procedure is indirect in the sense that it assumes that the flow resembles and then equation , or an equivalent, is used to determine $\lambda$ [@Watanabe; @Cheng; @Breuer; @Baudry; @Craig; @Bonaccurso; @Cottin; @Granick; @Granick2002; @Bonaccurso2].

The second way consists in performing direct velocity measurements in the fluid. We are only aware of two such previous works. Pit [*et al.*]{} [@Pit] measured velocities in shear flow of hexadecane over a smooth surface using a technique based on fluorescence recovery after photobleaching (see also [@Leger]). The measurements were performed down to 80 nm from the solid surface and averaged over a few tens of microns. Fluid slip was observed with $\lambda\sim 100$ nm in the case of lyophobic surfaces. Tretheway & Meinhart (2001) [@Meinhart] used micro-particle image velocimetry (PIV) techniques to measure the velocities of tracer nanoparticles (radius 150 nm) in pressure-driven channel flow of water. Measurements were made down to 450 nm from the solid surface and cross-correlated to increase signal-to-noise ratios. Results consistent with the no-slip condition were obtained in completely wetting conditions, but slip with $\lambda\sim1$ $\mu$m was obtained when the channel walls were treated to be hydrophobic.

In this paper, we wish to draw attention to some of 