Investigation of the Role of Diminishing Surface Area on Friction-Based Tactile Discrimination of Textures

The ability to discriminate among various tactual elements is crucial to any tactile communication system, such as in assistive technology for those with visual impairment. In previous work, the authors investigated the ability to differentiate textures having a large surface area. In the current work, the objective was to determine how diminishing surface area affects perception, and the extent to which limited area inhibits with the friction-based perception. A perception study in combination with friction measurement was performed to address this issue. Circular texture samples consisting of abrasive papers of P800, P1200 and P2500 grit, respectively, of three different sizes, 38.1 mm, 9.5 mm and 3.2 mm, were used as stimuli. Same size samples were presented in pairwise combinations to determine the mean probabilities of differentiation for an abrasive paper pair at different sizes. Results from the perception measurement indicated that decreasing size of the texture sample resulted in a decrease in the ability to both reliably differentiate different-grit abrasive pairs and reliably identify same-grit abrasive pairs. Finger friction measurements from the participants suggested a possible edge effect on the friction of the samples. Silicone-based probes were also employed for friction measurement of the texture samples to identify friction mechanisms as well as confirm the magnitude of the effect of sample edges on total friction

Parametric investigation of soft-body penetration into parallel-ridged textured surfaces for tactile applications

The tribological interactions between skin and textured surfaces has profound impact on both the tactile perception of the product being used, as well as the functionality of the product with regards to friction coefficient. Previous work has shown that parallel-ridged textures have vastly different friction coefficients with regards to the direction of skin sliding, and that penetration of the skin into the voids between ridges not only add contact area but also potential for interlocking. The ability to model skin penetration into textural elements would prove to be very useful for predicting friction; however, the mechanics of the problem are incredibly complex such that they rule out a closed-form analytical solution. The authors investigated soft-body penetration using a non-dimensional computational approach based on the elastic properties of skin, as well as the texture ridge geometry parameters, as well as the normal loading. Model results were verified experimentally. The model was applied to a number of different combinations of ridge parameters and it was found that the amount of penetration could be predicted very well using a simple exponential relationship among the nondimensional terms. Texture groove width and applied normal load played a dominant role in penetration. These results yield a quantitative mechanics model which can be integrated into an overarching frictional model to predict skin on texture behavior due to both adhesion and edge interlocking

Skin tribology phenomena associated with reading braille print: The influence of cell patterns and skin behavior on coefficient of friction

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 hysteresiscomponent 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

Influence of linear reciprocating and multi-directional sliding on PEEK wear performance and transfer film formation

Because of their light weight, chemical resistance, and self-lubricating properties, polymers are used in applications ranging from biomedical to aerospace. Some polymers exhibit significant differences in wear resistancebased on whether they are in unidirectional or multidirectional sliding. Shear induced polymer chain orientation is believed to be responsible for this behavior. Polyetheretherketone (PEEK) has excellent wear resistance, but its multidirectional sliding behavior has not been thoroughly investigated. A factorial multidirectional pin-on-plate wear study of PEEK was conducted with a focus on molecular weight and sliding path directionality. These factors were studied for their correlation to over all wear performance. Additionally, transfer film thickness was measured at locations along the wear path using white light interferometry. Wear in PEEK was shown to depend significantly on path shape and direction. The lowest wear configuration also resulted in quantifiably thinner and more continuous transfer films. A result of this work has been a greater understanding of PEEK wear mechanisms in various sliding configurations

Tactile Discrimination of Randomly Textured Surfaces: Effect of Friction and Surface Parameters

The aim of the current study is to identify the physical factors that could potentially be indicative of the tactile discriminability of textured surfaces. To this end, the tactile discrimination sensitivity for fine randomly textured surfaces was evaluated and its relation to the surface parameters and the coefficient of friction was investigated. Discrimination tasks were performed using a two-interval forced choice technique and the mean probability of perceiving a difference was measured for pairwise combinations of six fine-grit abrasive papers. The surface roughness parameters of the abrasive papers were measured using a contact profilometer and scanning electron microscopy images helped observe the surface microstructure. The coefficient of friction for each of the abrasive papers against human finger was measured for two sliding orientation of the finger: an ‘aligned’ orientation along the length of the finger and a ‘transverse’ orientation perpendicular to the length of the finger. Possible cases of theoretical probabilities of perceiving a difference for a given pair of tactile samples were proposed and the experimental probabilities were discussed within this context. Based on the evidence in existing literature, three measurable properties were chosen to investigate possible correlation with the mean discrimination probability: root mean square roughness, mean spacing of the profile peaks, and the mean coefficient of friction. Experimental evidence suggests that of the three parameters investigated, differences in the mean spacing and the mean friction coefficients were more indicative of the probability of perceiving difference between a pair of fine textured samples

