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 $(\pm 100\mathrm{nm})$, i.e., five times more accurately than previous work. In all cases, we find that the slip length is below $100\mathrm{nm}$.

• Received 22 November 2004

DOI:https://doi.org/10.1103/PhysRevE.71.035303