The Physics Behind Friction

Spring 2023

Cropped shot of man with artificial leg walking on treadmills at gym with other people

Without the force called friction, cars would skid off the roadway and objects would tumble off tables and onto the floor. Even so, how friction works at a molecular scale remains poorly understood.

Now, using complex modeling and computer simulations, a team that included Johns Hopkins mechanical engineer Jaafar El-Awady has shed new light on a particular feature of friction known as “aging.” The team’s findings, which appeared in ACS Nano, could inform the design of improved prosthetic devices and artificial joints, the researchers say.

Aging occurs “when one solid rests on another for a long time without sliding, and the force needed to slide them apart increases. We wanted to find out why,” says study leader Lucas Frérot, formerly a postdoctoral researcher at the Whiting School and now at Germany’s Albert-Ludwigs-Universität’s Institut for Mikrosystemtechnik. Previous experiments by researchers at France’s Laboratoire de Tribologie et Dynamique des Systèmes at École Centrale de Lyon gave a very detailed picture of the friction response of surfaces coated with fatty acids, an environmentally friendly family of lubricants.

But those alone couldn’t explain the phenomenon behind aging. Using measurements of surface roughness and the properties of the single-molecule-thick layer of fatty acid molecules, the Johns Hopkins team used molecular simulation to reproduce the aging process.

A graphic showing the microscopic view of a rough surface
Deposited on rough surfaces, fatty acid layers form contact junctions that explain frictional aging, as was highlighted by cutting-edge research combining friction experiments, molecular simulations, and a theoretical model.

“That allowed us to try things impossible in experiments, such as what would happen if the surfaces in contact were mathematically flat,” says El-Awady, program chair of mechanical engineering in the Whiting School’s Engineering for Professionals programs.

They found that the main cause of aging was surface roughness, concluding that even a small amount of roughness is enough to prevent the molecules from making contact over the whole surface, leaving the molecules on the edges of contact spots free to move. Over time, more molecules come in contact, resulting in aging.

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

“This is the case in biological systems like joints, and if we understand those systems better, we can design better and more durable prostheses,” Frérot says. “In a more general sense, understanding the physics behind friction is important in the design of sustainable systems. Some studies estimate that about 23% of the world’s energy consumption is lost to friction.”

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