Physical Variables Underlying Tactile Stickiness During Fingerpad Detachment

Abstract

One may notice a relatively wide range of tactile sensations even when touching the same hard, flat surface in similar ways. Little is known about the reasons for this variability, so we decided to investigate how the perceptual intensity of light stickiness relates to the physical interaction between the skin and the surface. We conducted a psychophysical experiment in which nine participants actively pressed their finger on a flat glass plate with a normal force close to 1.5 N and detached it after a few seconds. A custom-designed apparatus recorded the contact force vector and the finger contact area during each interaction as well as pre-and post-trial finger moisture. After detaching their finger, participants judged the stickiness of the glass using a nine-point scale. We explored how sixteen physical variables derived from the recorded data correlate with each other and with the stickiness judgments of each participant. These analyses indicate that stickiness perception mainly depends on the pre-detachment pressing duration, the time taken for the finger to detach, and the impulse in the normal direction after the normal force changes sign; finger-surface adhesion seems to build with pressing time, causing a larger normal impulse during detachment and thus a more intense stickiness sensation. We additionally found a strong between-subjects correlation between maximum real contact area and peak pull-off force, as well as between finger moisture and impulse.

Initial contact shapes the perception of friction

“…On a physical level, the overall so-called “frictional strength” of the contact is given by the number of asperities in intimate contact and their individual shear strength ( 4 – 6 ). It is the measure of the maximum lateral force on the contact that will lead to slippage.…”

Initial contact shapes the perception of friction

“…On a physical level, the overall so-called frictional strength of the contact is given by the number of asperities in intimate contact and their individual shear strength (4)(5)(6). It is the measure of the maximum lateral force on the contact that will lead to slippage.…”

Optimizing a Viscoelastic Finite Element Model to Represent the Dry, Natural, and Moist Human Finger Pressing on Glass

“…The light intensity contrast between the contact and non-contact fingerprint areas was emphasized by applying the prism-based frustrated total internal reflection (FTIR) principle [12]. The methods by which this apparatus achieves highly contrasted fingerprint images are explained in more detail by Nam et al [11]. Lastly, a capacitive-type moisture sensor (Corneometer CM 825 from Courage + Khazaka electronic) is installed near the contact platform to measure the fingerpad moisture; it measures the moisture value of the outermost layer of the fingerpad in arbitrary units (a.u.)…”

“…2. (a) Our apparatus for capturing normal forces and contact images over time while a finger is pressing the glass plate [11]. The nearby moisture sensor measures the fingerpad's moisture level before and after contact.…”