Role of friction and tangential force variation in the subjective scaling of tactile roughness

The present study examined the contribution of normal (Fz) and tangential (Fx) forces, and their ratio, kinetic friction (Fx/Fz), to the subjective magnitude estimations of roughness. The results suggested that the rate of variation in tangential stroking force is a significant determinant of roughness perception. In the first experiment, six volunteer subjects scaled the roughness of eight surfaces explored with a single, active scan of the middle finger. The surfaces were 7.5 x 2.4-cm polymer strips embossed with truncated cones 1.8 mm high with a spatial period of 2.0 mm in the transverse direction and 1.5-8.5 mm in the longitudinal, scanning direction. The surfaces were mounted on a six-axis force and torque sensor that measured the perpendicular, contact force (normal to the skin surface) and the tangential force along the axis of stroking. The results confirmed the findings of an earlier study that magnitude estimates of perceived roughness increase approximately linearly up to a longitudinal spatial period of 8.5 mm. Across subjects, no consistent correlations were found between perceived roughness and either the mean normal or tangential force alone. Although significant positive correlations were found between roughness and mean kinetic friction for all subjects, they were not as consistently robust as one might have expected. Furthermore, instantaneous kinetic friction varied widely over the course of a single stroke because of within trial oscillations in the tangential force. The amplitude of these oscillations increased with the longitudinal spatial period and their frequency was determined by a combination of the spatial period and the stroking velocity. These oscillations were even more conspicuous in the first derivative or rate of change of the tangential force (dFx/dt), which was quantified as the root mean square (RMS) of the tangential force rate. The mean normalized RMS proved to be strongly correlated with subjective roughness, averaging 0.88 for all subjects. In order to dissociate the fluctuations in tangential force from both the surface structure and the mean kinetic friction, a second experiment was performed on six additional subjects who estimated the roughness of identical lubricated and unlubricated (dry) surfaces. Lubrication with liquid soap reduced the mean kinetic friction by approximately 40%, the RMS of the tangential force rate by slightly more than 21% and the subjective estimates of roughness by 16.4%. Taken together, the results suggest that in tactile exploration, the RMS of the tangential force rate may be an important determinant of subjective roughness.