A review of fingerpad contact mechanics and friction and how this affects tactile perception

Abstract

In the sliding contact between the fingerpad and a rough surface when touching a product’s surface, friction plays a role in the perception of roughness, slipperiness and warmth. For product engineers who aim to control and optimize the sensorial properties of a product surface interacting with the skin, it is essential to understand this frictional behaviour. However, the friction of skin is yet poorly understood. The variation that is observed within or between skin friction studies can be assigned to gender, age and orientation of the finger. Analysing data collected from literature shows some consistent trends. The coefficient of friction increases considerably with increasing hydration level of the skin, due to softening of the top layer of the skin. The coefficient of friction of the fingerpad decreases with normal load to a constant value, which can be attributed to effects of normal adhesion and the deformation behaviour of the fingerpad. There is no consistent effect of velocity on the coefficient of friction. Friction decreases with increasing Ra roughness. When the Ra roughness increases further, the contribution of deformation causes an increase in the friction after which it remains constant. Some influence of the finishing method is reported. The type of material has a smaller influence than the surface roughness of the sample or the condition of the skin. Even though the coefficient of friction of the fingerpad shows some consistent trends, examining the friction behaviour at a more detailed level might explain the contribution of friction to tactile perception. The measuring signal contains relevant information and should be analysed thoroughly as opposed to taking the average coefficient of friction of the steady state part of the signal. Future work should involve the study of local friction behaviour at the scale of the surface roughness.

Frictional dynamics of finger pads are governed by four length-scales and two time-scales

“…The present images are consistent with those reported by Childs & Henson [16], but perhaps because of the poor quality of the ink print images, their significance was under-appreciated. Our results also indicate that friction, driven by the growth dynamics of the real area of contact, may play a determining role in the perception of roughness, slipperiness, and warmth, as recently reviewed in reference [12].…”

“…Hence, surfaces may be viewed to be composed of features at multiple length scales of roughness that are superimposed on each other [11]. A number of studies have examined the influence of the topography of counter-surfaces on the friction of the finger pad [12]. For regular counter surface textures, it was observed that the coefficient of friction increased with the tip radius and number density of the asperities as would be expected by the resulting increase in the real area of contact [3].…”

The role of the sliding direction against a grooved channel texture on tool steel: An experimental study on tactile friction

“…The study of friction and the role of surface textures in relation to touch perception is the subject of researches in both science and industry for a wide variety of applications (van Kuilenburg et al, 2013;Derler et al, 2009;. Tactility is directly related to the functional behavior and perception of products like haptic devices, smartphone cases, tool handles, personal care products and for example kitchenware.…”

Friction perception resulting from laterally vibrotactile stimuli

“…In our experiment, the surface was smoothly finished. The surface roughness and friction are closely related [45,46]. Because of the interlocking effects, surfaces with significant roughness develop large friction forces with a fingertip.…”