A microscale four-ball tribometer for characterization of lubrication by small volume samples

Four-ball tribometers are widely used to characterize the efficacy of lubricants in sliding contact. However, the standard apparatus requires 5 mL or more of fluid, which makes the characterization of biological fluids such as synovial fluid difficult. Here, we present a four-ball tribometer that requires less than a tenth of the sample volume and that is easy to replicate on any rheometer. An upper steel or polystyrene ball was attached to a parallel plate rheology geometry, then lowered onto three lower balls positioned and stabilized using an additively manufactured holder. Rheometer controls were then used to control axial force (contact pressure) and upper ball rotational velocity (sliding speed). The setup was validated on three well-characterized lubricant systems: motor oil, glycerol/water mixtures, and hyaluronic acid. Static and dynamic coefficients of Coulomb friction and wear "scar" test results were reproducible and consistent with trends from the literature. Simulated healthy and diseased synovial fluid was also characterized to demonstrate the setup's utility in measuring lubrication by small volumes of biological samples. The tribo-rheology setup shows promise for quantifying the lubrication properties of small volumes of precious samples.