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The tactile explorartion and perception of wrapping papers is investigated in terms of fingertip friction and rating of sensory, affective, and evaluative adjectives. Friction coefficients, which vary significantly between samples, are correlated with factors such as valence which are identified in a principal component analysis of subjective ratings. We found that affective appraisals of valence and arousal as well as evaluations of novelty, but not of value, decreased with increasing friction.
Fingertip friction and the related shear of skin are key mechanical mechanisms in tactile perception, but the perception of friction itself is rarely explored except for the flat surfaces of tactile displays. We investigated the perception of friction for tactile exploaration of a unique set of samples whose fabric-like surfaces are equpped with regular arrays of flexible micropillars.The measured fingertip friction increases with decreasing bending stiffness, where the latter is controlled by radius (20-75 µm) an aspect ration of the micropillars. In forced-choice tasks, participants noticed relative differences in friction as small as 0.2, and even smaller when a sample with less than 100µm distance between pillars is omitted from the analysis. In an affective ranking of sample upon active touch, the perception of pleasantness is anticorrelated with the measured friction. Our results offer insights towards a rational design of materials with well-controlled surface microstructure which elicit a dedicated tactile appeal.
Fingertip friction is a key component of tactile perception. In active tactile exploaration, friction forces depend on the applied normal force and on the sliding speed chosen. We have investigated wheter humans perceive the speed dependence of friction for textured surface of materials, which show either increase or decrease of the friction coefficient with speed. Participants perceived the decrease or increase when the relative difference in friction coefficient between fast and slow sliding speed was more than 20%. The friction of comparison judgments which were in agreement with the measured difference in friction coefficient did not depend on variations in the applied normal force. The results indicate a perceptual constancy for fingertip friction with respect to self-generated variations of sliding speed and applied normal force.
Friction between fingertip and surface is a key contribution to tactile perception during active exploration of materials. We explore the role of skin factors such as stratum corneum thickness and hydration, deformability, elasticity, or density of sweat glands and of Meissner corpuscles in friction and tactile perception. The skin parameters were determined non-invasively for the glabrous skin at the index finger pad of 60 participants. Sets of randomly rough plastic surfaces and of micro-structured fibrillar rubber surfaces were explored as model materials with well-defined parameterized textures. Friction varies greatly between participants, and this variation can be explained to 70% by skin factors for the randomly rough plastic surfaces. The predictability of friction by skin factors is much lower for micro-structured rubber surfaces with bendable fibrils, where 50% of variance is explained for the stiffest fibrils but only 20% for the most bendable fibrils. The participants’ age is the key predictor for their tactile sensitivity to perceive the fibrils, where age is negatively correlated to the density of Meissner corpuscles. The results suggest that stratum corneum hydration, skin deformability, and age are important factors for friction and perception in active tactile exploration of materials.
The perceived time can shrink or expand for emotional stimuli. Converging evidence suggests that emotional time distortions are rooted in the emotional states of the timing agents because emotional stimuli can influence the timing of simultaneous neutral events. As emotional states are transitory, we investigated if time modulating emotional states also influence timing of subsequent neutral events. In each trial, we induced different valence and arousal levels by using affective vibrotactile patterns before participants judged the duration of neutral auditory tones. Compared to neutral patterns, affective patterns modulated participants’ time perception of the subsequent tones. We observed an interaction between arousal and valence: Pleasant-Low arousal patterns expanded the timing of subsequent neutral events more than Unpleasant-Low arousal patterns while Pleasant and Unpleasant-High arousal led to a similar temporal expansion. Our results indicate time modulating effects of emotional stimuli are due to changed emotional states and influence time perception likely until the underlying state decays.
Friction was studied for the human finger pad during the spreading of viscous liquid samples in circular motion on a solid substrate. The samples included both Newtonian and shear-thinning liquids with a range of viscosity between 0.83 mPa s and 150 Pa s. During active touch, participants applied varying normal forces and sliding speeds depending on the sample and individual behavior. Friction coefficients vary greatly between participants, but fall on one Stribeck curve when shear-thinning effects were accounted for full-film lubrication. A comparison with the measured height variations during spreading demonstrates that the logarithm of the Hersey number is an instantaneous indicator of the film thickness in the full-film lubrication regime. Comparison of the measured friction coefficients with reported values of the perceived slipperiness for the same samples shows a close correspondence along the Stribeck curve.
