Gap-Dependent Hydrodynamic Lubrication in Conformal Contacts

We show that the hydrodynamic lubrication of contacting conformal surfaces with a typical texture height gives rise to a universal behaviour in the Stribeck curve in which the friction coefficient shows an anomalous power-law dependence on the Sommerfeld number, $\mu \sim S^{2/3}$. When the gap height drops below the `texture length scale', deviations from $S^{2/3}$ occur, which may resemble the onset of elasto-hydrodynamic and mixed lubrication. Within this framework, we analyse literature data for oral processing and find $S^{2/3}$ scaling with deviations consistent with measured lengthscales.