Superlubricity

Superlubrication is a state of extremely low friction that occurs when two atomically precise surfaces slide along each other in such a way that the "atomic bumps" do not mesh or more precisely when the lattices distances projected in the direction of movement are maximally incommensurate.

Examples:

• two coplanar sheets of graphene relatively rotated to one another
• two appropriately chosen coaxial nanotubes
• diamondoid molecular bearings and other DMEs with sliding interfaces.

If AP surfaces surfaces are designed or aligned to not mesh the "perceived bumps" become lower and their spacial frequency becomes higher. If the surface pressure isn't extremely high the characteristic thermal energy k_BT can become a lot higher than the bumps energy barriers. Thus the friction becomes so low that e.g. an unconstrained DMME bearing can be activated thermally and may starts turning randomly in a Brownian fashion.

Note that superlubrication works at room temperature and there is not a sharp cutoff in friction when decreasing the dergree of intermeshment like the cutoff in superconductivity.

Interestingly Van der Waals forces allow for stable designs in which the axle in DMME bearings is pulled outward in all directions instead of compressed inward

Oxygen or sulfur with their two bonds in a plane parallel to the relative sliding direction are a good choice for surface termination of bearing interfaces since this configuration gives maximal stiffness in sliding direction. If all the two bonds are instead in a plane normal to the sliding direction the lower stiffness may lead to higher energy dissipation. Hydrogen fluorine or chlorine passivations have low stiffness too.