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Tactual encoding of information is an essential aspect of communication in many aspects of life, but especially so for persons with visual impairments. The most common encoding is the use of topological features on a static medium such as paper. While Braille is widely used and effective for text transcription, the effective use of tactile graphics to communicate spatial or graphical information is less standardized. Tactile graphical techniques typically employ collections of individual topological features or texture patterns to communicate spatial information (maps, plots, diagrams), with the goal of minimizing reader confusion among the elements. In this study, the authors investigated the tribology and perception aspects of this complex issue in order to identify fundamental surface parameters which play a dominant role. Using texture specification guidelines published by the Braille Authority of North America (BANA), the investigators produced raised parallel-ridge textures on thermally activated tactile paper. The texture parameters modified included the width of ridges, spacing between ridges, and the inclination of the ridges to the pattern boundaries. Subjects reported their perception of tactile similarity or difference between members of texture pairs that were presented under a blind. Measurements of friction force were also collected for these textures to determine if friction mechanisms were most responsible for perceptive ability, or if other surface attributes played a dominant role. The study found that modifying pattern pitch improved perceptive ability to differentiate between adjacent mismatched texture patterns, but that ridge width had a deleterious effect. The results suggested that perceptive ability was not strongly related to various friction mechanisms proposed, but was closely correlated with changes in the number of pattern ridges in contact with the fingertip during tactile graphic exploration. • This psychophysical study involved the perceptive ability of sighted people to differentiate among tactile graphic textures. • Results provided evidence for texture grouping based on perceptive similarity. • Pattern pitch had a substantial positive impact in perceiving differences, while ridge width had a deleterious effect. • Friction mechanisms were ruled out as primary drivers of perceptive ability. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Efficient transmission of tactile information is vital for individuals who rely on their sense of touch to interact with and navigate their surroundings, including visually impaired persons. Somatosensory phenomena have been investigated with respect to surface topologies and neuron sensitivities in the skin, but there is little knowledge of the specific skin tribology when reading tactual-coded information such as braille. Braille is a tactual code that employs raised dome-shaped dots in six-position cells (2 columns by 3 rows per cell), with various dot patterns representing individual text characters, punctuation or mathematical operators. Due to the hypothesized significance of friction on tactile sensitivity, the authors investigated the effect of basic braille dot configurations on friction coefficient in fingertip sliding. Initial studies investigated the effect of multiple dot-row configurations and media type on friction coefficient, but the tribological effect of individual features and associated skin interactions was ill-defined. Subsequently, the frictional effect of an individual dot of varying radius was investigated and modeled against a multi-term frictional model implementing Hertzian contact, the Greenwood–Tabor hysteresis component of a spherical indenter against a soft surface, as well as Wolfram׳s traditional adhesion model. The results of the study show that macro-scale deformation of the fingerpad during fingertip-on-dot sliding is the primary friction mechanism and suggest that the contribution due to a macroscopic feature is largely independent of sample medium. Based on this understanding, the effect of braille dot spacing on a dot׳s friction contribution was investigated. The results from the spacing study indicate that the fingerpad׳s interaction with dot pairs is highly influenced by dot feature spacing. Further work is necessary to identify the fundamental sliding mechanics at the finger-dot interface, but the ability to identify the frictional mechanisms as well as the sliding interactions will provide a means to understand how much of a role friction plays in braille character recognition, as well as suggest potential friction-based methods to enhance the information density of braille codes. [ABSTRACT FROM AUTHOR]

Abstract: The surfaces of materials differ in topography and compatibility with human skin. Those differences were distinguished by measuring the friction coefficients between a human finger and different types of surfaces. Sample groups used in these tests were glass, plastic, and fabric. In the case of fabric, the following materials were used: linen, canvas, felt, silk, velvet, and cotton. Experiments were conducted using a specially-designed apparatus under constant temperature and relative humidity. Two factors were identified using the finger friction measurements. First was the friction level and second was the variance in friction. In Particular, it was found that different surfaces show significantly different variance in their friction coefficients. Fitting parameters from a linear regression of the relationship between friction coefficient and normal load were used to map materials' responses. [Copyright &y& Elsevier]

