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Stiffness is always one of two factors that determine frictional resistance. The other factor, the coefficient of friction, depends on the surface characteristics. Teflon coating reduces the sliding friction to one-half, and a hydrophilic plastic coat to about one-sixth compared with steel.
Urinary catheters are commonly used in chronic care facilities and geriatric homes. Problems associated with the catheters include encrustation, infection, physical trauma and inflammation. Many of these problems can be correlated to the catheter surface. Previous studies have dealt with problems associated with surface morphology and catheter composition and numerous catheter coatings have been developed to alter the surface including siliconized, Teflon and polyurethane hydrogel coatings. The hydrogel coating appears best, as it imparts a smooth, soft surface layer with lubricating properties. The paper describes a new poly(vinyl alcohol) hydrogel coating and compares its surface morphology and lubricity with other catheter coatings. Each catheter coating-type was examined by scanning electron microscopy for surface morphology. The new hydrogel coating was found to be smooth with no evidence of cracking. The apparent friction coefficient was used as an indication of surface lubricity. Friction measurement tests of the new hydrogen coating were run underwater and the friction coefficient was determined between the hydrated catheter surface and the hydrogel surface. The new hydrogel coating was found to have a high degree of lubricity in its hydrated state.
OBJECTIVES: A survey of general dental practitioners and dental surgery assistants was carried out to ascertain their preferences and opinions on powder-free hydrogel-coated gloves compared with starch-powdered gloves. The aim was to relate the survey findings to laboratory measurements of the frictional characteristics of glove inner surfaces and their water absorptive capability. METHODS: The survey was carried out using a questionnaire given to local dental practitioners. Glove friction and water absorption measurements were made using specially designed equipment. RESULTS: The survey showed that a selected group of dentist and dental surgery assistants preferred hydrogel-coated gloves, particularly for damp donning, durability and long-term wear comfort. Laboratory measurements showed that the hydrogel coating gave a low friction coefficient against damp skin. The coating was durable, and absorbed water more readily than other treatments. CONCLUSION: A survey of dental practitioners and dental surgery assistants and laboratory measurements indicates that hydrogel-coated gloves have superior properties, and are preferred to other non-sterile glove types.
INTRODUCTION: The aim of this study was to evaluate the changes in the transverse dimension and the perimeter of the maxillary arch produced by a low-friction system during the leveling and aligning phases of fixed appliance therapy. METHODS: The low-friction protocol consisted of a combination of preadjusted brackets, superelastic nickel-titanium archwires, and nonconventional elastomeric ligatures; it was used in 20 consecutively treated patients with mild crowding. RESULTS: Statistically significant increases were recorded for all dentoalveolar widths (with the exception of intermolar width measured lingually) and arch perimeter (3.5 mm on average). The increase in arch perimeter showed a significant negative relationship with the individual perimeter of the maxillary arch before treatment. No significant change was found for arch depth. CONCLUSIONS: The low-friction system produced statistically significant increases in the transverse dentoalveolar width and the perimeter of the maxillary arch during the leveling and aligning phases of treatment with an average duration of 6 months.
INTRODUCTION: The purposes of this article were to clarify the combined effect of friction and an archwire's flexural rigidity on canine movement in sliding mechanics, and to explain how to select a suitable archwire and force level for efficient bodily movement. METHODS: A numerical simulation was carried out by using a 3-dimensional finite element method. RESULTS: As the frictional force decreased, both the net force acting on and the moving speed of the canine increased. The elastic deformation of the archwire increased, and the moving pattern of the canine changed from bodily movement to tipping, although there was no clearance between the archwire and the bracket slot. When a light wire was used, wire deformation increased, and the canine experienced greater tipping. These effects of friction and wire properties on canine movement (tipping and rotational angles) are combined by using a single parameter, EI/P, where EI is the flexural rigidity of the archwire, and P is the net force acting on the canine. A suitable combination of EI and P will cause canine bodily movement. The EI is determined from the archwire's size and material. CONCLUSIONS: We propose a method for estimating a suitable combination in this article.
