Shear, friction, and lubrication forces between polymer-bearing surfaces

The forces between polymer-bearing surfaces in motion with respect to each other in a liquid medium are very different from their equilibrium interactions and control effects ranging from colloidal rheology to lubrication and wear. Several aspects of these forces are reviewed, both when surfaces are approaching or receding (lubrication forces) or when they slide past each other (shear and frictional forces). Lubrication forces are considered in different solvency conditions and for different types of polymer attachment to the surfaces, particularly in compressed layers, where viscous segmental drag on the flowing liquid dominates. Frictional forces between compressed, rubbing solid surfaces may be dramatically reduced by the attachment of solvated chain layers, which are able to sustain large normal loads while retaining a fluid interfacial layer. The coupling between sliding motion and normal forces is described, and the relation between the shear rates required to stretch chains and the chain relaxation rate is analyzed.