In the limit of clean surfaces, friction has its origins in the
microscopic, chemical interactions at the interface between the
two objects in question. One of the more amazing (to me, anyway)
consequences of this is the extremely important role played by
commensurability between the surfaces. Let me explain with an
example. Consider a gold crystal terminated at the (111)
surface, and another gold crystal also terminated at the (111)
surface. Now, if those two surfaces are brought into contact,
with the right orientation so that they match up as if they were
two adjacent layers of atoms inside a larger gold crystal, what
will happen? The answer is, in the absence of adsorbed
contaminants, the surfaces will stick. This is called "cold
welding". In contrast, if you bring together two ultraclean
surfaces that are incommensurate, they can slide past each other
with essentially no friction. This is called "superlubricity".
Here are two great examples (pdf of first one; pdf of second
one) of this. In this new paper