Quantifying touch-feel perception: tribological aspects

We report a new investigation into how surface topography and friction affect human touch-feel perception. In contrast with previous work based on micro-scale mapping of surface mechanical and tribological properties, this investigation focuses on the direct measurement of the friction generated when a fingertip is stroked on a test specimen. A special friction apparatus was built for the in situ testing, based on a linear flexure mechanism with both contact force and frictional force measured simultaneously. Ten specimens, already independently assessed in a 'perception clinic', with materials including natural wood, leather, engineered plastics and metal were tested and the results compared with the perceived rankings. Because surface geometrical features are suspected to play a significant role in perception, a second set of samples, all of one material, were prepared and tested in order to minimize the influence of properties such as hardness and thermal conductivity. To minimize subjective effects, all specimens were also tested in a roller-on-block configuration based upon the same friction apparatus, with the roller materials being steel, brass and rubber. This paper reports the detailed design and instrumentation of the friction apparatus, the experimental set-up and the friction test results. Attempts have been made to correlate the measured properties and the perceived feelings for both roughness and friction. The results show that the measured roughness and friction coefficient both have a strong correlation with the rough-smooth and grippy-slippery feelings.