Initial contact shapes the perception of friction.

Humans efficiently estimate the grip force necessary to lift a variety of objects, including slippery ones. The regulation of grip force starts with the initial contact and takes into account the surface properties, such as friction. This estimation of the frictional strength has been shown to depend critically on cutaneous information. However, the physical and perceptual mechanism that provides such early tactile information remains elusive. In this study, we developed a friction-modulation apparatus to elucidate the effects of the frictional properties of objects during initial contact. We found a correlation between participants' conscious perception of friction and radial strain patterns of skin deformation. The results provide insights into the tactile cues made available by contact mechanics to the sensorimotor regulation of grip, as well as to the conscious perception of the frictional properties of an object.

Heat source and sink effects on periodic mixed convection flow along the electrically conducting cone inserted in porous medium.

The system of partial differential equations governing the unsteady hydromagnetic boundary-layer flow along an electrically conducting cone embedded in porous medium in the presence of thermal buoyancy, magnetic field, heat source and sink effects are formulated. These equations are solved numerically by using an implicit Finite-Difference Method. The effects of the various parameters that are source/sink parameter, porous medium parameter, Prandtl number, mixed convection parameter and magnetic Prandtl number on the velocity, temperature profiles, transverse magnetic field are predicted. The effects of heat source and sink parameter on the time-mean value as well as on transient skin friction; heat transfer and current density rate are delineated especially in each plot. The extensive results reveal the existence of periodicity and show that periodicity becomes more distinctive for source and sink in the case of the electrically conducting cone. As the source and sink contrast increases, the periodic convective motion is invigorated to the amplitude and phase angle as reflect in the each plot. The dimensionless forms of the set of partial differential equations is transform into primitive form by using primitive variable formulation and then are solved numerically by using Finite Difference Scheme which has given in literature frequently. Physical interpretations of the overall flow and heat transfer along with current density are highlighted with detail in results and discussion section. The main novelty of the obtained numerical results is that first we retain numerical results for steady part and then used in unsteady part to obtain transient skin friction, rate of heat transfer and current density. The intensity of velocity profile is increased for increasing values of porosity parameter Ω, the temperature and mass concentration intensities are reduced due heat source effects.

Friction Dynamics of Wood Coated with Vegetable Oil.

Surface treatment of wood surface is an effective method to improve the physical properties. The friction dynamics of wood coated with vegetable oil were evaluated and compared to wood treated with polyurethane and untreated wood. The kinetic friction coefficient, µ k , was 0.39±0.01, which was smaller than the values for polyurethane-treated wood and untreated oak. The effect of the surface treatment was also observed in the dependence of velocity on the friction coefficient. The friction profile of the wood surface treated with vegetable oil was similar to that of untreated wood, and the friction coefficient was nearly constant, except in the static friction region of sliding out. These results suggest that wood treated with vegetable oil is suitable for inducing a smooth feel.

Physics of martial arts: Incorporation of angular momentum to model body motion and strikes.

We develop a physics-based kinematic model of martial arts movements incorporating rotation and angular momentum, extending prior analyses. Here, our approach is designed for a classroom environment; we begin with a warm-up exercise introducing counter-intuitive aspects of rotational motion before proceeding to a set of model collision problems that are applied to martial arts movements. Finally, we develop a deformable solid-body mechanics model of a martial arts practitioner suitable for an intermediate mechanics course. We provide evidence for our improved model based on calculations from biomechanical data obtained from prior reports as well as time-lapse images of several different kicks. In addition to incorporating angular motion, our model explicitly makes reference to friction between foot and ground as an action-reaction pair, showing that this interaction provides the motive force/torque for nearly all martial arts movements. Moment-of-inertia tensors are developed to describe kicking movements and show that kicks aimed high, towards the head, transfer more momentum to the target than kicks aimed lower, e.g. towards the body.

Frictional behaviour of molten chocolate as a function of fat content.

Soft tribology is used to probe the lubrication behaviour of molten chocolate between soft contacts, analogous to in-mouth interactions between the tongue and palate. Molten chocolate is a concentrated suspension of solid particles (sugar, cocoa and milk solids) in cocoa butter. We hypothesise that the complex frictional behaviour of molten chocolate depends on its particulate nature and thus solid volume fraction (sugar & cocoa solids/fat content). In this work, we assess the properties of molten chocolate as a function of fat content by diluting milk chocolate containing 26, 27 and 29% fat with cocoa butter. The tribological behaviour of molten chocolate deviates notably from the typical Stribeck curve of Newtonian fluids. Additional transitions are observed in mixed and elastohydrodynamic lubrication which are respectively attributed to the effect of shear-thinning rheology (i.e. breakdown of aggregates) and the selective entrainment or exclusion of particles depending on interfacial gap height. These transitions are more pronounced in chocolate of high solid fraction, and correlate with the influence of particle aggregation on rheology. In addition, we assess oral lubrication by preparing model chocolate boluses with aqueous buffer, which produces a ternary system of oil droplets and insoluble cocoa solids dispersed within a continuous aqueous phase. The frictional behaviour of chocolate boluses is determined by the viscosity ratio between cocoa butter and aqueous phase, in agreement with previous findings for oil-in-water emulsions. We provide a conceptual model to interpret how fat content influences the oral lubrication and mouthfeel of chocolate during consumption.

