Large variation in the boundary-condition slippage for a rarefied gas   flowing between two surfaces
  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. 
J. Laurent, A. Drezet, H. Sellier, J. Chevrier, S. Huant
physics.flu-dyn cond-mat.mes-hall