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The creep of polycrystalline ice in uniaxial compression has been studied over a very wide range of strain rates. The creep is far too slow to explain the friction of ice observed at very low sliding speeds, but experiments with single crystals show that sliding produces recrystallization at the interface, the ice presenting an orientation favourable to easy glide in directions tangential to the interface.
The Kuiper belt is a disk of icy bodies that orbit the Sun beyond Neptune; the largest known members are Pluto and its companion Charon. A few per cent of Kuiper-belt bodies have recently been found to be binaries with wide separations and mass ratios of the order of unity. Collisions were too infrequent to account for the observed number of binaries, implying that these binaries formed through collisionless interactions mediated by gravity. These interactions are likely to have been most effective during the period of runaway accretion, early in the Solar System's history. Here we show that a transient binary forms when two large bodies penetrate one another's Hill sphere (the region where their mutual forces are larger than the tidal force of the Sun). The loss of energy needed to stabilize the binary orbit can then occur either through dynamical friction from surrounding small bodies, or through the gravitational scattering of a third large body. Our estimates slightly favour the former mechanism. We predict that five per cent of Kuiper-belt objects are binaries with apparent separations greater than 0.2 arcsec, and that most are in tighter binaries or systems of higher multiplicity.
For years, engineers have used elastic and plastic models to describe the properties of granular solids, such as sand piles and grains in silos. However, there are theoretical and experimental results that challenge this approach. Specifically, it has been claimed that stress in granular solids propagates in a manner described by wave-like (hyperbolic) equations, rather than the elliptic equations of static elasticity. Here we report numerical simulations of the response of a two-dimensional granular slab to an external load, revealing that both approaches are valid--albeit on different length scales. For small systems that can be considered mesoscopic on the scale of the grains, a hyperbolic-like, strongly anisotropic response is expected. However, in large systems (those typically considered by engineers), the response is closer to that predicted by traditional isotropic elasticity models. Static friction, often ignored in simple models, plays a key role: it increases the elastic range and renders the response more isotropic, even beyond this range.
The present study was designed to investigate the nature of friction between orthodontic wire and various ceramic brackets. The amount of tooth movement with metal and ceramic brackets was measured, and the wire surfaces were examined microscopically immediately after artificial tooth movement. The amount of tooth movement produced by the ceramic brackets was significantly less than that produced by metal bracket. The wire surfaces were scratched more obviously by ceramic brackets than by metal bracket. Slot surfaces and edges of the ceramic brackets were substantially more porous and rougher than those surfaces of the metal bracket. These material differences between metal and ceramic brackets significantly affect the efficiency of orthodontic tooth movement.
This is a retrospective survivorship analysis review of 5089 Charnley low-friction arthroplasties performed as a primary procedure at 8 hospitals around the world before December 31, 1980. Data collected were simple with a well-defined endpoint: death or revision. There was no clinical or radiographic evaluation. The mean age at operation was 63 years; 57% of hips were in women, and 43% were in men. The preoperative diagnosis was primary osteoarthrosis in 74% of hips. Kaplan-Meier survivorship analysis using failure as revision of any component for any cause showed probability of survival at 20 years of 83% (women, 86%, and men, 78%). In patients aged 70 to 80 at surgery, probability of survival was 92%; in patients aged < or =40, probability of survival was 67%. This study showed almost identical durability of the femoral versus acetabular component-87%.
We use Langevin molecular dynamics (MD) simulations to improve the picture of the processes that contribute to QHAS broadening, as a function of momentum transfer at the crystal. We use a simulation of realistic damped vibrational motion in a harmonic well to establish the contributions to QHAS measurements due to both vibrational motion and intracell diffusion (usually referred to as vibrational dephasing). We demonstrate the experimental conditions under which such contributions are likely to be important. These results are compared with those from a simulation of thermal diffusion over a sinusoidally corrugated potential. We show that resolution and atom-surface "friction" play an important role in determining the form of QHAS measurements and we demonstrate that there is no simple relationship between the "activation energy" derived from an Arrhenius plot of QHAS data and the adiabatic potential barrier height. MD simulations are therefore necessary to perform a complete analysis of the data. Finally, we discuss the application of our results to more sophisticated potentials, particularly those with multiple adsorption sites.
The friction of rocks in the laboratory is a function of time, velocity of sliding, and displacement. Although the processes responsible for these dependencies are unknown, constitutive equations have been developed that do a reasonable job of describing the laboratory behavior. These constitutive laws have been used to create a model of earthquakes at Parkfield, CA, by using boundary conditions appropriate for the section of the fault that slips in magnitude 6 earthquakes every 20-30 years. The behavior of this model prior to the earthquakes is investigated to determine whether or not the model earthquakes could be predicted in the real world by using realistic instruments and instrument locations. Premonitory slip does occur in the model, but it is relatively restricted in time and space and detecting it from the surface may be difficult. The magnitude of the strain rate at the earth's surface due to this accelerating slip seems lower than the detectability limit of instruments in the presence of earth noise. Although not specifically modeled, microseismicity related to the accelerating creep and to creep events in the model should be detectable. In fact the logarithm of the moment rate on the hypocentral cell of the fault due to slip increases linearly with minus the logarithm of the time to the earthquake. This could conceivably be used to determine when the earthquake was going to occur. An unresolved question is whether this pattern of accelerating slip could be recognized from the microseismicity, given the discrete nature of seismic events. Nevertheless, the model results suggest that the most likely solution to earthquake prediction is to look for a pattern of acceleration in microseismicity and thereby identify the microearthquakes as foreshocks.
