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Beyond the sense of sight, the sense of touch is one of the primary ways that individuals experience their surrounding environment. Fundamentally understanding the relationship of skin-surface tribology and its elicited tactile attributes could provide a breakthrough in improving the ability to efficiently transmit tactile information to those who rely on the sense of touch to interact with their surroundings, such as the blind and visually impaired (BVI) community. The tactile language of braille has been adopted by the BVI community, employing configurations of raised dome-shape dots to convey what is ordinarily presented in text and image form. The coefficient of friction caused by skin sliding across a these dot features is hypothesized to affect the reader's tactile sensitivity, and skin-on-braille coefficient of friction has been investigated in previous work, where macro-scale deformation of the human fingerpad sliding over the dot contour was identified as the dominant friction mechanisms. This investigation succeeds that study by examining a simplified large-scale, two-dimensional representation of skin-on-braille sliding to characterize the underlying contact mechanisms in the loading behaviors that dictate the resulting coefficient of friction. This was accomplished by using a multi-axis tribometer to sliding a 25.4 mm radius cylindrical polyurethane(representing a human fingerpad) rod over a lubricated 3.17 mm aluminum half rod (representing a braille dot) under displacement-displacement-controlled conditions. The results from the tribometer study indicate that the presence of the dot feature drastically affects the vertical and lateral loading behavior by vertically displacing the body's elastic bulk, generating rubber-like Poisson effect contributions. Most importantly, the Poisson effect rapidly increases the lateral load when the body contacts the dot's leading edge, and rapidly decreases when the body rests largely in contact with the dot's trailing edge. This rapid decrease is caused by a “propulsion” effect, where vertical compression expands the material laterally, and when situated on the trailing edge of the dot, propels it into the direction of sliding, virtually negating adhesive surface friction. Computational modeling of this system discovered that while normal contact pressures dominated the fluctuations seen in the vertical loading, effects due to both normal contact pressures and frictional shears nearly equally drove the lateral loading behavior.
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Darden, Matthew A. and Schwartz, Christian J., "Study of the temporal characteristics of friction and contact behavior encountered during braille reading" (2017). Mechanical Engineering Publications. 334.