  The friction of a sliding plate on a thin immersed granular layer obeys
Amonton-Coulomb law. We bring to the fore a large set of experimental results
which indicate that, over a few decades of values, the effective dynamical
friction-coefficient depends neither on the viscosity of the interstitial fluid
nor on the size of beads in the sheared layer, which bears out the analogy with
the solid-solid friction in a wide range of experimental parameters. We
accurately determine the granular-layer dilatancy, which dependance on the
grain size and slider velocity can be qualitatively accounted by considering
the rheological behaviour of the whole slurry. However, additional results,
obtained after modification of the grain surface by a chemical treatment,
demonstrate that the theoretical description of the flow properties of granular
matter, even immersed, requires the detailed properties of the grain surface to
be taken into account.
