Dry friction is probably the friction the reader is most accustomed to. The friction that occurs when two solid surfaces that are in contact are sliding past each other. The ability to control friction is very important. Machine parts, cogs and wheels all need to have as little friction as possible while car brakes and pulleys both require it. Frictional force is normally calculated as simply being the normal contact force (the force the ground exerts on the object) multiplied by a constant. This means that surface area has no effect on frictions as, when you think about it, the rough surfaces of materials only touch in protruding places. But what happens when surfaces that are flat down to the atomic level are allowed to slide over each other?
The results fail to be as simple as linear in relation to the normal force as predicted by Amontons-Coulomb law. Mathematical models for the nonlinear dynamics are used to analyse the results of these frictional phenomenon. One of the more accurate models out of those that have been created is called the Frenkel–Kontorova model which models the interaction as a chain of connected particles sliding over a solid and undeformable substrate. As technology has progressed it became possible to support these models through direct observations using equipment such as friction force microscopes which use an incredibly sharp spike to test the friction of a material down to the individual atomic scale.
Of course knowing the value of the friction at a point is not the same as understanding the dynamics that cause it. In this study the mechanism of two deformable chains, perhaps representing biological molecules, was investigated. Rather that actually dragging two polymers over each other the system was instead experimentally modelled by laser cooling ytterbium-142 (72Yb+) ions down to only a few milliKelvin. These means they then crystallise and can be imaged by laser light near the ytterbium’s resonance causing them to fluoresce. The results show that this system can be used for examining the scenarios where classical mechanics problems such as friction transition into quantum physics problems on such small scales.