Lubricated friction on microtextured soft substrates

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 controlled dimensions and spacings to study the influence of surface geometry on their frictional and lubrication properties. Tribological tests performed with a thin layer of aqueous glycerol solution between the PDMS substrates show that the frictional behavior does not follow the type of Stribeck curve that is typically observed with flat surfaces. This major difference can be explained by a scaling theory we developed, in which the friction force and the normal force are expressed with a combination of lubricant properties, experimental conditions and surface geometries. Our study establishes a design framework for the friction of elastomers based on their surface microtextures, and paves the way for engineering soft materials in technological applications such as wearable electronics, antifouling coatings, and synthetic implants.