(175d) Effect of Surface Geometry on the Frictional Properties of Poly(dimethyl siloxane)

Natural cartilage is durable and elastic, providing a low friction coefficient to moving joints under frequent applications of heavy loads. However, the physical mechanisms contributing to this low friction coefficient is not well understood. We hypothesize that the non-ideal surface geometry of cartilage gives rise to its low friction coefficient in certain directions of motion. We design microtextured soft poly(dimethyl siloxane) (PDMS) substrates with lithography to study the influence of surface geometry on their frictional and lubrication properties. The PDMS surfaces consist of stripes with controlled dimensions and spacings between each other. Tribological tests performed with a thin layer of aqueous glycerol solution at different concentrations between the PDMS substrates show that the friction coefficient is a function of the sliding velocity. However, this tribological behavior does not follow the type of Stribeck curve that is typically observed with flat surfaces. Two major differences are observed at different velocity ranges. First, a significant frictional reduction is observed in the boundary regime where the sliding speed is relatively low. Second, a velocity-dependent friction peak is observed in the elastohydrodynamic lubrication (EHL) regime. We hypothesize that this friction peak arises due to the competition between micro-EHL and EHL at the microtextured surfaces. To test this hypothesis, we develop scaling theories that show how the localized maximum friction is a function of the differential lubrication generated by the geometry of the microtextured surfaces.