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PubMed2006

Theory of the effects of multiscale surface roughness and stiffness on static friction.

It is shown on the basis of simple scaling arguments that an interface between two three-dimensional elastic solids, consisting of completely flat disordered surfaces, which interact with interatomic hard core interactions, will be in a weak pinning regime, and hence exhibit negligibly small static friction. It is argued, however, that the presence of roughness on multiple length scales can lead to much larger friction (i.e., static friction coefficients not too much smaller than 1), as is characteristic of most solid surfaces. This approach suggests a possible way of understanding why coatings of materials with high elastic constants are often excellent lubricants.

Journal Article↗
PubMed2000

Anomalous frequency dependence of the internal friction of vitreous silica

The internal friction Q-1 and the sound velocity deltav/v of vitreous silica were measured at very low temperatures using mechanical double paddle resonators operated at frequencies ranging from 0.33 to 14 kHz. Below approximately 40 mK the internal friction showed an unexpected temperature and frequency dependence, with absolute values of Q-1 clearly exceeding those predicted by the standard tunneling model. Even though the most plausible origin of the observed excess internal friction appears to be the mutual interaction between tunneling states, the results are difficult to reconcile quantitatively with present theories taking into account this interaction.

Journal Article↗
PubMed2003

Creeping friction dynamics and molecular dissipation mechanisms in glassy polymers.

The dissipation mechanism of nanoscale kinetic friction between an atomic force microscopy tip and a surface of amorphous glassy polystyrene has been studied as a function of two parameters: the scanning velocity and the temperature. Superposition of the friction results using the method of reduced variables revealed the dissipative behavior as an activated relaxation process with a potential barrier height of 7.0 kcal/mol, corresponding to the hindered rotation of phenyl groups around the C-C bond with the backbone. The velocity relationship with friction F(v) was found to satisfy simple fluctuation surface potential models with F proportional to const-ln(v) and F proportional to const-ln(v)2/3.

Journal Article↗
PubMed2004

Friction through dynamical formation and rupture of molecular bonds.

We introduce a model for friction in a system of two rigid plates connected by bonds (springs) and experiencing an external drive. The macroscopic frictional properties of the system are shown to be directly related to the rupture and formation dynamics of the microscopic bonds. Different regimes of motion are characterized by different rates of rupture and formation relative to the driving velocity. In particular, the stick-slip regime is shown to correspond to a cooperative rupture of the bonds. Moreover, the notion of static friction is shown to be dependent on the experimental conditions and time scales. The overall behavior can be described in terms of two Deborah numbers.

Journal Article↗
PubMed2005

Impact of substrate corrugation on the sliding friction levels of adsorbed films.

We report a quartz crystal microbalance (QCM) study of sliding friction for solid xenon monolayers at 77 K on Cu(111), Ni(111), graphene/Ni(111), and C(60) substrates. Simulations have predicted a strong dependence of phononic friction coefficient (eta) on surface corrugation in systems with similar lattice spacing, eta approximately U(2)(0), but this has never before been shown experimentally. In order to make direct comparisons with theory, substrates with similar lattice spacing but varying amplitudes of surface corrugation were studied. QCM data reveal friction levels proportional to U(2)(0), validating current theoretical and numerical predictions. Measurements of Xe/C(60) are also included for comparison purposes.

Journal Article↗
PubMed2005

Simple model of microscopic rolling friction.

Rolling friction at a microscopic scale is studied with the help of a simple two-dimensional model. Molecular dynamics simulations show that rolling of spherical lubricant molecules exists only for concentrations lower than the concentration of a close-packed layer. At concentrations higher than a critical one due to jamming of lubricant molecules the rolling of nearest neighboring molecules is hindered. An optimal concentration exists which provides the minimum of kinetic friction. Methods for avoiding jamming and increasing the range of operation of rolling mechanism of friction are discussed.

Journal Article↗
PubMed2006

Granular species segregation under vertical tapping: effects of size, density, friction, and shaking amplitude.

We present extensive molecular dynamics simulations on species segregation in a granular mixture subject to vertical taps. We discuss how grain properties, e.g., size, density, friction, as well as shaking properties, e.g., amplitude and frequency, affect such a phenomenon. Both the Brazil nut effect (larger particles on the top, BN) and the reverse Brazil nut effect (larger particles on the bottom, RBN) are found and we derive the system comprehensive "segregation diagram" and the BN to RBN crossover line. We also discuss the role of friction and show that particles which differ only for their frictional properties segregate in states depending on the tapping acceleration and frequency.

Journal Article↗
PubMed2006

High friction from a stiff polymer using microfiber arrays.

High dry friction requires intimate contact between two surfaces and is generally obtained using soft materials with an elastic modulus less than 10 MPa. We demonstrate that high-friction properties similar to rubberlike materials can also be obtained using microfiber arrays constructed from a stiff thermoplastic (polypropylene, 1 GPa). The fiber arrays have a smaller true area of contact than a rubberlike material, but polypropylene's higher interfacial shear strength provides an effective friction coefficient of greater than 5 at normal loads of 8 kPa. At the pressures tested, the fiber arrays showed more than an order of magnitude increase in shear resistance compared to the bulk material. Unlike softer materials, vertical fiber arrays of stiff polymer demonstrate no measurable adhesion on smooth surfaces due to high tensile stiffness.

