A hydration-dependent minimum in finger pad friction on rough surfaces

Abstract Finger pad friction is central to object manipulation and tactile perception, and surface roughness is a key parameter for its control. In a systematic investigation of finger pad friction on aluminum surfaces with varying roughness, measurements from 61 participants revealed a robust minimum in friction at intermediate roughness. To interpret this non-monotonic behavior, we model friction as the sum of adhesive and deformation contributions. Increasing roughness reduces the real area of contact between skin and surface, leading to a decrease in adhesive friction, while enhanced asperity–skin interactions increase deformation friction. The relative contributions of these mechanisms depend strongly on hydration of the stratum corneum . Specifically, the roughness corresponding to minimum friction shifts from a mean surface slope of 0.2 for dry skin to 0.4 for moist skin. This transition between dominant friction mechanisms also alters the load dependence of friction: the exponent of the friction–load power law increases from 0.79 on polished surfaces to 0.94 on perceptibly rough surfaces. Together, these results establish quantitative links between surface roughness, skin physiology, and frictional behavior, providing design principles for surfaces with tailored tactile and frictional properties.