Without the force called friction, cars would skid off the pavement, humans couldn’t walk on the sidewalk, and objects would fall off your kitchen counter and fall to the floor. Even so, how friction works at the molecular level remains poorly understood.

Using complex modeling and computer simulations, a team led by a postdoctoral researcher from the Johns Hopkins Whiting School of Engineering and the Krieger School of Arts and Sciences studied friction at the molecular and macroscopic scale. The results of the team’s study, which appear in ACS Nanonot only shed light on friction in general, but could also shed light on the design of improved prosthetic devices and artificial joints.

A graph showing friction aging

Legend: Deposited on rough surfaces, layers of fatty acids form contact junctions that explain friction aging, as evidenced by cutting-edge research combining friction experiments, molecular simulations and theoretical modeling.

Picture credit: Courtesy of the research team

“Friction is puzzling because it is not governed by a single interaction, such as the attraction between charges; it emerges from a combination of processes that can occur at different scales. In our work, we have tried to relate the nano and macro worlds for a particular characteristic of friction called aging, i.e. when one solid rests on another for a long time without slipping, the force required to pull them apart increases. We wanted to find out why,” said Lucas Frérot, now postdoctoral researcher at Albert-Ludwigs-Institut de l’Universität for Mikrosystemtechnik.

Research team members from the Laboratory of Tribology and Systems Dynamics at the Ecole Centrale de Lyon in France had previously conducted experiments that provided a very detailed picture of the frictional response of fatty acid-coated surfaces, a family of eco-friendly lubricants, but these alone could not explain the phenomenon behind aging. Using measurements of the surface roughness and single layer properties of fatty acid molecules, the Johns Hopkins team was able to reproduce the aging process in a molecular simulation.

“Our simulation allowed us to try things that weren’t possible in experiments, like what would happen if the surfaces in contact were mathematically flat,” said team member Jaafar El-Awady, professor of mechanical engineering at the Whiting School.

They found that the main cause of aging was surface roughness. In fact, models without roughness have not aged at all, according to Frérot.

“This was surprising because the surface roughness measured by the Lyon team was extremely small; the highest mountain and the deepest valley on the surface would be separated by about the length of a fatty acid molecule,” said Frerot.

The team concluded that even such a small amount of roughness is enough to prevent molecules from making contact across the entire surface, leaving molecules at the edges of the contact points free to move. Over time, more molecules come into contact, which leads to aging.

Although the discovered mechanism is not the only one that can explain why frictional systems age, the team believe it can be applied to a wide range of systems where chain-like molecules such as fatty acids that they have studied form a protective layer on a surface.

“This is the case in biological systems like joints, and if we understand these systems better, we can design better and more durable prostheses. More generally, understanding the physics behind friction is important in the design of systems Some studies estimate that around 23% of global energy consumption is lost due to friction,” Frérot said.

Research team members note that the design of the simulations used in this study was originally envisioned by Mark O. Robbins, a professor in the Johns Hopkins Department of Physics and Astronomy, who died in 2020.

“Lucas and I then continued to push the simulations with our collaborators to successfully achieve its goals and dedicate it to Mark’s memory,” El-Awady said.

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