Study of the temporal characteristics of friction and contact behavior encountered during braille reading

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\u27s 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\u27s elastic bulk, generating rubber-like Poisson effect contributions. Most importantly, the Poisson effect rapidly increases the lateral load when the body contacts the dot\u27s leading edge, and rapidly decreases when the body rests largely in contact with the dot\u27s 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

Effects of contact pressure, molecular weight, and supplier on the wear behavior and transfer film of polyetheretherketone (PEEK)

Polyetheretherketone (PEEK) is a designation given to materials of the polyaryletherketone family having a characteristic distribution of ether and ketone groups in the polymer backbone. PEEK materials have high strength and chemical resistance as well as very high melting points and glass transition temperatures. Because of this combination of properties, PEEK materials find use for wear application in extreme environments where they provide a light-weight and corrosion resistant bearing material that often does not require lubrication. This study focused on determining the effects of supplier and molecular weight on the wear of particular PEEK materials, in addition to the effect of contact pressure. Multidirectional wear testingwas performed on four PEEK materials. The materials were obtained from two different suppliers, and two molecular weights were chosen for each supplier. Extensive analysis of transfer films produced during wear testing was performed using optical microscopy. White light profilometry was used to measure transfer film thickness in order to calculate a mean film thickness for given experimental conditions. Dynamic mechanical analysis, as well as gel permeation chromatography and differential scanning calorimetry were used to characterize each material\u27s viscoelastic behavior, molecular weight, and crystallinity, respectively. It was found that the wear of PEEK materials was significantly affected by both contact pressure and molecular weight, but not by supplier. However, an interaction was observed that showed the low molecular weight material from one of the suppliers was more vulnerable to wear at high contact pressures than the other three materials. Results of transfer film analysis showed that film thickness was greatest in locations where pin sliding direction was perpendicular to the counterface roughness direction, but that mean transfer film thickness did not correlate to wear amounts. This work is significant because it highlights the fact that tribologically relevant polymers, such as PEEK materials, vary greatly in terms of their polymer morphology and processing history, and this variation must be recognized by investigators when reporting wear data

Precision direct photon and W-boson spectra at high p_T and comparison to LHC data

The differential p_T spectrum for vector boson production is computed at next-to-leading fixed order and including the resummation of threshold logarithms at next-to-next-to-leading logarithmic accuracy. A comparison is made to ATLAS data on direct photon and W production at high transverse momentum p_T, finding excellent agreement. The resummation is achieved by factorizing contributions associated with different scales using Soft-Collinear Effective Theory. Each part is then calculated perturbatively and the individual contributions are combined using renormalization group methods. A key advantage of the effective theory framework is that it indicates a set of natural scale choices, in contrast to the fixed-order calculation. Resummation of logarithms of ratios of these scales leads to better agreement with data and reduced theoretical uncertainties.Comment: 24 pages, 10 figures; v2: journal version; v3: corrections in (20), (37), (38

Improving jet distributions with effective field theory

We obtain perturbative expressions for jet distributions using soft-collinear effective theory (SCET). By matching SCET onto QCD at high energy, tree level matrix elements and higher order virtual corrections can be reproduced in SCET. The resulting operators are then evolved to lower scales, with additional operators being populated by required threshold matchings in the effective theory. We show that the renormalization group evolution and threshold matchings reproduce the Sudakov factors and splitting functions of QCD, and that the effective theory naturally combines QCD matrix elements and parton showers. The effective theory calculation is systematically improvable and any higher order perturbative effects can be included by a well defined procedure.Comment: 4 pages, 1 figure; typos corrected and notation updated to match hep-ph/060729