Atoms slip against one another, eventually sticking in various combinations. Tectonic plates do the same, sliding across each other until they stick in a stationary state. Everything from the tiniest particles to unfathomably large landmasses possesses this fundamental stick and slip characteristic, but only now are scientists beginning to understand the mechanics of the friction underpinning this property.
"The intermittent motion in sliding systems is termed stick-slip since the two surfaces in contact seem to repeat the stick and slip states. However, several precise measurements have found that extremely slow slip occurs in even apparently stick states before every stick-to-slip transition," said Toshiki Watanabe, doctoral student in YOKOHAMA National University's Graduate School of Environmental and Information Sciences, who recently co-authored a paper describing a new model to explain the puzzling switch. "This strange phenomenon, termed the static friction paradox, has remained an unsolved problem for decades."
The problem the model helps answer is that to explain these slow slips, other researchers reached for artificial friction laws, or explanations that included specific but fictional parameters like state variables. The new model — which was developed by Watanabe and co-author Ken Nakano, a professor in YOKOHAMA National University's Faculty of Environmental and Information Sciences, and is available online now and was published in Physical Review E on June 18— offers a simplified solution, without relying on artificial friction laws.
"The simple mechanical model we propose here, a viscoelastic toy model, provides a novel scenario to explain the static friction paradox in stick-slip instability without artificial friction laws," Nakano said, explaining the team validated their model theoretically. "Although one tends to think friction phenomena are complicated in general, their essence could be much simpler, as the present model shows."
Viscoelasticity describes how a material can react like a liquid or a solid to different stresses and deformations. A classic example of a viscoelastic material is silly putty. Left on its own, it'll conform to whatever surface it rests on, like a viscous liquid. Quickly pulled in two directions, however, it resists stretching and then breaks, like an elastic solid. Several different models describing viscoelasticity exist, the researchers said, but they turned to the simplest version, called the Kelvin-Voigt viscoelastic foundation. The idea is that a Kelvin-Voigt material appears only to have elastic properties over the long-term but resists quick changes. To this model, the researchers added a rigid probe that moves vertically and oscillates horizontally.
The sliding system does not employ static friction, the researchers said, but rather creates two slip states: slow-slip and fast-slip.
"This toy model provides a purely mechanical explanation for the static friction paradox," Watanabe said, noting that the rigid probe lifts vertically to control what's known as a gradual slow-slip growth, or slow creep motion. The slow-slip gradually increases in speed as it builds the stress of the friction. "During slip velocity growth, the more abruptly the timescale changes, the more intensively the slow-to-fast slip transition occurs."
Next, the researchers said they plan to further explore their model and investigate how the slow slip phenomenon may provide clues pointing to earthquakes. "Our ultimate goal is to understand complicated friction phenomena as intuitively as possible and provide criteria for predicting and controlling various friction-related systems, from atomic to geological scales," Nakano said.
The Japan Science and Technology Agency supported this research.
##
YOKOHAMA National University (YNU) is a leading research university dedicated to academic excellence and global collaboration. Its faculties and research institutes lead efforts in pioneering new academic fields, advancing research in artificial intelligence, robotics, quantum information, semiconductor innovation, energy, biotechnology, ecosystems, and smart city development. Through interdisciplinary research and international partnerships, YNU drives innovation and contributes to global societal advancement.
