SUBJECTIVE SCALING OF SMOOTH SURFACE FRICTION

1. Six men and four women, 30-51 yr of age, were asked to use the tip of the washed and dried index finger to stroke six different featurele ss, flat surfaces mounted on a three-dimensional force platform. The s ix surfaces were rosin-coated glass, glass, satin-finished aluminum, p oly-vinyl chloride (PVC) plastic, Teflon, and nyloprint (polyamide pla stic). The subjects were requested to indicate where the sensation pro duced by each surface should be placed on an unidimensional scale repr esented by an 18-cm line labeled at one end by the words ''most slippe ry'' and at the other end by the words ''most sticky.'' The coefficien ts of friction for each surface and for each subject were subsequently assessed by asking each subject to stroke the surfaces as if they wer e assessing its slipperiness for 5 s. 2. The finger forces normal and tangential to the stroked surfaces were digitized at 250 Hz and stored on a laboratory computer. The ratio of the mean tangential force to t he mean perpendicular force during stroking was used to calculate the mean coefficient of kinetic friction. The mean friction for all subjec ts ranged from 0.43 for the nyloprint surface to 2.79 for the rosin-co ated glass. Correlation coefficients calculated between the subjective estimates of friction aad the measured coefficients of friction for e ach subject individually resulted in a mean correlation of 0.85 (n = 1 0, P < 0.001). 3. These data indicate that subjects can accurately sca le relative differences in the friction of macroscopically smooth, fla t surfaces, by modulating the tangential force applied to the finger w hile keeping the normal force relatively constant. The fact that subje cts maintained a relatively constant normal force and instead varied t he tangential force across different surfaces suggests that receptors sensitive to these tangential forces are important in the perception o f smooth surface friction.