Dynamic Friction and Lubrication in Soft Hydrogels*

Hydrogel's use ranges from synthetic cartilage and contact lenses, to gel-based soft robotics. Across its applications, hydrogel interfaces experience dynamic and static loads, hence minimizing friction increases longevity. A gel's frictional behavior is strongly dependent on factors such as contact area, sliding velocity, normal force, and gel surface chemistry. Using a custom low-force tribometer, we have probed the single-contact frictional properties of spherical hydrogel particles on flat surfaces under a variety of environmental conditions. On hard surfaces, (aluminum, glass, etc.) we have examined a dynamic frictional transition at a critical sliding velocity (≈ 0.5 cm/s) which is in agreement with previous research. We have varied both the slider and surface materials, and the solvent. Interestingly, upon inverting our system to that of solid particles (steel, glass, etc.) on hydrogel surfaces, this frictional phenomena disappears. These results imply a connection between the relaxation time of the polymers and this dynamic transition. A physical model utilizing the hydrodynamic lubrication layer between the deformed surfaces and the polymer interactions will be discussed.