(92g) 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 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 speed. However, we find that the tribological behavior does not follow the type of Stribeck curve that is typically observed in soft materials with flat surfaces. The hysteresis from the surface geometry could be a reason for this deviation. To test this hypothesis, we develop a spring-dashpot model that represents viscoelastic contributions from the multiple surface length scales.