The role of attached polymer molecules in wall slip

High molecular weight polymer liquids are well known viscoelastic materials. Does this viscoelasticity affect the frictional properties against a solid wall? The question is of great practical importance (as in extrusion or in injection moulding for example), but the answer is not obvious and has long remained a matter for debate. in this review, the recently developed experimental techniques that have allowed the direct measurement of the local velocity at the wall are described These techniques have demonstrated the importance of polymer molecules attached to the surface for the appearance of slip boundary conditions, that is, non-zero flow velocity at the wall. Molecular models, which describe how both the deformability of these surface chains and their degree of interdigitation with the bulk polymer are the important quantities that drive the amount of slip at the wall, will also be discussed. These ideas should lead in the near future to the design of surfaces with optimized friction properties.