  The role of the polymer volume fraction, $\phi$, on steady state slippage and
interfacial friction is investigated for a semi-dilute polystyrene solutions in
diethyl phthalate in contact with two solid surfaces. Signicant slippage is
evidenced for all samples, with slip lengths b obeying a power law dependence.
The Navier's interfacial friction coecient, k, is deduced from the slip length
measurements and from independent measurements of the solutions viscosity
$\eta$. The observed scaling of k versus $\phi$ clearly excludes a molecular
mechanism of friction based on the existence of a depletion layer. Instead, we
show that the data of $\eta$($\phi$) and k($\phi$) are understood when taking
into account the dependence of the solvent friction on $\phi$. Two models,
based on the friction of blobs or of monomers on the solid surface, well
describe our data. Both points out that the Navier's interfacial friction is a
semi-local phenomenon.
