Understanding the friction mechanisms between the human finger and flat contacting surfaces in moist conditions

Human hands sweat in different circumstances and the presence of sweat can alter the friction between the hand and contacting surface. It is, therefore, important to understand how hand moisture varies between people, during different activities and the effect of this on friction. In this study, a survey of fingertip moisture was done. Friction tests were then carried out to investigate the effect of moisture. Moisture was added to the surface of the finger, the finger was soaked in water, and water was added to the counter-surface; the friction of the contact was then measured. It was found that the friction increased, up until a certain level of moisture and then decreased. The increase in friction has previously been explained by viscous shearing, water absorption and capillary adhesion. The results from the experiments enabled the mechanisms to be investigated analytically. This study found that water absorption is the principle mechanism responsible for the increase in friction, followed by capillary adhesion, although it was not conclusively proved that this contributes significantly. Both these mechanisms increase friction by increasing the area of contact and therefore adhesion. Viscous shearing in the liquid bridges has negligible effect. There are, however, many limitations in the modelling that need further exploration

Skin friction at the interface between hands and sports equipment

The friction between the finger pad/palm and items of sports equipment strongly influences how well an athlete is able to perform. It not only determines how well equipment can be gripped and manipulated, but also how the equipment feels to use and the perceived level of performance. In this paper various fundamental aspects of finger pad friction are reviewed, including the effects of applied force, skin moisture, material, surface texture etc., and the influence that they have on friction mechanisms such as adhesion, deformation, interlocking and hysteresis. A number of applied case studies are then outlined. The first is rugby balls and the effect the ball surface pimple pattern has on friction. Initially high speed video was used to establish how the hand interacts with a ball. Friction tests were then carried out with different hand conditions and pimple patterns and the link between friction and pass accuracy was explored. The second study relates to friction modifiers used in sports such as rock climbing and athletics. These can be affected by hand and environmental conditions so a focus was placed on tests with moist hands or wet surfaces. Finally Frisbee interactions were investigated. The impact of loss of feel as a result of wearing gloves was studied to see if any improvements in wet conditions with gloves were offset by the reduced feedback from the Frisbee interface. The fundamentals of skin tribology can play a key role in developing optimised sports equipment, gaps still exist, however, in the understanding and modelling of surface texture and how important feel/comfort are, which are both important for sports equipment design

Friction at the tennis shoe-court interface: how biomechanically informed lab-based testing can enhance understanding

This paper presents some of the methodology, observations and findings from a 30-month study, aiming to improve the understanding of tennis shoe-court interactions and the biomechanical implications of changes in friction between the shoe and surface. A detailed programme of biomechanical player testing on different court surfaces provided the boundary conditions with which to develop a lab-based rig capable of simulating the key aspects of shoe-surface interaction that are required for acceptable performance (e.g. push-off to accelerate) within expected levels of consistency (e.g. for a controlled slide). Large- scale parametric testing could then be carried out for a variety of surface types and components under a range of loading conditions, without the risk of injury to human participants. Two case studies are described to demonstrate the value of a combined approach of biomechanical field testing and lab-based rigs that simulate shoe-court interactions. These include a study that compared different artificial clay court designs; and a study that examined the effect of different acrylic hard court parameters on friction and the tribological mechanisms that explain the observed interactio

Effect of varying the volume infill sand on synthetic clay surfaces in terms of the shoe-surface friction

The friction developed by the shoe-surface interface on artificial clay has not been widely studied, and can influence player's performance and injury risk. The aim of this research was to investigate the effect of varying the quantity of infill sand on shoe- surface friction on an artificial clay court tennis surface. A laboratory-based mechanical test rig was used to measure the friction force developed at the shoe-surface interface. Additionally, the perception of a group of participants, performing a turning movement on the same surface under dry conditions, was collected in order to compare against the mechanical results. The relationship between the normal force and friction generated by the shoe-surface interaction was examined for surfaces with different sand in-fill volumes. The mechanical testing was performed under dry and wet conditions, showing strong and significant differences. Results indicated that the normal force significantly influenced the static and dynamic frictional forces. For lower sand infill volumes, as normal loading increased, the dry condition was found to exhibit the lowest peak static friction force and highest average dynamic friction force. However, for higher sand infill volume conditions, the opposite behaviour was observed. Strong and significant positive linear relationships were found between peak friction force and average dynamic friction force for all infill sand volumes and conditions. The mechanical results were in agreement with the perception data, which suggests that the participants were sensitive to the small changes in sand infill volumes. The findings of this study will therefore aid the understanding of tennis players’ perceived response to a tennis court surface. In order to get a better understanding of friction behaviour, further testing needs to be performed, and once the mechanisms involved are understood, surface properties could be modified to increase performance and reduce injury risk

An assessment of the performance of grip enhancing agents used in sports applications

