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

Area of Science:

• Tribology
• Rheology
• Biomaterials Science

Background:

• Standard four-ball tribometers require large fluid volumes (>5 mL), limiting biological fluid analysis.
• Characterizing lubrication properties of biological fluids like synovial fluid is challenging due to sample volume limitations.

Purpose of the Study:

• To develop and validate a low-volume four-ball tribometer adaptable to rheometers.
• To enable characterization of friction and wear properties using minimal sample volumes.

Main Methods:

• Adapted a parallel plate rheometer with a custom holder for three lower balls.
• Utilized rheometer controls for axial force (contact pressure) and rotational velocity (sliding speed).
• Validated the setup with motor oil, glycerol/water mixtures, hyaluronic acid, and simulated synovial fluids.

Main Results:

• Achieved reproducible static and dynamic coefficients of Coulomb friction and wear scar measurements.
• Results were consistent with established literature trends for well-characterized lubricants.
• Successfully characterized simulated healthy and diseased synovial fluid, demonstrating utility for biological samples.

Conclusions:

• The developed tribo-rheology setup significantly reduces sample volume requirements for tribological testing.
• This method facilitates the quantification of lubrication properties in small volumes of precious biological samples.
• The setup is easily replicable on standard rheometers, offering broad applicability.