  Using molecular dynamics (MD) we study the dependency of the contact
mechanics on the sliding speed when an elastically soft slab (block) is sliding
on a rigid substrate with a ${\rm sin} (q_0 x)$ surface height profile. The
atoms on the block interact with the substrate atoms by Lennard-Jones
potentials. We consider contacts with and without adhesion. We found that the
contact area and the friction force are nearly velocity independent for small
velocities ($v < 0.25 \ {\rm m/s}$) in spite of the fact that the shear stress
in the contact area is rather non-uniform. For the case of no adhesion the
friction coefficient is very small. For the case of adhesion the friction
coefficient is higher, and is mainly due to energy dissipation at the opening
crack tip, where rapid atomic snap-off events occur during sliding. Adhesion
experiments between a human finger and a clean glass plate were carried out,
and for a dry finger no macroscopic adhesion occurred. We suggest that the
observed decrease in the contact area with increasing shear force results from
non-adhesive finger-glass contact mechanics, involving large deformations of a
complex layered material.