The performances of four grip enhancing agents, powdered and liquid chalk, rosin and Venice turpentine, were assessed using a bespoke finger friction rig and compared against an agent-free finger. The effectiveness of these agents was measured in dry, damp and wet conditions, to simulate the different environments in which the agents are used. The tests were first done on a polished steel surface and then the powdered and liquid chalk and agent-free finger were tested on sandstone. The tests on the steel showed that in a dry condition, only the Venice turpentine significantly increased the coefficient of friction, compared to no application of agent, with the rosin and powdered chalk actually decreasing the coefficient of friction. It is thought that the reduction in the coefficient of friction is caused by the solid particles acting as a lubricant between the two surfaces. When the fingers were wet, only the granular powder-based agents increased the coefficient of friction. This is because the Venice turpentine cannot adhere well to a wet finger, and therefore is not as effective. When the surface is wet, there is very little difference between the agents due to the water separating the finger surface from the steel. The tests on the sandstone showed no real difference between the lubricants or the different conditions, except for the dry, chalk-free finger, which had a decreased coefficient of friction due to the lubricating properties of the sandstone particles. These results highlight that the use of grip enhancing agents should take into account the moisture in the contact, as in dry conditions, the grip may be optimum when there is no agent used. It also shows that in different sports, different grip enhancing agents should be used

Human finger contact with small, triangular ridged surfaces

Ridges are often added to surfaces to improve grip of objects such as sports equipment, kitchen utensils, assistive technology, etc. Although considerable work has been carried out to study finger friction generally, not much attention has been paid to understanding and modelling the effects of surface texture. Previous studies indicate that at low roughness values friction decreases as roughness increases, but then a sharp increase is seen after a threshold level of roughness is reached. This is thought to be due to interlocking. In this study an analytical model was developed to analyse the different mechanisms of friction of a fingerpad sliding against triangular-ridged surfaces that incorporated adhesion, interlocking and hysteresis. Modelling was compared with experimental results from tests on five different triangular-ridged surfaces, manufactured from aluminium, brass and steel. Model and experiment compared well. The study showed that at low ridge height and width the friction was dominated by adhesion. However, above a ridge height of 42.5 μm, interlocking friction starts to contribute greatly to the overall friction. Then at a height of 250 μm, a noticeable contribution from hysteresis, of up to 20% of the total friction, is observed

Finite element model to simulate impact on a soft tissue simulant

A finite element model of an impact test on a soft tissue simulant, used as part of a shoulder surrogate, was developed in Ansys© LS-DYNA®. The surrogate consisted of a metal hemicylindrical core, with a diameter of 75 mm, covered with a 15 mm thick relaxed muscle simulant. The muscle simulant consisted of a 14 mm thick layer of silicone covered with 1 mm thick chamois leather to represent skin. The material properties of the silicone were obtained via quasi-static compression testing (curve fit with hyperelastic models) and compressive stress relaxation testing (curve fit with a Prony series). Outputs of the finite element models were compared against experimental data from impact tests on the shoulder surrogate at energies of 4.9, 9.8 and 14.7 J. The accuracy of the finite element models was assessed using four parameters: peak impact force, maximum deformation, impact duration and impulse. A 5-parameter Mooney-Rivlin material model combined with a 2-term Prony series was found to be suitable for modelling the soft tissue simulant of the shoulder surrogate. This model had under 10% overall mean deviation from the experimental values for the four assessment parameters across the three impact energies. Overall, the model provided a repeatable test method that can be adapted to help predict injuries to skin tissue and the performance/efficacy of personal protective equipment

A critical review of the assessment of medical gloves

Medical protective gloves must be assessed to adequate standards before becoming available for commercial use. Strength, integrity and contamination are assessed at the manufacturing stage. However, concerns are raised over the standardisation of the testing; should more be done to assess how gloves affect the performance? Over the years, studies have demonstrated how the design of gloves have reduced tactile sensitivity in medical staff, ultimately leading to poor patient care through missed information. Studies have also demonstrated that a loss of grip control and dexterity through glove use are detrimental to medical tasks. However, it remains relatively unexplored as to how wearing medical gloves affects the users and the patients. Gaps in research have been identified around the frictional and grip properties of gloves. Linking the key performance parameters to the manufacturing processes, which affect material properties, will provide more insight into the behaviour of medical gloves and how to properly assess materials

Probing the liquid-t-gas like phase transition in a cluster via a caloric curve

IP

Vibration transmissibility measurement of glove materials under different grip forces

The transmission of vibration from tools, through work gloves and into the hands, is affected by many factors such as glove material properties, tool vibration conditions, temperature, and grip force. This study investigated how glove material properties affect tool vibration transmission into the index finger. Three samples of material (two taken from differently designed anti-vibration gloves and another for comparison that was designed for mounting vibration-sensitive equipment) were assessed using stepped sinusoidal vertical vibration excitations covering a range of a one-third octave band (from 20 to 400 Hz). Twelve human subjects were used for the testing. For all samples and subjects, measurements were obtained for: (I) dynamic mechanical analysis (DMA) of the samples; (II) the transmissibility of vibration to the index finger at a grip force of 30 N, across the range of frequencies; and (III) transmissibility of vibration to the index finger at a frequency of 125 Hz for finger grip forces of 15, 30, and 45 N. No significant vibration attenuation was provided at frequencies below 150 Hz. The two materials taken from the gloves that passed the ISO 10819:1996 test showed resonance at frequencies of 150 and 160 Hz a, but the material that did not pass the ISO test showed resonance at 250 Hz. The attenuation for all three materials was occurred at 400 Hz. There was no significant change of transmissibility across the range of finger grip forces for any of the material samples. The level of transmissibility was found to vary between samples and subjects