A matter of morphology: The role of asperity characteristics in hydrogel friction

Hydrogels can be easily modified to have a particular surface roughness. This modularity in hydrogels is an important feature, as roughness affects friction. Here, we systematically vary the surface morphology by separately altering the height, diameter and distance between surface asperities shaped as cylindrical pillars. Using a custom- made 3D-printed tribometer, we show that slender pillars conjure a higher friction coefficient, which we attribute to their bending and hence larger exposed contact area. In contrast, we measure low friction coefficients when the small diameter of asperities or their sparse presence on the surfaces reduces the total effective sliding contact area. We investigate two surface patterns with different spatial symmetry and demonstrate that the mechanisms behind probe-pillar friction depend on the spatial location of the pillars on the surface with respect to the sliding path. Our findings provide a path towards optimization of frictional properties of soft hydrogel surfaces.