  We study the influence of the surface energy and contamination films on
rubber adhesion and sliding friction. We find that there is a transfer of
molecules from the rubber to the substrate which reduces the work of adhesion
and makes the rubber friction insensitive to the substrate surface energy. We
show that there is no simple relation between adhesion and friction: adhesion
is due to (vertical) detachment processes at the edge of the contact regions
(opening crack propagation), while friction in many cases is determined mainly
by (tangential) stick-slip instabilities of nanosized regions, within the whole
sliding contact. Thus while the pull-off force in fluids may be strongly
reduced (due to a reduction of the work of adhesion), the sliding friction may
be only slightly affected as the area of real contact may be dry, and the
frictional shear stress in the contact area nearly unaffected by the fluid.
