picture for its origin has yet to be given. The ongoing debate stems from the fact that it is an assumption which cannot be derived from first principles. It has been shown that on length scales much larger than the scale of surface heterogeneities, the no-slip condition might be a macroscopic consequence of inevitable microscopic roughness [@Richardson; @Jansons], but the case of perfectly smooth surfaces has yet to be explained. In particular, the physico-chemical properties of both the fluid and the solid surface certainly are important.

Only a few experimental studies have addressed the no-slip condition in the past [@Schnell; @Churaev], and it is only the recent advances in the controlled fabrication of micro- and nano-devices and in the corresponding measurement techniques that have allowed the problem to be reconsidered. Over the last few years, a number of pressure-driven flow [@Watanabe; @Cheng; @Meinhart; @Breuer], shear-flow [@Pit], and squeeze-flow experiments [@Baudry; @Craig; @Bonaccurso; @Cottin; @Granick; @Granick2002; @Bonaccurso2] showing a response interpretable as some degree of slip for partially wetting liquids have been reported. Molecular dynamics simulations of Lennard-Jones liquids have also shown that slip can occur, but only at unrealistically high shear rates [@Nature; @Barrat].

Fluid slip is usually quantified by 