 Slip length

The slip length measures tangential wall mobility in the Navier slip boundary condition. Where the wall shear rate is nonzero,The numerator is a velocity and the denominator has units ${\mathrm{s}}^{-1}$, so $b$ has units of length. Geometrically, take a local inward normal coordinate $s$, with the wall at $s=0$. The tangent line to the relative velocity profile isIt reaches zero at $s=-b$, a fictitious distance $b$ inside the solid. This extrapolation does not assert that fluid exists there. At a moving wall, extrapolate to the wall's velocity, rather than to zero laboratory velocity. If both the wall shear rate and relative velocity vanish, their ratio cannot determine $b$.

The useful comparison is $b/H$, where $H$ is the channel gap. A small absolute slip length can noticeably change volumetric flow rate in a micrometre-scale channel. Measured effective slip length may summarize unresolved interfacial structure; it need not mean literal molecular sliding at every point. The review distinguishes intrinsic, apparent and effective slip and discusses how surface conditions affect measurements. ^[ref]