid substrates, under a controlled normal stress and traction force. The same phenomenon of opening impulse propagation was observed, and measurements revealed that these impulses propagate at high speeds, comparable to the speed of shear waves in the soft material (approximately 22-24 m/s or 72-79 ft/s for the silicone used).
When the sliding surface of the silicone block is flat, the impulses are disordered. Their complex dynamics results in broadband acoustic emission: a noise without a dominant frequency. But when the researchers created thin parallel ribs, crenellations, on the sliding surface of the soft block, they observed that the opening impulses are spatially confined and propagate along these crenellations.
Schematic diagram of the experimental setup designed for visualizing waves at the friction interface between a shoe sole and a solid plate.
© V. Tournat
The impulses are now generated regularly, with a constant rate, and the crenellations act as waveguides. The squeaking sound is clear and tonal. The geometry of the crenellations has little influence on the emitted fundamental frequency, which instead depends on the height of the silicone block.
The team thus manufactured a series of crenellated blocks, each producing a different note when sliding on glass. Similar observations were obtained with other soft materials, such as an elastomer and a thermoplastic polyurethane.
These results, beyond explaining the mechanism at the origin of squeaking, could allow it to be controlled, whether to suppress it or to choose its frequency, for detection or diagnostic purposes. The structuring of surfaces also opens new ways to reduce or modulate friction through fine management of interface rupture.
Finally, this study provides new experimental insight into friction and rupture between two materials, which may be of interest to research in geophysics and earthquake dynamics.
Schematic representation of the phenomena at play in the squeaking experiment when a soft solid is slid on a rigid surface.
© V. Tournat