Soft lubrication of cornstarch-based shear-thickening fluids

The behavior of complex fluids in thin films under the isoviscous elastohydrodynamic lubrication regime is a topic of current interest. A vast majority of the materials used to lubricate compliant contacts are shear-thinning fluids and, in comparison, very little is known about the lubrication properties of shear-thickening (ST) fluids. In this work we carried out both rheological and tribological experiments on ST fluids based on cornstarch suspensions. We investigated the influence of the polarity and viscosity of the carrier fluid and the cornstarch particles' concentration. From a rheological perspective, the ST response is diminished (as the shear-thinning region enlarges) when non-polar carriers are used (e.g. silicone oil) while a marked thickening is observed for polar carriers (e.g. glycerol-water mixtures). From a tribological perspective, the friction coefficient at slow sliding speeds is strongly affected by the nature of the carrier fluid, and in particular its wettability in contact with hydrophobic PDMS tribopairs. We demonstrate that experimental data, plotted in terms of Stribeck curves, provide useful information on the entrainment of particles/fluid through the contact. Finally, experimental results are also compared with numerical solutions of the Reynolds equation for inelastic non-Newtonian fluids in the isoviscous elastohydrodynamic regime. A Carreau-Yasuda constitutive equation for the viscosity-shear rate relationship is used.