Molecular dynamics simulations were performed under conditions of constant volume and temperature and of constant pressure and temperature to elucidate the structure activity relationships of a series of non-ionic surfactant molecules derived from vegetable fat and employed as friction modifiers in commercial engine oils. The simulations show the extent to which intermolecular hydrogen bonding is important in determining the stability of the monolayer formed by the surfactant molecules and show that mono-alkanoyl glyceride molecules are able to pack more efficiently, forming significantly more intermolecular hydrogen bonds and occupying approximately half the volume needed by di-alkanoyl glyceride molecules. Density profiles are presented which show significant mixing of the hydrophobic tail groups and a non-polar solvent. The distribution of torsion angles in the tail groups shows that the conformation is consistent with a liquid at finite temperature rather than a crystal structure. The measured friction coefficients of equimolar solutions of the glycerides show that the efficacy as friction modifiers varies in the order mono-, di- and the tri-oleyl glyceride, which is consistent with the efficacy of film formation predicted by the molecular dynamics calculations.
Iliotibial band (ITB) syndrome is regarded as an overuse injury, common in runners and cyclists. It is believed to be associated with excessive friction between the tract and the lateral femoral epicondyle-friction which 'inflames' the tract or a bursa. This article highlights evidence which challenges these views. Basic anatomical principles of the ITB have been overlooked: (a) it is not a discrete structure, but a thickened part of the fascia lata which envelops the thigh, (b) it is connected to the linea aspera by an intermuscular septum and to the supracondylar region of the femur (including the epicondyle) by coarse, fibrous bands (which are not pathological adhesions) that are clearly visible by dissection or MRI and (c) a bursa is rarely present-but may be mistaken for the lateral recess of the knee. We would thus suggest that the ITB cannot create frictional forces by moving forwards and backwards over the epicondyle during flexion and extension of the knee. The perception of movement of the ITB across the epicondyle is an illusion because of changing tension in its anterior and posterior fibres. Nevertheless, slight medial-lateral movement is possible and we propose that ITB syndrome is caused by increased compression of a highly vascularised and innervated layer of fat and loose connective tissue that separates the ITB from the epicondyle. Our view is that ITB syndrome is related to impaired function of the hip musculature and that its resolution can only be properly achieved when the biomechanics of hip muscle function are properly addressed.
One of the major causes of loosening of cementless acetabular cup implants is insufficient initial stability. This study used three-dimensional finite element models of the pelvis and acetabular components to investigate the effects of the number of screws, bone quality, and friction coefficient of the acetabular cup on the initial stability under normal walking. A commercially available hemispherical acetabular cup with five screw holes was used as the default model. The stiffness of the pelvis and the friction coefficient of the cup were systematically varied, within a realistic range, to assess the initial stability of the acetabular cup. The simulations showed that the inserted screws provide only a localized reduction in the relative micromotion between the cup and pelvis therefore inserting several screw closed together might not be useful. Changes in the pelvic stiffness have a non-linear effect on the initial stability of acetabular cup and the subchondral bone provides good support for fixation of the cementless cup. The friction coefficient of the acetabular cup plays a limited role, comparing with the factor of bone quality, in resisting relative micromotion in the cup-pelvis interface.
STATEMENT OF PROBLEM: All-ceramic and metal-ceramic dental restorations are potentially harmful because they may wear opposing enamel. Wear is a multifactorial process; however, laboratory procedures can minimize the wear process, thus benefiting clinicians and manufacturers. PURPOSE: This in vitro investigation of wear evaluated the physical and tribological characteristics of 2 low-fusing ceramics and dental enamel. MATERIAL AND METHODS: Quantitative and qualitative analyses of 2 low-fusing ceramics (IPS d.Sign and Vita Omega 900) and enamel were performed using scanning electron microscopy, x-ray diffraction, and energy-dispersive analyses. Each ceramic was tested using 20 disks (10 mm x 3 mm), of which 10 were glazed and 10 were polished. Ten polished enamel specimens (6 mm x 7 mm) served as the control. The specimens were submitted to a Vickers hardness test for 30 seconds using a 200 g load. The coefficient of friction was determined by using a pin-on-disk test without lubrication (5 N load, 600 seconds, 120 RPM). The data were analyzed using a 1-way analysis of variance and Sheffe analysis (alpha=.05). RESULTS: The same chemical elements were found in both ceramics, and tetragonal leucite was the main component of the crystalline phases. The microhardness of the enamel was significantly different ( P <.05) between the glazed and polished ceramics. There was no significant difference found in the friction coefficient (mean +/- SD) of the enamel (0.286 +/- 0.174), glazed Vita Omega 900 (0.309 +/- 0.085), and glazed IPS d.Sign (0.253 +/- 0.068) specimens. The mean friction coefficient of the polished Vita Omega 900 (0.486 +/- 0.036) and IPS d.Sign (0.446 +/- 0.037) specimens was found to be significantly higher than the enamel and glazed specimens ( P <.05). CONCLUSIONS: Both ceramics tested presented similar chemical-structural and tribological characteristics. When glazed, they showed tribological similarity to dental enamel.
