Hardness Perceived When Sliding Over Roughened Surfaces

The objective of this study was to investigate the influence of roughened surface features on the perceived hardness of various materials. Thirteen participants used a visual analog scale to evaluate the hardness of ten 3D-printed specimens by sliding a fingertip on them. The specimens had two types of surface features: flat and smooth, or with microscopic rectangular gratings. They were fabricated from two types of plastic with different Young's moduli—2.46 and 9.35 MPa. We found that both surface pattern and mechanical hardness significantly contributed to the perceived hardness of a material individually and without interaction. The roughened surfaces with rectangular gratings were judged to be harder than the flat and smooth surfaces of the same material. Among the parameters of the rectangular gratings, the groove width or periodic surface wavelength significantly contributed to the perceived hardness. Although the root cause of this phenomenon is unknown, friction caused by surface roughness is considered a potential mediator that influences the perceived hardness. The findings of this study can facilitate the manipulation of softness perception through surface design.