Abstract: The most fundamental interaction between a user and a product is physical touch. In fact, a significant amount of the perceived value of a product results from the initial touch experience by the potential customer, whether the product is an automobile interior or a portable music player. Therefore, the ability to engineer a product''s tactile character to produce favorable sensory perceptions has the potential to revolutionize product design. Another major consideration is the potential for products to produce friction-induced injuries to skin such as blistering. Progress in this field has been hindered by the difficulties in drawing correlations between human sensory outcomes (e.g. softness, smoothness, leather-like feel) and quantitative physical properties (e.g. friction coefficient, elastic modulus). In this paper, a framework is proposed to address this issue with regards to polymer fabrics, which are used everywhere from clothing to protective wound-care products, and results are reported. Human evaluators were used in a modified Quantitative Descriptive Analysis (QDA) approach in order to identify four specific tactile attributes of fabric materials—sensible texture, abrasiveness, slipperiness, and fuzziness. The friction coefficients of the same fabrics against skin were measured by employing a three-axis dynamometer. The results were then correlated to sensory attributes by means of standard statistical methods. It was concluded from this work that human evaluators can score various fabrics along the above four tactile descriptors with significant statistical repeatability if “high” and “low” benchmark reference fabrics are made available during the evaluation. It was also observed that friction between fabric and skin is heavily dependent on both material type and fiber geometry, but that the impact of friction coefficient on tactility is still unclear. However, the methods presented in the paper may be used to further identify key fabric properties that are responsible for tactility. [Copyright &y& Elsevier]

Accurate depiction of visual information presents a formidable obstacle to the inclusion of people with visual impairments in standardized educational assessment. This challenge is often addressed by using tactile graphics, such as the use of a collection of tactile patterns representing various zones similar to the use of colors to represent political boundaries on a visual map. This study investigated friction mechanisms involved during fingertip exploration of tactile graphics, as well as the role that friction played in forming perceptions. Perception experiments were run using four texture types published by the Braille Authority of North America (BANA): two parallel-ridge textures of different ridge widths and spacings, and two ordered-dot textures of different dot spacings. To minimize tactile confusion, BANA guidelines prohibit the use of similar textures adjacent to each other. In this study, texture pairs were printed adjacent to each other on thermally activated ('swell touch') tactile paper. Sighted human subjects participated in a blinded task of determining if the two textures in the pair were the same or different. Friction measurements of the fingertip exploration of the texture pairs was recorded using a dynamometer in a range of normal loads from 0.4 to 0.8 N. A generalized linear model (GLM) with a logistic link function was used to predict the influence of texture pair and participant on the likelihood of perceiving each texture pair as the same or different. Results showed that the particular texture pair had a strong correlation to the ability of participants to discern between textures. These quantitative results support the BANA guidelines. The friction data showed that there was not a significant amount of difference among the coefficients of friction of various textures, such that COF was not a strong predictor of perceptive ability. However, there was some evidence that penetration of the fingertip between raised texture elements, and interlocking with these elements, allowed for dissimilar dot patterns to be better discerned than dissimilar ridge patterns. The results indicate that there are other aspects, possibly still tribological in nature, that govern tactile perception when exploring tactile graphics. These phenomena may include such things as finger deformation and vibrational response produced during contact with individual texture elements. • Psychophysical studies were done on sighted people to evaluate the tactile discrimination of tactile graphic textures. • The probability of discerning textures was well predicted using the General Linear Model with a logistic link function. • Sequence order effects in similar ridged textures appeared to be related to finger penetration into the textures. • The ability to discern between these texture types did not appear to be strongly influenced by coefficient of friction. [ABSTRACT FROM AUTHOR]