Friction remembers its past
In this photo from a demonstration of the study authors’ friction-measuring technique, a plastic cylinder (green) is pressed against a glass disk. LEDs on the outside of the disk inject light into the glass. Total internal reflection keeps the light inside the glass—except at microscopic points where the plastic and glass touch.
Shmuel Rubinstein
The friction between two surfaces increases with time because microscopic regions on the surfaces gradually deform and come into closer contact with each other. That finding was made 80 years ago. Now Sam Dillavou and Shmuel Rubinstein at Harvard University have found that frictional aging depends not only on how long the surfaces remain in contact, but also on the interface’s history.
Dillavou and Rubinstein investigated how friction at the interface between two stacked translucent plastic blocks changes when the blocks are subjected to a normal force. They shined a light at a very low angle into the top of the bottom block. Light remained inside the block by total internal reflection, only passing into the upper block at microscopic points where the two surfaces contacted each other. By imaging the upper block and measuring the transmitted light, the researchers determined the true total area that came into contact, which is considered a proxy for frictional strength.
With a weighted spring, the researchers applied a constant force to the upper block. The contact area between the blocks increased logarithmically over time. After times ranging from a few seconds to a few hours, the researchers abruptly removed part of the weight. As expected, the contact area dropped instantaneously, reduced by an amount proportional to the weight removed. But to the researchers’ surprise, the contact area continued to decrease gradually, reached a minimum, and then abruptly began increasing logarithmically. The time it took for the contact area to begin its recovery depended on how long the initial weight had remained on the system.
The researchers also measured the coefficient of friction, or how much force is needed to make the interface slip laterally. That value also decreased when the weight was removed and then crept back up, though not in tandem with the contact area. The researchers attribute the discrepancy to nonuniform evolution of the contact surfaces.
Scientists know that many systems, like crumpled paper or polymer glass, age and relax to their original forms after being subjected to stress. Dillavou and Rubinstein demonstrated that frictional interfaces follow the same behavior. The study provides new evidence that memory of past conditions is a more universal phenomenon than previously thought, a finding that could help improve models of systems such as earthquakes and industrial machinery. (S. Dillavou, S. M. Rubinstein, Phys. Rev. Lett. 120, 224101, 2018 .)
