  Perturbation theory, simulations and scaling arguments predict that there
should be no static friction for two weakly interacting flat atomically smooth
clean solid surfaces. The absence of static friction results from the fact that
the atomic level interfacial potential energy is much weaker than the elastic
potential energy, which prevents the atoms from sinking to their interfacial
potential minima. Consequently, we have essentially two rigid solids, for which
the forces at randomly distributed "pinning sites" cancel. It is shown here
that even fluctuations in the concentration of atomic level defects at the
interface do not account for static friction. It is also argued that the
sliding of contacting asperities, which occurs when the problem is studied at
the multi-micron length scale, relative to each other involves the shearing of
planes of atoms. Since this results in a force for the interaction of two
asperities which varies over sliding distances of the order of an atomic
spacing, the contacting asperities at the surface are able to sink to their
interfacial potential minima, with negligible cost in elastic potential energy.
This results in static friction.