Super-hydrophobic surfaces as well as low adhesion and friction are desirable for various industrial applications. Certain plant leaves are known to be hydrophobic in nature. These leaves are hydrophobic due to the presence of microbumps and a thin wax film on the surface of the leaf. The purpose of this study is to fully characterize the leaf surface and to separate out the effects of the microbumps and the wax on the hydrophobicity. Furthermore, the adhesion and friction properties of the leaves, with and without wax, are studied. Using an optical profiler and an atomic/friction force microscope (AFM/FFM), measurements on the hydrophobic leaves, both with and without wax, were made to fully characterize the leaf surface. Using a model that predicts contact angle as a function of roughness, the roughness factor for the hydrophobic leaves has been calculated, which is used to calculate the contact angle for a flat leaf surface. It is shown that both the microbumps and the wax play an equally important role in the hydrophobic nature as well as adhesion and friction of the leaf. This study will be useful in developing super-hydrophobic surfaces.
The aim of this study was to propose and validate a post-hoc correction method to obtain maximal power values taking into account inertia of the flywheel during sprints on friction-loaded cycle ergometers. This correction method was obtained from a basic postulate of linear deceleration-time evolution during the initial phase (until maximal power) of a sprint and included simple parameters as flywheel inertia, maximal velocity, time to reach maximal velocity and friction force. The validity of this model was tested by comparing measured and calculated maximal power values for 19 sprint bouts performed by five subjects against 0.6-1 N kg(-1) friction loads. Non-significant differences between measured and calculated maximal power (1151+/-169 vs. 1148+/-170 W) and a mean error index of 1.31+/-1.20% (ranging from 0.09% to 4.20%) showed the validity of this method. Furthermore, the differences between measured maximal power and power neglecting inertia (20.4+/-7.6%, ranging from 9.5% to 33.2%) emphasized the usefulness of power correcting in studies about anaerobic power which do not include inertia, and also the interest of this simple post-hoc method.
Measured interface nonlinear friction properties are used to develop models to study the short-term fixation response of smooth- and porous-surfaced posts, bone screws, and plates fixed with and without posts/screws. Experimental studies are carried out to validate the model predictions and identify the relative role of posts and screws in fixation of a plate on a polyurethane block under symmetric/eccentric axial compression loads. The idealized Coulomb's friction is also used for the sake of comparison. The incorporation of measured nonlinear, rather than the idealized Coulomb, friction is essential to compute realistic results. For plate fixation, the experimental and finite element results show that the screw fixation yields the stiffest response followed by the smooth- and then porous-coated post fixation. For example, under 1000 N eccentric axial compression, the edge of the plate opposite the loaded edge is measured to lift by 1147 +/- 72, 244 +/- 38, or 112 +/- 28 microns, respectively, for the cases with no fixation, with smooth-surfaced posts, or with screws. The corresponding models predict, respectively, values of 1538, 347, or 259 microns and also 556 microns for the plate fixed with porous coated posts. The satisfactory agreement between numerical and experimental results confirms the importance of proper interface modelling for the analysis of posts, screws, and complex fixation systems. This becomes further evident when considering cementless implants in which the bone-implant interface exhibits relatively large displacements as the maximum resistance force is reached. The developed models can be used to investigate the post-operative short-term stability of various cementless implant designs.
A computer model was generated which modelled the bearing surfaces of total knees, and predicted the kinematics for a set of input forces and moments. The model included friction at the bearing surfaces and soft tissue restraint forces, including the effect of cruciate resection. Predictions from the model were compared with data from a Knee Simulating Machine. There was close agreement in the shapes of the curves and in the magnitudes of the displacements and rotations under most conditions. The model predicted major differences in kinematics when friction between metal and polyethylene was included, the differences being even greater at the friction levels associated with small embedded acrylic particles. Soft tissue restraint was shown to reduce the displacements and rotations for tibial surfaces of low constraint but for moderate to high constraint, the soft tissues affected the kinematics only slightly. When the model was used to predict the motions for different condylar geometries, widely different contact paths on the tibial surface were determined. This suggested that condylar geometries which appeared to be generally similar, could have important differences in kinematics, function and wear.
