  The understanding of sliding friction for wet, patterned surfaces from first
principles is challenging. While emerging applications have sought design
principles from biology, a general framework is lacking because soft interfaces
experience a multiphysics coupling between solid deformation and fluid
dissipation. We investigate the elastohydrodynamic sliding of >50 patterned
sliding pairs comprising elastomers, thermosets, and hydrogels, and discover
that texturing induces a critical transition in the macroscopic friction
coefficient. This critical friction scales universally, without any fitting
parameters, with the reduced elastic modulus and the pattern geometry. To
capture the frictional dissipation, we separate the flow curve into two regimes
and account for the contributions of shear and normal forces applied by the
fluid on the patterns. Our model combines Reynolds' equations and elastic
deformation to provide physical insights that allow engineering of the
elastohydrodynamic friction in a class of soft tribopairs using pattern
geometry, material elasticity, and fluid properties.
