a slip length $\lambda$. Let us consider for simplicity a unidirectional flow past a solid surface. Following Navier [@Navier], the slip length linearly relates the surface slip velocity to the shear rate of the fluid evaluated at the surface $$\label{navier}
u=\lambda \frac{\partial u}{\partial n}\cdot
$$ The slip length can also be interpreted as the fictitious distance below the surface at which the velocity would be equal to zero if extrapolated linearly: the no-slip boundary condition is equivalent to $\lambda=0$ and a no-shear boundary condition is equivalent $\lambda=\infty$.

Consider pressure-driven flow in a two-dimensional channel of height $2h$. If we assume that the boundary condition on the channel walls ($z=\pm h$) is given by , the axial velocity profile in the channel is $$\label{slip}
U_{\rm slip}(z)= -\frac{h^2}{2\mu}\frac{\d p}{\d
x}\left[1-\frac{z^2}{h^2}  +\frac{2\lambda}{h}\right],$$ which is a Poiseuille flow augmented by a finite plug velocity, which augmented flow rate ${Q_{\rm slip}}$ is given in a non-dimensional form by $$\label{rate}
\frac{Q_{\rm slip}}{Q_{\rm no\mbox{\footnotesize
-}slip}}=1+\frac{3\lambda}{h}\cdot$$

Experimentalists have usually addressed the issue of fluid slip in two distinct ways. The first consists in performing indirect measurements, such as pressure-drop versus flow rate or squeezing rate versus resistance, and then use such measurements to infer a slip length. This 