Journal Article↗
PubMed1991

A new device for measuring oral mucosal surface friction--reference values.

Sialometry and dryness measurements performed with a sliding friction instrument were carried out in healthy subjects and patients with oral dryness due to salivary gland diseases. The salivary flow rates were within normal reference values. The mean friction value in the healthy population was 1.73 +/- 0.37 in the lip mucosa and 1.25 +/- 0.31 in the buccal mucosa and did not show any age or sex differences. In the group of patients with salivary gland diseases, none produced measurable amounts of resting saliva but a few of them did produce small amounts of paraffin wax chewing-stimulated saliva. The friction instrument proved to register lubrication produced even by these very small amounts of saliva.

Adult↗
PubMed2006

Electronic control of friction in silicon pn junctions.

A remarkable dependence of the friction force on carrier concentration was found on doped silicon substrates. The sample was a nearly intrinsic n-type Si(100) wafer patterned with 2-micrometer-wide stripes of highly B-doped p-type material. The counter surface was the tip of an atomic force microscope coated with conductive titanium nitride. The local carrier concentration was controlled through application of forward or reverse bias voltages between the tip and the sample in the p and the n regions. Charge depletion or accumulation resulted in substantial differences in friction force. The results demonstrate the capability to electronically control friction in semiconductor devices, with potential applications in nanoscale machines containing moving parts.

Journal Article↗
PubMed1983

Frictional forces and ankle fractures in sport.

The frictional forces between the sports footwear and the ground can induce distorsion trauma to the lower extremity. Two cases are reported in which fractures about the ankle are related to this friction. An adjustment and reduction of the frictional forces are recommendable.

Adolescent↗
PubMed2006

Non-Amontons behavior of friction in single contacts.

We report on the frictional properties of a single contact between a glassy polymer lens and a flat silica substrate covered either by a disordered or by a self-assembled alkylsilane monolayer. We find that, in contrast to a widely spread belief, the Amontons proportionality between frictional and normal stresses does not hold. Besides, we observe that the velocity dependence of the sliding stress is strongly sensitive to the structure of the silane layer. Analysis of the frictional rheology observed on both disordered and self-assembled monolayers suggests that dissipation is controlled by the plasticity of a glass-like interfacial layer in the former case, and by pinning of polymer chains on the substrate in the latter one.

Journal Article↗
PubMed2003

Friction and rotation modes in a packing of cylinders under shear stress.

We study the influence of collective processes such as rotations on the effective friction of sheared granular media made of two-dimensional array of cylinders. An original experimental device allows us to measure simultaneously grain rotations and the global friction force between the packing and the basal plane. It is shown that the correlation between these two quantities can be analyzed at two different time scales: 1. Averaging over the duration of a whole experiment, the mean sliding behavior of the first row on the base of the packing describes satisfactorily the global friction force. 2. At short-time, description of this correlation requires the knowledge of the propagation of rotation perpendicularly to the shear direction.

Journal Article↗
PubMed2004

Molecular-dynamics simulations with explicit hydrodynamics I: on the friction coefficients of deformed polymers.

We implement large-scale Molecular-Dynamics (MD) simulations which incorporate hydrodynamic interactions via the inclusion of explicit Lennard-Jones solvent to examine the behaviour of polymer chains in sieving media. We begin by examining the friction coefficients of polymers in long-lived states responsible for inducing length-dependent mobility, i.e., allowing separation of polymers (or polyelectrolytes) by molecular weight. In particular, the conformations we examine occur in devices which utilize arrays of molecular obstacles or dilute solutions of polymers. We compare the results from our MD simulations with expressions from macroscopic hydrodynamics for four specific cases: i) a random coil excluded-volume Zimm polymer, ii) a rigid polymer moving perpendicular to its major axis iii) a rigid polymer moving parallel to its major axis and iv) a rigid polymer, folded at different points along its contour. We also examine the behaviour of the friction coefficient of a fully flexible molecule pulled by its middle monomer as a function of an applied force F and show that there are several distinct frictional regimes.

Journal Article↗
PubMed1995

Skin surface lipid and skin friction: relation to age, sex and anatomical region.

Differences in the skin surface lipid content (SSL) and the dynamic friction coefficient (mu) were investigated with respect to age, sex, and anatomical region in 29 volunteers. The group consisted of (a) 7 young adult females (24.9 +/- 1.1 years old, mean +/- SD), (b) 7 old females (75.3 +/- 2.4 years old), (c) 7 young adult males (28.7 +/- 0.5 years old), and (d) 8 old males (73.8 +/- 1.9 years old). Measurements were obtained on 11 anatomical regions, namely, the forehead, upper arm, volar and dorsal forearm, postauricular, palm, abdomen, upper and lower back, thigh, and ankle. Skin surface lipid content data were compared with mu measurements to determine the relative contribution of the former to frictional properties of skin. mu and SSL were not statistically different between age groups on all regions except for the ankle, where lipid content was lower in the elderly. Similarly, mu did not vary between sex groups. Skin surface lipid content was statistically lower on the forehead, dorsal forearm and postauricular area in females. Both parameters, however, showed considerable regional variability. A significant linear correlation was established between mu and SSL combining all regions from all volunteers. When mu was plotted against SSL among individual anatomic sites, only the forehead and postauricular area showed significant linear correlations between the two parameters. These data suggest that surface lipid content plays a limited role in frictional properties of skin.

