  The tribology between surfaces can have a profound impact on the response of
a mechanical system, such as how granular particles are driven to flow. In this
work, we perform experiments that time-resolve the tangential and normal
components of the force between two semi-cylindrical PDMS
(polydimethylsiloxane) samples immersed in fluid, as they slide against each
other in a range of controlled speeds. The time-averaged friction force shows a
non-monotonic dependence on the sliding speed over four decades, which is
consistent to the paradigmatic Stribeck diagram and three dynamical regimes
associated with it. Our specially designed fixed-depth setup allows us to study
the fluctuation of force that exhibits strong stick-slip patterns in one of the
regimes. Data from repetitive experiments reveal that both the "onset speed"
for the stick-slip patterns and its spatial location along the sample change
gradually during the course of our experiments, indicating changes on the
sample surfaces. In addition, we conduct counterpart experiments by using
spherical samples rubbing against each other, to make a direct connection of
the inter-particle tribology to the granular flow reported in our previous work
[Phys. Rev. Lett. 126,128001 (2021)].
