Natural and induced surface roughness determine frictional regimes in hydrogel pairs

Hydrogels display extremely complex frictional behavior with surprisingly slippery surfaces. We measure the sliding behavior of hydrogels submerged in water using a custom-made tribotool. Samples with an imposed surface roughness give two distinct frictional regimes. Friction coefficients in the first regime change with asperity sizes and Young's moduli. Under increased normal force, a second frictional regime emerges likely due to smoothening of asperities. Friction coefficients in the second regime remain constant across length scales of roughness and appear to be material specific. The hydrogel polymer network also directly influences the surface topography, and with that, the frictional behavior of hydrogels. We highlight the tribological importance of surface roughness at different length scales, which provides potential to engineer functional frictional behavior.