Stick and Slip

What do the sounds of a creaky old hinge and a cello have in common? Both rely on the same kind of friction: two surfaces that alternately stick and slide against one another. This physical phenomenon is called stick-slip and, in the case of the creaky hinge, it is often mitigated by the application of a lubricant between the surfaces. It has long been accepted that such a thin layer of lubrication between sliding surfaces alternates along with the cycles of sticking and slipping; it starts as a solid, turns to liquid in the slipping phase and then back to a solid when the surfaces stick once again. But a recent paper in the Proceedings of the National Academy of Sciences (PNAS) suggests this model is incorrect. The findings arose out of research by the group of Prof. Jacob Klein of the Weizmann Institute’s Materials and Interfaces Department, with the collaboration of Prof. Arie Yeredor of Tel Aviv University. 

The group – including research student Irit Rosenhek-Goldian and associate staff scientist Dr. Nir Kampf, both members of Klein’s lab – tested this assumption in an ideal system: two perfectly smooth surfaces separated by a thin layer of lubricating material. The material in question contains molecules organized in layers, totaling around four to five nanometers in thickness. The idea was that as the surfaces stuck and then slipped, the lubricant molecules would also stick – as a solid – and then liquefy and slip over one another fluidly.

The shift from solid to liquid should entail another change: “When a material is solid, it is generally denser than its liquid form,” says Klein. “Thus, when the lubricating material turns liquid, it should physically expand by around ten percent; that is, the thin lubricant layer should expand by around 0.5 nanometers.” But where there should have been a bit more space between the surfaces in the slip stage, there was none. The measurements, which were accurate down to 0.1 nanometers, revealed no change at all in the volume of the lubricant as it went from stick to slip and back again. The conclusion: The lubricant does not change from solid to liquid after all.

Since friction and wear account for billions of dollars of loss annually, a better understanding of the basic science underlying lubrication may lead to significant improvements in both biomedical and industrial applications.
 
Prof. Jacob Klein's research is supported by the European Research Council. Prof. Klein is the incumbent of the Hermann Mark Professorial Chair of Polymer Physics.