Analysis of required coefficient of friction in running and walking.

The popularity of running has increased over the past few years. However, just a few studies in running have focused on the friction between surface and shoe/foot. Changes in friction can affect aspects of human motion, such as safety, motion pattern and efficiency among others. The aim was to investigate the effects of cadence (walk, self-selected running and imposed-running), stance sub-phases (absorption and propulsion) and footwear (barefoot and shod) on the required coefficient of friction (RCOF) of regular runners. Twenty healthy runners (12 males, 8 females, 29.4 ± 4.9 years, 70.4 ± 9.6 kg) participated in this study. Two force plates were used to measure the ground reaction forces (GRF) in order to calculate the RCOF for each condition and the stance phase was divided in sub-phases. In walk, the RCOF was smaller in the absorption than in propulsion phase (p < 0.001). Results evidenced effects of the cadence (p < 0.001), stance sub-phases (p < 0.001) and footwear (p < 0.001) on the RCOF. There was interaction effect in cadence with stance sub-phases (p < 0.001) and footwear with stance sub-phases (p < 0.001). Our results show RCOF is influenced by cadence and footwear condition in the absorption phase.

Fabrication and friction characteristics of arbitrary biosurfaces.

There are many different types of surfaces found in nature which can increase or reduce friction, such as the well-studied frog toe or lotus leaf. However, methods for replicating these surfaces on a large scale for use in industrial applications are needed in order to take advantage of this natural friction engineering. Most replication processes rely on molding that requires an input surface size comparable to the desired output surface. We present a novel approach of replicating large-scale biosurfaces using a laser scanning confocal microscope for surface digitization and 3D two-photon lithography for the fabrication of the digitized surface. Two different natural surfaces (banana skin and daffodil petal) were replicated. An intermediary tiling process was used to cover a target area of arbitrary size independent of the input texture size. The surfaces were coated with a thin layer of ZnO, and the frictional and wettability characteristics of the replicated surfaces were then examined, demonstrating significant friction reduction up to 42% and increased hydrophobicity due to the presence of texture.

Tactile perception of randomly rough surfaces.

Most everyday surfaces are randomly rough and self-similar on sufficiently small scales. We investigated the tactile perception of randomly rough surfaces using 3D-printed samples, where the topographic structure and the statistical properties of scale-dependent roughness were varied independently. We found that the tactile perception of similarity between surfaces was dominated by the statistical micro-scale roughness rather than by their topographic resemblance. Participants were able to notice differences in the Hurst roughness exponent of 0.2, or a difference in surface curvature of 0.8 [Formula: see text] for surfaces with curvatures between 1 and 3 [Formula: see text]. In contrast, visual perception of similarity between color-coded images of the surface height was dominated by their topographic resemblance. We conclude that vibration cues from roughness at the length scale of the finger ridge distance distract the participants from including the topography into the judgement of similarity. The interaction between surface asperities and fingertip skin led to higher friction for higher micro-scale roughness. Individual friction data allowed us to construct a psychometric curve which relates similarity decisions to differences in friction. Participants noticed differences in the friction coefficient as small as 0.035 for samples with friction coefficients between 0.34 and 0.45.

Coulomb friction in twisting of biomimetic scale-covered substrate.

Biomimetic scale-covered substrates provide geometric tailorability via scale orientation, spacing and also interfacial properties of contact in various deformation modes. No work has investigated the effect of friction in twisting deformation of biomimetic scale-covered beams. In this work, we investigate the frictional effects in the biomimetic scale-covered structure by developing an analytical model verified by finite element simulations. In this model, we consider dry (Coulomb) friction between rigid scales surfaces, and the substrate as the linear elastic rectangular beam. The obtained results show that the friction has a dual contribution on the system by advancing the locking mechanism due to change of mechanism from purely kinematic to interfacial behavior, and stiffening the twist response due to sharp increase in the engagement forces. We also discovered, by increasing the coefficient of friction potentially using engineering scale surfaces to a critical coefficient, the system could reach to instantaneous post-engagement locking. The developed model outlines analytical relationships between geometry, deformation, frictional force and strain energy, to design biomimetic scale-covered metamaterials for a wide range of applications.

