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New Scientific Study Published on February 25 Investigated the Known Squeak of Shoes Observed on Sports Courts and Hard Surfaces, Revealing That the Phenomenon Involves Complex Microscopical Physical Processes That Challenge Traditional Explanations About Friction and Movement Between Soft and Solid Materials
The squeak produced by shoes sliding over hard surfaces was investigated by researchers in a study published on February 25 in the journal Nature, indicating that the sound results not only from friction but from extremely rapid microscopic movements in the rubber.
Research Identifies New Physical Mechanism Behind the Squeak of Shoes
The study showed that shoes do not slide uniformly over the ground. Instead, small regions of the sole detach and quickly reconnect with the surface, forming structures called opening slip pulses.
These pulses behave like rapid fronts similar to wrinkles that move across the contact area. The process occurs repeatedly and generates vibrations that are perceived by the human ear as the characteristic squeak of shoes.
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According to the researchers, this behavior challenges the traditional explanation based only on the adhesion-sliding friction, a model widely used to explain noises in rigid systems, such as hinges or brakes.
Soft materials, like the rubber present in shoes, exhibit distinct dynamics when coming into contact with hard surfaces, requiring new physical models to explain the phenomenon.
Experiments with Rubber Reveal Rapid Pulses and Microscopical Electrical Discharges
To analyze the behavior of the shoes, scientists from the John A. Paulson School of Engineering and Applied Sciences at Harvard collaborated with researchers from the University of Nottingham and the French National Centre for Scientific Research.
The team used high-speed optical imaging combined with synchronized audio recordings to observe the movement of rubber sliding quickly over smooth glass.
The experiments demonstrated that the movement does not occur continuously. It happens in cycles of starts and stops concentrated in small areas, while other parts remain fully adhered to the ground.
In some tests, the researchers observed small flashes generated by friction. These electrical discharges were described as tiny sparks similar to lightning.
Although not the main source of the sound, these sparks showed that electric energy can accumulate when shoes move over hard surfaces, influencing the behavior of the slip pulses.
Shape of the Rubber Directly Influences the Sound Produced by the Shoes
The results indicated that the shape of the rubber plays a more important role than the movement itself in the tone of the noise produced by the shoes.
When flat blocks of rubber slid over the glass, the pulses occurred irregularly, producing a broader sound similar to a hum.
By adding fine grooves to the surface of the rubber, the researchers were able to confine the slip pulses. This caused them to repeat at regular intervals, stabilizing the sound frequency.
The protrusions acted as guides that channel the movement, fixing the squeak at a specific tone. The observed frequency became primarily dependent on the height of these structures in the rubber.
The pattern proved to be so predictable that the team built blocks with different heights and used the system to manually reproduce the theme from the Imperial March, from the Star Wars franchise.
According to the study’s lead author, Adel Djellouli, three consecutive days of rehearsals were required to synchronize rhythm and technique during the recording made in the laboratory.
Phenomenon Observed in Shoes Shows Similarities to Earthquakes
In addition to explaining the squeak of shoes, the study pointed out connections between the observed behavior and geological processes associated with earthquakes.
The identified slip pulses share characteristics with rupture fronts present in geological faults, where parts of the Earth’s crust suddenly break and slide at very high speeds.
According to co-author Shmuel Rubinstein, friction in soft materials is usually considered slow. However, experiments showed that the squeak of a sneaker can propagate as quickly as, or even faster than, geological ruptures.
The similarity suggests that studies involving shoes and rubber could help to enhance the understanding of the physics involved in earthquakes.
Findings Could Guide the Development of Adjustable Adhesive Surfaces
The researchers claim that the results have applications that go beyond shoe design.
Understanding how surface geometry controls slip pulses could allow for the development of materials capable of switching between slippery and adhesive states as needed.
According to Katia Bertoldi, a professor of applied mechanics at Harvard, adjusting frictional behavior in real-time represents an age-old goal of engineering.
The new analysis paves the way for the creation of adjustable frictional metamaterials, capable of transitioning between low friction and high adhesion on demand, using geometric control of surfaces.
Thus, the study that investigated the simple squeak of shoes revealed complex physical mechanisms involving supersonic motion, localized vibration, and accumulation of electrical energy on microscopic scales.
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