  Frictional properties of contacts between a smooth viscoelastic rubber and
rigid surfaces are investigated using a torsional contact configuration where a
glass lens is continuously rotated on the rubber surface. From the inversion of
the displacement field measured at the surface of the rubber, spatially
resolved values of the steady state frictional shear stress are determined
within the non homogeneous pressure and velocity fields of the contact. For
contacts with a smooth lens, a velocity dependent but pressure independent
local shear stress is retrieved from the inversion. On the other hand, the
local shear stress is found to depend both on velocity and applied contact
pressure when a randomly rough (sand blasted) glass lens is rubbed against the
rubber surface. As a result of changes in the density of micro-asperity
contacts, the amount of light transmitted by the transparent multi-contact
interface is observed to vary locally as a function of both contact pressure
and sliding velocity. Under the assumption that the intensity of light
transmitted by the rough interface is proportional to the proportion of area
into contact, it is found that the local frictional stress can be expressed
experimentally as the product of a purely velocity dependent term, $k(v)$, by a
term representing the pressure and velocity dependence of the actual contact
area, $A/A_0$. A comparison between $k(v)$ and the frictional shear stress of
smooth contacts suggests that nanometer scale dissipative processes occurring
at the interface predominate over viscoelastic dissipation at micro-asperity
scale.