Variation of the frictional anisotropy on ventral scales of snakes caused by nanoscale steps.

The ventral scales of most snakes feature micron-sized fibril structures with nanoscale steps oriented towards the snake's tail. We examined these structures by microtribometry as well as atomic force microscopy (AFM) and observed that the nanoscale steps of the micro-fibrils cause a frictional anisotropy, which varies along the snake's body in dependence of the height of the nanoscale steps. A significant frictional behavior is detected when a sharp AFM tip scans the nanoscale steps up or down. Larger friction peaks appear during upward scans (tail to head direction), while considerably lower peaks are observed for downward scans (head to tail direction). This effect causes a frictional anisotropy on the nanoscale, i.e. friction along the head to tail direction is lower than in the opposite direction. The overall effect increases linearly with the step height of the micro-fibrils. Although the step heights are different for each snake, the general step height distribution along the body of the examined snakes follows a common pattern. The frictional anisotropy, induced by the step height distribution, is largest close to the tail, intermediate in the middle, and lower close to the head. This common distribution of frictional anisotropy suggests that snakes even optimized nanoscale features like the height of micro-fibrils through evolution in order to achieve optimal friction performance for locomotion. Finally, ventral snake scales are replicated by imprinting their micro-fibril structures into a polymer. As the natural prototype, the artificial surface exhibits frictional anisotropy in dependence of the respective step height. This feature is of high interest for the design of tribological surfaces with artificial frictional anisotropy.

Using mechanical testing to assess texturing of prosthetic sockets to improve suspension in the transverse plane and reduce rotation.

Creating a secure and comfortable linkage between the residual limb and prosthetic socket in persons with lower limb amputation is a critical factor for successful rehabilitation, including ambulation and other activities of daily living. Unwanted rotation within the socket can be a clinical problem for prosthesis users. One way of addressing issues experienced with transverse plane control of the socket may be through increased friction interface forces. It has been proposed that friction at the residual limb/socket interface may be increased by adding texture to interface components. Three-dimensional (3D) printing may be used to fabricate sockets with texture patterns added to the inner socket surface. Hence, the aim of this study was to investigate the effects of socket texturing on transverse plane rotation of the socket on a mock residual limb under two suspension conditions: passive suction and active vacuum. To conduct this study, we developed a mechanical testing protocol as no standardized tests currently exist to assess prosthetic sockets. Sockets with 14 different texture patterns were fabricated using the Squirt-Shape™ 3D printer. Textured sockets were compared to an Original Squirt-Shape (OSS) socket and a smooth thermoformed socket. Sockets were fitted with a mock residual limb and bi-axially loaded to 350 N compression with simultaneous rotation (2.5°, 5° and 7.5°) using a custom rotation assembly attached to a uniaxial hydraulic material testing system. There was a statistically significant three-way interaction between suspension, angle and texture (p < 0.0005). Torques between textured and reference sockets, for all rotation angles and both suspension conditions, were significantly different (p < 0.0005). Using newly developed testing protocols, it was demonstrated that some texture patterns significantly increased torque (i.e., resistance against unwanted rotation) in the transverse plane compared to both OSS and smooth sockets, especially for passive suction. Rotation testing of sockets may provide insight into socket design to improve suspension in the transverse plane.

Hip implant performance prediction by acoustic emission techniques: a review.

Nowadays, acoustic emission (AE) has its applications in various areas, including mechanical, civil, underwater acoustics, and biomedical engineering. It is a non-destructive evaluation (NDE) and a non-intrusive method to detect active damage mechanisms such as crack growth, delamination, and processes such as friction, continuous wear, etc. The application of AE in orthopedics, especially in hip implant monitoring, is an emerging research field. This article presents a thorough literature review associated with the implementation of acoustic emission as a diagnostic tool for total hip replacement (THR) implants. Structural health monitoring of an implant via acoustic emission and vibration analysis is an evolving research area in the field of biomedical engineering. A review of the literature reveals a lack of reliable, non-invasive, and non-traumatic early warning methods to evaluate implant loosening that can help to identify patients at risk for osteolysis prior to implant failure. Developing an intelligent acoustic emission technique with excellent condition monitoring capabilities will be an achievement of great importance that fills the gaps or drawbacks associated with osteolysis/implant failure. Graphical abstract.

Etiology of knee pain in elite cyclists: A 14-month consecutive case series.