Adult↗
PubMed2004

Comparison of effects of exercise programme, pulsed ultrasound and transverse friction in the treatment of chronic patellar tendinopathy.

OBJECTIVE: To compare the effectiveness of an exercise programme, pulsed ultrasound and transverse friction in the treatment of chronic patellar tendinopathy. DESIGN: Randomized controlled trial. SETTING: Rheumatology and Rehabilitation Centre. SUBJECTS: This study was carried out with 30 patients who had chronic patellar tendinopathy. They were randomized into three groups. INTERVENTIONS: Group A (n = 10) was treated with exercise programme. Pulsed ultrasound was given to group B (n = 10). Group C (n = 10) received transverse friction. All patients received three treatments per week for four weeks. OUTCOMES: Patients' pain was evaluated at the end of the four-week course of treatment (week 4), one month (week 8) and three months (week 16) after the end of treatment. RESULTS: The exercise programme was statistically significantly better than the other two treatments at the end of treatment (chi2 = 12.21, p < 0.01), one month (chi2 = 23.2, p < 0.001) and three months (chi2 = 23.2, p < 0.001) after the end of the treatment. CONCLUSION: Although the results suggested that the exercise programme was more effective treatment than ultrasound and transverse friction at the end of the treatment as well as at the follow-ups, future controlled studies are needed to establish the relative and absolute effectiveness of each of the three treatment interventions.

Adult↗
PubMed1995

Friction blisters. Pathophysiology, prevention and treatment.

Blisters occur frequently, especially in vigorously active populations. Studies using respective rubbing techniques show that blisters result from frictional forces that mechanically separate epidermal cells at level of the stratum spinosum. Hydrostatic pressure causes the area of the separation to fill with a fluid that is similar in composition to plasma but has a lower protein level. About 6 hours after formation of the blister, cells in the blister base begin to take amino acids and nucleosides; at 24 hours, there is high mitotic activity in the basal cells; at 48 and 120 hours, new stratum granulosum and stratum corneum, respectively, can be seen. The magnitude of frictional forces (Ff) and the number of times that an object cycles across the skin determine the probability of blister development - the higher the Ff, the fewer the cycles necessary to produce a blister. Moist skin increases Ff, but very dry or very wet skin necessary to produce a blister. Moist skin increases Ff, but very dry or very wet skin decreases Ff. Blisters are more likely in skin areas that have a thick horny layer held tightly to underlying structures (e.g. palms of the hands or soles of the feet). More vigorous activity and the carrying of heavy loads during locomotion both appear to increase the likelihood of foot blisters. Antiperspirants with emollients and drying powders applied to the foot do not appear to decrease the probability of friction blisters. There is some evidence that foot blister incidence can be reduced by closed cell neoprene insoles. Wearing foot socks composed of acrylic results in fewer foot blisters in runners. A thin polyester sock, combined with a thick wool or polypropylene sock that maintains its bulk when exposed to sweat and compression reduces blister incidence in Marine recruits. Recent exposure of the skin to repeated low intensity Ff results in a number of adaptations including cellular proliferation and epidermal thickening, which may reduce the likelihood of blisters. More well-designed studies are necessary to determine which prevention strategies actually decrease blister probability. Clinical experience suggests draining intact blisters and maintaining the blister roof results in the least patient discomfort and may reduce the possibility of secondary infection. Treating deroofed blisters with hydrocolloid dressings provides pain relief and may allow patients to continue physical activity if necessary. There is no evidence that antibiotics influence blister healing. Clinical trials are needed to determine the efficacy of various blister treatment methods.(ABSTRACT TRUNCATED AT 250 WORDS)

Blister↗
PubMed1991

The effect of a chewing gum on salivary secretion, oral mucosal friction, and the feeling of dry mouth in xerostomic patients.

A new chewing gum (PTC) with the ability to release flavoring substances for a rather long time was tested for its ability to stimulate saliva secretion. The chewing gum is mildly flavored and contains non-cariogenic sweeteners (xylitol and sorbitol). Measurements of saliva secretion rate and oral mucosal sliding friction and subjective evaluations on visual analog scales were made in relation to chewing and compared with those when chewing a commercially available gum (V6). The study was a randomized, crossover comparison with blind evaluation. Both chewing gums stimulated saliva secretion and decreased oral mucosal friction. The PTC gum gave consistently higher mean values of saliva secretion rate and lower oral mucosal friction values than V6. The difference was statistically significant for the saliva secretion rate and most pronounced after 5-10 min of chewing. There was also a statistically significant difference between the subjective evaluations of V6 and PTC with regard to saliva-stimulating ability and taste in favor of the PTC gum.

Aged↗