How to Measure the Area of Real Contact of Skin on Glass

Abstract

The contact between the fingertip and an object is formed by a collection of micro-scale junctions, which collectively constitute the real contact area. This real area of contact is only a fraction of the apparent area of contact and is directly linked to the frictional strength of the contact (i.e., the lateral force at which the finger starts sliding). As a consequence, a measure of this area of real contact can help probe into the mechanism behind the friction of skin on glass. In this article, we present two methods to measure the variations of contact area; one that improves upon a tried-and-true fingertip imaging technique to provide ground truth, and the other that relies on the absorption and reflection of acoustic energy. To achieve precise measurements, the ultrasonic method exploits a recently developed model of the interaction that incorporates the non-linearity of squeeze film levitation. The two methods are in good agreement (ρ = 0.94) over a large range of normal forces and vibration amplitudes. Since the real area of contact fundamentally underlies fingertip friction, the methods described in the article have importance for studying human grasping, understanding friction perception, and controlling surface-haptic devices.

Rapid change of friction causes the illusion of touching a receding surface

“…This real contact area is the sum of all the junctions between the asperities of the skin and glass plate. The real area of contact is known to be linearly correlated to the frictional strength of the contact, therefore providing a measurement of the friction at the interface without resorting to force measurement during gross slippage [ 19 , 21 ]. A larger contact area has more junctions to adhere to the surface and therefore experiences more friction, assuming a constant shear strength of the interface.…”

“…The light is emitted through a beam splitter which ensures coaxiality with the return path. This illumination uses frustrated reflection of the glass–air interface to highlight the micro-junctions where the asperities of the skin are in intimate contact with the plate [ 19 ]. In the absence of the finger, 4% of the incident light is reflected toward the sensor, but if part of the skin is in intimate contact with the glass plate, the reflection is frustrated, leading to a dark area on the image.…”

The Biomechanics of the Tactile Perception of Friction

“…Images of the fingertip were captured at 300 frames per second by a high-speed camera (Phantom Miro M110). Frustrated Total Internal Reflection (F.T.I.R) was used to highlight the asperities of the skin in intimate contact with the glass plate, while darkening everything that is not touching the plate [100]. This technique create highly contrasted images of the skin asperities at pixel resolution, that is 0.0535 mm.…”

“…During my thesis, we developed a new method based on the principle of absorption and reflection of acoustic waves to estimate the frictional strength of a contact on glass plates [100]. When using an ultrasonic friction reduction device, a portion of the acoustic energy is transferred to the fingertip while the number of junctions between the skin and the glass plate is reducing.…”

Efficient Tactile Encoding of Object Slippage

“…The deformation of the skin was extracted from the images of the contact illuminated by frustrated total internal reflection (FTIR). This illumination technique highlights the asperities of the skin that are in intimate contact with the plate while darkening everything that is not touching the plate, resulting in a highly contrasted image 32 . An illustration of the apparatus can be found Figure 2A and typical images for a high-and lowfriction case are shown in figure 2C.…”