Limits of the hydrodynamic no-slip boundary condition

The importance of surface roughness and fluid surface intermolecular interactions in determining the no-slip boundary condition was discussed. Surface spacing was measured with optical interferometry and dynamic forces were measured using piezoelectric methods. The critical shear stress and shear rate to observe deviations using the no-slip boundary condition increased with increasing roughness. The local intermolecular interactions dominated when the surface was very smooth, but roughness dominated in other case.