A computational head-neck model was developed to test the hypothesis that increases in friction between the head and impact surface will increase head and neck injury risk during near-axial impact. The model consisted of rigid vertebrae interconnected by assemblies of nonlinear springs and dashpots, and a finite element shell model of the skull. For frictionless impact surfaces, the model reproduced the kinematics and kinetics observed in near-axial impacts to cadaveric head-neck specimens. Increases in the coefficient of friction between the head and impact surface over a range from 0.0 to 1.0 resulted in increases of up to 40, 113, 9.8, and 43% in peak post-buckled resultant neck forces, peak moment at the occiput-C1 joint, peak resultant head accelerations, and HIC values, respectively. The most dramatic increases in injury-predicting quantities occurred for COF increases from 0.0 to 0.2, while further COF increases above 0.5 generally produced only nominal changes. These data suggest that safety equipment and impact environments which minimize the friction between the head and impact surface may reduce the risk of head and neck injury in near-vertex head impact.
Frictional forces during simulated sliding tooth movement were measured with a model that was representative of the clinical condition. The model allowed tipping of the tooth until contact was established between the arch wire and diagonally opposite corners of the bracket wings; it also allowed rotation until the wire contacted opposite corners of the ligature tie, or the buccal shield with self-ligating brackets, and the base of the slot. Conventional and self-ligating stainless steel brackets as well as conventional ceramic brackets, and ceramic brackets with a stainless steel slot, all with 0.022 inch bracket slot, were tested with 0.019 x 0.025 inch arch wires of stainless steel, nickel titanium, and beta titanium. Each of the 12 bracket-arch wire combinations was tested 10 times. No significant interaction was detected between brackets and arch wires (P = .89), but the bracket and arch wire effects were significant (P < .001). The pairwise differences between conventional and self-ligating stainless steel brackets and ceramic brackets with stainless steel slot were not significant. However, the conventional ceramic brackets generated significantly higher friction than the other brackets tested. Beta titanium arch wires produced higher frictional forces than nickel titanium arch wires, but no significant differences were found between each of the two and stainless steel arch wires. Attempts to identify differences in surface scratches of the arch wires produced by the different brackets were unsuccessful.
Dielectric fluctuations are shown to be the dominant source of noncontact friction in high-sensitivity scanning probe microscopy of dielectric materials. Recent measurements have directly determined the friction acting on custom-fabricated single-crystal silicon cantilevers whose capacitively charged tips are located 3-200 nm above thin films of poly(methyl methacrylate), poly(vinyl acetate), and polystyrene. Differences in measured friction among these polymers are explained here by relating electric field fluctuations at the cantilever tip to dielectric relaxation of the polymer.
Hydrogels of poly(2-hydroxyethyl methacrylate) (PHEMA) with well-defined polyelectrolyte brushes of poly(sodium 4-styrenesulfonate) (PNaSS) of various molecular weights were synthesized, keeping the distance between the polymer brushes constant at ca. 20 nm. The effect of polyelectrolyte brush length on the sliding friction against a glass plate, an electrorepulsive solid substrate, was investigated in water in a velocity range of 7.5 x 10(-5) to 7.5 x 10(-2) m/s. It is found that the presence of polymer brush can dramatically reduce the friction when the polymer brushes are short. With an increase in the length of the polymer brush, this drag reduction effect only works at a low sliding velocity, and the gel with long polymer brushes even shows a higher friction than that of a normal network gel at a high sliding velocity. The strong polymer length and sliding velocity dependence indicate a dynamic mechanism of the polymer brush effect.
Poly(N-isopropylacrylamide) (pNIPAAm) gels are stimulus-responsive hydrogels that exhibit large reversible changes in their volume and surface physicochemical properties near the lower critical solution temperature (LCST) in response to external stimuli, such as a change in temperature or solvent composition. Here we report how different phase states, induced isothermally by changes in the solvent composition, affect the tribological properties of pNIPAAm hydrogels. Our measurements indicate that gels in a collapsed conformation (above the LCST) exhibit significantly more friction than swollen gels (below the LCST) at low shear rates. These differences arise from changes in the surface roughness, adhesive interactions, and chain entanglements of the gel surfaces associated with the phase transition. Importantly, we show that the changes in friction, triggered by an external stimulus, are reversible. These reversible and possibly tunable changes in friction may have a significant impact on the design of coatings for biosensors and for actuation devices based on stimulus-responsive hydrogels.