Scaling arguments are developed for the load balance in hydrodynamic
lubrication, and applied to non-Newtonian lubricants with a shear-thinning
rheology typical of a structured liquid. It is argued that the shear thinning
regime may be mechanically unstable in lubrication flow, and consequently the
Stribeck (friction) curve should be discontinuous, with possible hysteresis.
Further analysis suggests that normal stress and flow transience (stress
overshoot) do not destroy this basic picture, although they may provide
stabilising mechanisms at higher shear rates. Extensional viscosity is also
expected to be insignificant unless the Trouton ratio is large. A possible
application to recent theories of shear thickening in non-Brownian particulate
suspensions is indicated.Comment: 6 pages, 5 figures, RevTeX 4.