Generally sealing systems are designed by means of elasto-hydrodynamics. In this case, the no slip boundary condition is applied to the solid walls for solving the Reynolds equation. Nowadays engineers apply the no-slip boundary condition with the most matter of course. But in the 19th century the behaviour of fluids close to the solid wall was discussed by personalities as Stokes, Poisson, Maxwell or Navier. Navier [1] e.g. introduced a slippage factor in his derivation of the Navier-Stokes equations.

In the middle of the 19th century Stokes was commissioned by the Royal Society, to discuss the issue regarding the slipping behaviour of fluids close to solid walls. And he concluded in [2] & [3] that the effect of slippage on solid walls, if it exists, was up to his time too small to measure and could be neglected. Hence for sufficiently large flow geometries the effect of wall sliding was neglected. Even today measuring the slip velocity close to solid walls is a challenging task and no general approach has figured out. This conclusion yields up to now and is sometimes treated like a principle. But in the case of sealing systems where commonly gaps are in the order of some microns and below the effect of wall slippage has to be taken into account. Experimental investigations of sealing manufacturers show that the leakage behav- iour of geometrically identical sealing systems depends on the material paring and hence on the surface energy. As a cause for the discrepancy between calculations and experimental investigations a breakdown of the no slip boundary condition is suspected.