  A scaling analysis is undertaken for the load balance in sliding friction in
the hydrodynamic lubrication regime, with a particular emphasis on power-law
shear-thinning typical of a structured liquid. It is argued that the
shear-thinning regime is mechanically unstable if the power-law index n < 1/2,
where n is the exponent that relates the shear stress to the shear rate.
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 shear thickening in non-Brownian particulate
suspensions is indicated.
