  We study the lubrication of fluid-immersed soft interfaces and show that
elastic deformation couples tangential and normal forces and thus generates
lift. We consider materials that deform easily, due to either geometry (e.g. a
shell) or constitutive properties (e.g. a gel or a rubber), so that the effects
of pressure and temperature on the fluid properties may be neglected. Four
different system geometries are considered: a rigid cylinder moving parallel to
a soft layer coating a rigid substrate; a soft cylinder moving parallel to a
rigid substrate; a cylindrical shell moving parallel to a rigid substrate; and
finally a cylindrical conforming journal bearing coated with a thin soft layer.
In addition, for the particular case of a soft layer coating a rigid substrate
we consider both elastic and poroelastic material responses. For all these
cases we find the same generic behavior: there is an optimal combination of
geometric and material parameters that maximizes the dimensionless normal force
as a function of the softness parameter = hydrodynamic pressure/elastic
stiffness = surface deflection/gap thickness which characterizes the
fluid-induced deformation of the interface. The corresponding cases for a
spherical slider are treated using scaling concepts.
