  In this paper, we report on new experimental results on the effects of
in-plane surface stretching on the friction of Poly(DiMethylSiloxane) (PDMS)
rubber with smooth rigid probes. Friction-induced displacement fields are
measured at the surface of the PDMS substrate under steady-state sliding. Then,
the corresponding contact pressure and frictional stress distributions are
determined from an inversion procedure. Using this approach, we show that the
local frictional stress $\tau$ is proportional to the local stretch ratio
$\lambda$ at the rubber surface. Additional data using a triangular flat punch
indicate that $\tau(\lambda)$ relationship is independent on the contact
geometry. From friction experiments using pre-stretched PDMS substrate, it is
also found that the stretch-dependence of the frictional stress is isotropic,
i.e. it does not depend on the angle between stretching and sliding directions.
Potential physical explanations for this phenomenon are provided within the
framework of Schallamach's friction model. Although the present experiments are
dealing with smooth contact interfaces, the reported $\tau(\lambda)$ dependence
is also relevant to the friction of statistically rough contact interfaces,
while not accounted for in related contact mechanics models.
