Challenges and opportunities in soft tribology

Despite the ubiquitous use of soft materials in everything from transportation to biomedicine, there remain tremendous opportunities for fundamental studies on the governing principles behind their tribological performance. One of the greatest challenges in performing tribological studies of friction and wear on soft materials is the low modulus, which necessitates low forces for convenient and accessible contact areas widely used in experimental tribology. Many excellent and established tribological instruments that have been optimized over the years for use with hard materials are essentially unusable for low modulus materials like soft elastomers, hydrogels, tissues, and cells. This critical need for fundamental measurements of soft contacts has led to a growing field of instrumentation development, stronger connections between tribology and rheology, increased in situ studies of contact deformation using optical microscopy, and new models for rate-dependent effects on friction and wear. Improvements in the ability to measure the real area of contact, assess time-dependent responses of soft interfaces, and reduce uncertainty in shear stress measurements are critical for friction measurements on soft materials. Some recent efforts in soft tribology are outlined herein with an eye towards performing non-destructive experiments on living cell layers to foster stronger interactions with biology and biomedicine.