Model non-Newtonian fluids are used to determine the influence of fluid rheology on friction between viscoelastic substrates. We uniquely designed two groups of fluids to be iso-viscous at low- and high-shear rate respectively using nanocellulose dispersions. While high-shear viscosity defines the elastohydrodynamic (EHL) region, below this limit a secondary transition in the Stribeck curve is found to be influenced by the low-shear viscosity; this is more pronounced for rough substrates and absent for Newtonian lubricants. We consider this to indicate that viscosity at low shear rates surprisingly has a strong influence on micro-EHL as well as fluid entrainment and complex fluid-asperity interactions in the boundary-mixed regimes. A conceptual model is proposed for non-Newtonian fluids in soft-lubrication.
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