On fluid/wall slippage

Certain (nonpolymeric) fluids show an anomalously low friction when flowing against well-chosen solid walls. We discuss here one possible explanation, postulating that a gaseous film of small thickness h is present between fluid and wall. When h is smaller than the mean free path l of the gas (Knudsen regime), the Navier length b is expected to be independent of h and very large (micrometers).