In (Computational) Fluid Dynamics lectures, we assume fluid gets trapped
within small cavities that define the surface roughness and use this
assumption to claim that the fluid's velocity at the surface equals the
velocity of the surface, i.e. zero-velocity boundary condition for fixed
surfaces. For large-scale CFD simulations, numerical methods cannot
resolve the surface roughness, so a zero-velocity boundary condition is
assumed. Surprisingly, the Darmstadt Tribometer experiment at TU
Darmstadt (FST Institute) indicates that the surface roughness introduces a
(small) partial slip at the surface, which can be modelled using the Navier-
Slip boundary condition, with the slip-length related to the characteristics of
surface roughness. This effect possibly plays a significant role in wetting
processes investigated in the Collaborative Research Center 1194 at TU
Darmstadt.
More details and contact information are in the attached PDF.