When we touch an object, complex frictional forces are produced, aiding us in perceiving surface features that help to identify the object at hand, and also facilitating grasping and manipulation. However, even during controlled tactile exploration, sliding friction forces fluctuate greatly, and it is unclear how they relate to the surface topography or mechanics of contact with the finger. We investigated the sliding contact between the finger and different relief surfaces, using high-speed video and force measurements. Informed by these experiments, we developed a friction force model that accounts for surface shape and contact mechanical effects, and is able to predict sliding friction forces for different surfaces and exploration speeds. We also observed that local regions of disconnection between the finger and surface develop near high relief features, due to the stiffness of the finger tissues. Every tested surface had regions that were never contacted by the finger; we refer to these as "tactile blind spots". The results elucidate friction force production during tactile exploration, may aid efforts to connect sensory and motor function of the hand to properties of touched objects, and provide crucial knowledge to inform the rendering of realistic experiences of touch contact in virtual reality. The tools we touch, grasp and manipulate often have topographical features such as ridges, bumps and texture that are meant to guide our gestures and provide crucial information on their nature and function. As an example, the rubbery curve on a quality pen provides grip and indicates where to position our fingers for optimal manipulation. These features are often recognized by touch alone from the deformation imposed by the skin in contact with the feature. While static shape perception can be attributed to the indentation (i.e. normal deformation) of the skin, the dynamic perception of these features is due to the complex interactions that result from sliding, in particular to the modification of frictional forces. Complex frictional forces are involved in both static and sliding contact between the finger pad and a touched surface. Friction plays an essential role in object grasping and manipulation, interrelating the grip and load forces that are exerted by the fingers and that must be coordinated in order to prevent slipping 1 . Surface curvature, roughness, bumps, and ridges can enhance friction forces, aiding grasping and fine manipulation 2,3 . Sensing these features is facilitated by dynamic tactile signals from thousands of mechanoreceptive afferents that innervate the finger pads 4 , allowing the brain to identify surface features and select an appropriate grasp. Friction forces are crucial for such acts of discriminatory touch. Sliding our fingers over the surface of an object, we obtain perceptual information about roughness 5 , shape 6 , adhesion 7 , and texture 8 . Local surface features, such as edges, are also reliably encoded in population responses of tactile afferents in the fingertip 9 . Variations in friction forces yield high-frequency, low-amplitude skin deformations that propagate throughout the hand 10 , exciting mechanoreceptive afferents within and beyond the finger pad, and further facilitating surface texture discrimination 8, 11 . While the relevance of friction forces to sensory and motor functions of the hand is well established, our understanding of how time-varying friction forces are produced during contact with non-flat surfaces is incomplete. This can partly be attributed to the complex dynamics governing the tribology of the finger pad 12 , including the moisture content of the fingertip skin, which can affect the mechanics and dynamics of frictional sliding sliding 12-14 , static to sliding transitions [15] [16] [17] , and the presence of non-Coulombic behavior 18 . However, effects due Published: xx xx xxxx OPEN www.nature.com/scientificreports/
Its results showed that bumps and upward steps elicit frictional modulations that can be modelled based on the surface shape and the contact mechanics of the finger and therefore implemented in haptic displays REF .
Contact geometry and mechanics predict friction forces during tactile surface exploration
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