Expanding Fluidized Zones: A Model of Speed-Invariant Lubricity in Biology

A simple model for speed-invariant shear stress in biological aqueous lubrication has been developed based on previous observations of these fluid films and our foundational understanding of these complex fluids. The model revealed that the exponent of the shear-thinning behavior is not a critical parameter in speed invariance. Additionally, due to the localization of fluidization as a result of gradients in concentration through the aqueous gel network, the shear-thinning behavior results in a monotonic decline in viscosity to the high shear-rate viscosity plateau, η_∞. Finally, and perhaps somewhat surprising, was the finding that the optimal gradient was the weakest possible gradient. These findings are consistent with our understanding of aqueous lubrication across soft biological interfaces, which is sensitive to the macromolecular quality of the gel-spanning networks and the water content but not the sliding speed. The finding that this speed-invariant shear stress does not require a unique concentration profile reveals a built-in mechanism of biological resilience to maintain lubricity over a wide range of conditions. Graphical Abstract: Gradients in concentration localize fluidization at the lowest concentration regime during sliding. This leads to an expanding shear zone with proportionally thicker fluidized zones for increasing values of sliding speed resulting in roughly equivalent shear rate and shear stress for all sliding speeds [Figure not available: see fulltext.]