  We consider some basic principles of fluid-induced lubrication at soft
interfaces. In particular, we show how the presence of a soft substrate leads
to an increase in the physical separation between surfaces sliding past each
other. By considering the model problem of a symmetric non-conforming contact
moving tangentially to a thin elastic layer, we determine the normal force in
the small and large deflection limit, and show that there is an optimal
combination of material and geometric properties which maximizes the normal
force. Our results can be generalized to a variety of other geometries which
show the same qualitative behavior. Thus they are relevant in the
elastohydrodynamic lubrication of soft elastic and poroelastic gels and shells,
and in the context of bio-lubrication in cartilaginous joints.
