  A microscopic theory for the ubiquitous phenomenon of static friction is
presented. Interactions between two surfaces are modeled by an energy penalty
that increases exponentially with the degree of surface overlap. The resulting
static friction is proportional to load, in accordance with Amontons' laws.
However the friction coefficient between bare surfaces vanishes as the area of
individual contacts grows, except in the rare case of commensurate surfaces. An
area independent friction coefficient is obtained for any surface geometry when
an adsorbed layer of mobile atoms is introduced between the surfaces. The
predictions from our simple analytic model are confirmed by atomistically
detailed molecular dynamics simulations.
