Soft and rough: Tribology of naturally and artificially rough hydrogels

Frictional properties of materials in contact depend on an array of parameters including surface roughness, normal force, and sliding speed. For soft materials, bulk properties such as the elastic modulus influence the friction coefficient as well. The friction coefficient is therefore not fixed for a specific combination of materials but varies with each parameter, making the friction coefficient a system parameter. We use a 3D-printed tribometer and measure the friction coefficient of 4 hydrogels: polyacrylamide, physically and chemically cross-linked gelatin, and agar. For these hydrogels, we find two normal force-dependent frictional regimes caused by the flattening of surface asperities. Imaging of these asperities using cryo-SEM revealed that the asperities are around 1 µm in size. The nature of the asperities and the friction coefficient depend on the type of polymer network. We vary the asperity size using 3D-printed molds and find that the friction coefficient in the first regime changes with surface roughness. The friction coefficient in the second regime is constant for different asperity sizes and varies with polymer type. Our findings highlight the importance of surface properties and bulk properties in soft material friction.