Rheology and tribology

Rheology is the study of the deformation and flow of matter (1929). A brief description is given of the rheology of steadily shearing materials, including shear and normal stresses. Some difficulties with the measurement of yield stresses are mentioned. Application of these classical ideas to lubrication in very thin films occasionally gives results at variance with observation. It is suggested, from evidence obtained from nonequilibrium molecular dynamics calculations, that very thin films (nanometre scale thickness) have rheological properties different from thick films-their viscosity is bigger, and they may also slip at solid walls. Wall slip is a feature of non-Newtonian flow in polymers not usually seen in classical fluid mechanics when dealing with small molecule liquids. We show examples of slip under certain conditions. Transient phenomena, especially stress overshoot, are shown to explain some paradoxical results in squeeze-film lubrication. Finally, the main differences between elongational and shear flows are pointed out.