Overuse injuries of the knee are a common cause of missed training and competition days in elite cyclists, however the underlying conditions causing this knee pain are not well defined. We conducted a diagnostic study, investigating a consecutive series of 53 high level cyclists with non-traumatic knee pain over a 14 month period. Demographic data on the participants' cycling specialty and training level was noted. Clinical information concerning knee pain intensity, location and occurrence were collected using a questionnaire. Our results show 7 different overuse injuries were identified. The prepatellar friction syndrome accounted for the majority of these overuse injuries (46%), while medial plica syndrome (15%), biceps femoris tendinopathy (7.5%), patellar tendinopathy (9.4%), infrapatellar plica friction syndrome (7.5%), infrapatellar fat pad impingement (5.7%) and iliotibial band syndrome (3.7%) were other causes of knee pain in these athletes. In contrast to current belief, our results show that instead of patellofemoral cartilage overload, friction related overuse injuries are the most frequent and underestimated cause of knee pain in high level cyclists.

The effect of dynamic friction with wet fabrics on skin wetness perception.

Purpose. To investigate the effect of cold stimulation (contacting wet fabric) and tactile stimulation (dynamic friction) on skin wetness perception: (a) wetness perception of fabric with different water transfer performance; (b) wetness perception of fabric with different water content; (c) wetness perception of fabric at different friction velocity with skin. Methods. Twenty participants were screened as reliable assessors using a 7-point wetness rating scale. The dynamic friction between skin and fabric was simulated by a self-made device with an adjustable speed controller. Each participant was asked to finish 18 assessments: 3 types of knitted fabric (cotton, polyester/spandex and Coolmax®) × 3 water contents (low, medium and high) × 2 friction velocities (30 and 90 mm/s). The subjective wetness perception and skin cooling rate were recorded during the experiments. Conclusions. The fabric type, water content and friction velocity significantly influence skin perceptual wetness. Fabric with good liquid moisture management property is perceived as significantly less wet, especially under a heavy sweaty state of the human body. The stronger wetness perception is perceived when fabric with a small frictional coefficient rubs skin at a lower friction velocity. The maximum transient thermal flow of fabric has a significantly positive correlation with wetness perception (r = 0.972).

Shaping the zebrafish myotome by intertissue friction and active stress.

Organ formation is an inherently biophysical process, requiring large-scale tissue deformations. Yet, understanding how complex organ shape emerges during development remains a major challenge. During zebrafish embryogenesis, large muscle segments, called myotomes, acquire a characteristic chevron morphology, which is believed to aid swimming. Myotome shape can be altered by perturbing muscle cell differentiation or the interaction between myotomes and surrounding tissues during morphogenesis. To disentangle the mechanisms contributing to shape formation of the myotome, we combine single-cell resolution live imaging with quantitative image analysis and theoretical modeling. We find that, soon after segmentation from the presomitic mesoderm, the future myotome spreads across the underlying tissues. The mechanical coupling between the future myotome and the surrounding tissues appears to spatially vary, effectively resulting in spatially heterogeneous friction. Using a vertex model combined with experimental validation, we show that the interplay of tissue spreading and friction is sufficient to drive the initial phase of chevron shape formation. However, local anisotropic stresses, generated during muscle cell differentiation, are necessary to reach the acute angle of the chevron in wild-type embryos. Finally, tissue plasticity is required for formation and maintenance of the chevron shape, which is mediated by orientated cellular rearrangements. Our work sheds light on how a spatiotemporal sequence of local cellular events can have a nonlocal and irreversible mechanical impact at the tissue scale, leading to robust organ shaping.

Effect of heel base area and walking speed on the utilized coefficient of friction during high-heeled walking.

BACKGROUND: The utilized coefficient of friction (uCOF) and the risk of slipping are known to increase as the heel height of shoes increases. The heel base area of shoes can also affect the uCOF. OBJECTIVE: The purpose of this study is to investigate the effect of the heel base area of high heels and walking speed on the uCOF during walking and their interaction effect. METHODS: The walking experiment was conducted at the speed of 1.0 m/s and 1.25 m/s using four 9-cm high heels having different heel areas (narrow, moderate, wide, and wedge heels). RESULTS: The peak uCOF was significantly lower when wearing the wide heels than when wearing the other heels. Wearing the narrow and moderate heels reduced the vertical ground reaction force (GRF) owing to the early timing of the peak anterior-posterior GRF and increased the peak uCOF. As the walking speed became faster, the peak uCOF became greater with more increases by the interaction effect when wearing the narrow and moderate heels than when wearing the wide and wedge heels. CONCLUSIONS: These results imply that wearing narrow high heels should be considered carefully, as the potential for a slip could be high owing to the increase in the peak uCOF. If it is inevitable to wear narrow high heels, it is critical to walk at a slower speed than usual. It is better to wear high heels with a wide heel area, e.g., 3 cm*3 cm, rather than narrow high heels or even wedge heels to reduce the possibility of slipping.

