  We study the slippage of a gas along mobile rigid walls in the sphere-plane
confined geometry and find that it varies considerably with pressure. The
classical no-slip boundary condition valid at ambient pressure changes
continuously to an almost perfect slip condition in a primary vacuum. Our study
emphasizes the key role played by the mean free-path of the gas molecules on
the interaction between a confined fluid and solid surfaces and further
demonstrates that the macroscopic hydrodynamics approach can be used with
confidence even in a primary vacuum environment where it is intuitively
expected to fail.
