Simple lubricants transform from liquid to a solid-like film under extreme conditions, according to findings by a GW research team.
By Kristen Mitchell
A School of Engineering and Applied Science research team recently published a paper on the structure of lubricants that they believe settles a long-debated fundamental science question: How do simple lubricants act under extreme conditions?
Yongsheng Leng, SEAS professor, and Rong-Guang Xu, a doctoral student, used computational modeling to examine the question and found that on a nanometer scale, lubricants change from liquid to a solid-like structure when they experience excess friction. This solid-like structure creates a boundary film where two surfaces meet and plays a critical role in lubrication engineering.
How lubricants act under extreme confinement has long divided researchers. Some believe lubricants transform from liquid to a glass-like substance, while others believe they change to a solid crystal-like structure. The work in Dr. Leng’s lab confirms the findings of previous work done by another research team.
A simple lubricant will form a boundary film to protect surfaces from damage when an object under pressure goes from still to moving, Dr. Leng said. This creates a jerky motion called a stick-slip. A stick-slip is what causes sound when squeaky door hinges creak or when a musician plays the violin.
Understanding lubricants is fundamental to engineering, Dr. Leng said. Knowing more about how these lubricants react to squeezing and friction can lead to more efficient energy usage and surface protection.
“People widely believe that when there is friction, the film turns back to liquid. We changed their view that it keeps the solid-like film,” Dr. Leng said. “I think the very, very important thing is that somehow, this kind of lubricant essentially resolves this long-standing debate, this scientific fundamental debate about the thermodynamic state of lubricant under extreme confinement.”
SEAS professor Yongsheng Leng (left) and Rong-Guang Xu, a doctoral student, used computational modeling to examine the structure of lubricants on a nanometer scale. (Logan Werlinger/ GW Today)
Michael Plesniak, SEAS Mechanical and Aerospace Engineering Department chair, said the molecular dynamics simulations conducted in Dr. Leng’s lab provides deep insight on the fundamentals of lubrication processes.
“Dr. Leng’s studies will allow new ‘designer’ lubricants to be engineered at the molecular level to achieve improved efficiency and better durability,” he said. “Such new lubricants will lead to significant energy and cost savings in many sectors and will enable new technologies.”
This research could help engineers better understand the dynamics of lubricants in cars. When a vehicle is behaving as normal on the road, the oil in the vehicle remains a liquid. Under extreme conditions, however, like when the driver slams on the brakes or accelerates quickly, the transition happening in the engine might change the lubrication to a solid.
“Sometimes the lubricant in your car behaves dramatically different from what you thought,” Dr. Leng said. “But it’s fine. It would go back to a liquid when normal conditions return.”
Dr. Leng was awarded a five-year National Science Foundation Faculty Early Career Development (CAREER) grant in 2012 to pursue this work. It’s important that researchers continue to explore fundamental questions of science and engineering as national funding stagnates, he said.
Dr. Leng’s lab plans to expand on his current research and explore how the properties of more complicated lubricants behave under extreme conditions.
The paper, “Squeezing and stick–slip friction behaviors of lubricants in boundary lubrication,” was published in PNAS in June.