Effects of Loading Conditions on Articular Cartilage in a Metal-on-Cartilage Pairing.

The aim of this in vitro study was to investigate the response of articular cartilage to frictional load when sliding against a metal implant, and identify potential mechanisms of damage to articular cartilage in a metal-on-cartilage pairing. Bovine osteochondral cylinders were reciprocally slid against metal cylinders (cobalt-chromium-molybdenum alloy) with several variations of load and sliding velocity using a microtribometer. The effects of different loads and velocities, and the resulting friction coefficients on articular cartilage, were evaluated by measuring histological and metabolic outcomes. Moreover, the biotribocorrosion of the metal was determined. Chondrocytes stimulated with high load and velocity showed increased metabolic activity and cartilage-specific gene expression. In addition, higher load and velocity resulted in biotribocorrosion of the metal implant and damage to the surface of the articular cartilage, whereas low velocity and a high coefficient of friction increased the expression of catabolic genes. Articular cartilage showed particular responses to load and velocity when sliding against a metal implant. Moreover, metal implants showed tribocorrosion. Therefore, corrosion particles may play a role in the mechano-biochemical wear of articular cartilage after implantation of a metal implant. These findings may be useful to surgeons performing resurfacing procedures and total knee arthroplasty. © 2019 The Authors. Journal of Orthopaedic Research® published by Wiley Periodicals, Inc. on behalf of Orthopaedic Research Society J Orthop Res 37:2531-2539, 2019.

The Amphoteric and Hydrophilic Properties of Cartilage Surface in Mammalian Joints: Interfacial Tension and Molecular Dynamics Simulation Studies.

In this paper, we explain the amphoteric character of the cartilage surface by studying a lipid bilayer model built from phospholipids. We examined the interfacial tension values and molecular dynamics simulation in solutions of varying pH. The effects of negative and positive charge density (or fixed charges) on the (cartilage/cartilage) friction coefficient were investigated. In physiological (or synovial) fluid, after the isoelectric point (pI), the curve of interfacial tension decreases rapidly as it reaches pH 7.4 and then approaches a constant value at higher pH. It was shown that the curve of the interfacial tension curve exhibits a maximum value at the isoelectric point with a Gaussian shape feature. The phospholipid bilayers facilitate an almost frictionless contact in the joint. Moreover, the slippage of the bilayer and the short-range repulsion between the surfaces of the negatively charged cartilage surfaces are the main determinants of the low frictional properties of the joint.

Foot callus thickness does not trade off protection for tactile sensitivity during walking.

Until relatively recently, humans, similar to other animals, were habitually barefoot. Therefore, the soles of our feet were the only direct contact between the body and the ground when walking. There is indirect evidence that footwear such as sandals and moccasins were first invented within the past 40 thousand years1, the oldest recovered footwear dates to eight thousand years ago2 and inexpensive shoes with cushioned heels were not developed until the Industrial Revolution3. Because calluses-thickened and hardened areas of the epidermal layer of the skin-are the evolutionary solution to protecting the foot, we wondered whether they differ from shoes in maintaining tactile sensitivity during walking, especially at initial foot contact, to improve safety on surfaces that can be slippery, abrasive or otherwise injurious or uncomfortable. Here we show that, as expected, people from Kenya and the United States who frequently walk barefoot have thicker and harder calluses than those who typically use footwear. However, in contrast to shoes, callus thickness does not trade-off protection, measured as hardness and stiffness, for the ability to perceive tactile stimuli at frequencies experienced during walking. Additionally, unlike cushioned footwear, callus thickness does not affect how hard the feet strike the ground during walking, as indicated by impact forces. Along with providing protection and comfort at the cost of tactile sensitivity, cushioned footwear also lowers rates of loading at impact but increases force impulses, with unknown effects on the skeleton that merit future study.

Inner thigh friction as a cause of acne mechanica.

Acne mechanica is defined as being any acneiform eruption in areas of friction, pressure, stretching, rubbing, pinching, or occlusion of the skin in any individual, regardless of preexisting acne. Various causes have been reported, including prolonged back rest against a chair or bed, occlusive clothing, pressure from a prosthetic limb, and others. This is the first reported case of bilateral open comedones caused by inner thigh friction.