Direct measurement of the apparent slip length.

We measure velocity profiles in water flowing through thin microchannels, using particle image velocimetry combined with a nanopositioning system. From the velocity profiles, we determine the slip lengths in two cases: Smooth hydrophilic glass surfaces, and smooth hydrophobic glass surfaces, grafted with a monolayer of silane. The slip length is determined within (+/-100 nm) , i.e., five times more accurately than previous work. In all cases, we find that the slip length is below 100 nm.