Small, single-domain proteins typically fold via a compact transition-state ensemble in a process well fitted by a simple, two-state model. To characterize the rate-limiting conformational changes that underlie two-state folding, we have investigated experimentally the effects of changing solvent viscosity on the refolding of the IgG binding domain of protein L. In conjunction with numerical simulations, our results indicate that the rate-limiting conformational changes of the folding of this domain are strongly coupled to solvent viscosity and lack any significant "internal friction" arising from intrachain collisions. When compared with the previously determined solvent viscosity dependencies of other, more restricted conformational changes, our results suggest that the rate-limiting folding transition involves conformational fluctuations that displace considerable amounts of solvent. Reconciling evidence that the folding transition state ensemble is comprised of highly collapsed species with these and similar, previously reported results should provide a significant constraint for theoretical models of the folding process.
Assessments of maximal intensity exercise which determine peak power output on friction-baked cycle ergometers have fallen into two categories: correction procedures which account for changes in momentum of the ergometer's flywheel and optimization procedures which attempt to satisfy muscle force-velocity relationships. The aim of this study was to compare performance in each procedure and so investigate assumptions which underpin the tests. Nineteen males aged 20.9 +/- 0.4 years and 18 females aged 22.2 +/- 0.7 years (mean +/- S.E.M.), who were fully accustomed to the procedures, participated in a single experimental protocol. After a 5 min warm-up, the subjects performed four bouts of all-out exercise on a Monark 814E cycle ergometer against randomly assigned loads. The loads were selected to produce peak pedalling rates in the range 100-200 rev min-1 and each bout lasted 10 s. From the inverse linear relationship between applied load and peak pedalling rate, optimized peak power output (PPopt) and the accompanying pedalling rate (RPMopt) were calculated. One of the bouts used a loading equivalent to 7.5% of body weight and for this bout corrected peak power output (PPcorr) and its corresponding pedalling rate (RPMcorr) were calculated. The PPopt was less than PPcorr in the males (915 +/- 35 vs 1005 +/- 32 W) and females (673 +/- 33 vs 777 +/- 39 W) (both P < 0.001). Similarly, RPMopt was less than RPMcorr (111 +/- 1 vs 128 +/- 2 rev min-1 and 101 +/- 1 vs 111 +/- 2 rev min-1 in the males and females, respectively; P < 0.001). The results demonstrate that optimization and correction procedures produce different values of performance. These differences are probably attributable to the mechanical principles which underpin the tests.
There has been considerable interest during the past 30 years regarding the potential for exposure to asbestos fibers generated during asbestos-containing automobile and light-duty truck brake replacement and repair operations. Less attention has been directed to asbestos fiber exposures resulting from servicing of brakes on heavy-duty trucks and other machinery. Potential for the generation of respirable fibers as a result of using a rotary bench grinder and the drilling of asbestos-containing friction pads was investigated to assess the characteristics of the fibers generated from these procedures. The data from the grinding process revealed that this material was either resin matrix or fibers bound to the resin matrix. Fibers generated from the drilling procedure revealed morphology consistent with chrysotile asbestos. In each case, nonfibrous materials remained bound to the fibers released from these processes. These fibers had resin deposits attached. This investigation offers evidence that drilling and grinding of brake pads generates fibers that, for the most part, remain bound to the underlying matrix, suggesting that grinding and drilling may not pose an important source of exposure to respirable asbestos fibers.
The phenomenological permeation coefficients were determined for two artificial cellulose membranes of known thickness and water content. Transport of non-electrolytes was studied with tagged water, urea, glucose, and sucrose. The effect of the unstirred layers on the experimental determination of the coefficients is discussed. Frictional coefficients between membrane matrix and solutes, and between solvent and solutes in the membrane are calculated. It was shown that the geometrical tortuosity does not correspond to a physical tortuosity.
A small-scale study was carried out on friction and reinforcing materials containing Kevlar and glass fibres. Kevlar, an aramid fibre, and synthetic silicate fibres are being increasingly used in brake pads and shoes for cars and trucks. The fibre levels given out under abrasion in the laboratory were low. As a comparison, a brake pad containing asbestos was also tested and gave high levels of airborne fibres.
An early pericardial friction rub was noted in 23.4% of a population of 1264 consecutive patients admitted with acute myocardial infarction. The incidence of the rub did not vary with age, sex or past cardiac history. The pericardial rub, however, was more often a complication of Q- than non-Q-wave infarcts (25.5% vs 10.5%, P greater than 0.001) and of anterior than inferior infarcts (35.3% vs 20.8%, P greater than 0.001). In comparing the 297 patients with a pericardial rub to the 967 others, we noted that the former group had a higher CK peak (1706 +/- 1110 UI l-1 vs 1189 +/- 1038 UI l-1, P greater than 0.001) and a higher incidence of Killip class greater than 1 (47.5% vs 33.2%, P greater than 0.001), atrial flutter or fibrillation (22.2% vs 9.3%, P greater than 0.001), second or third degree atrioventricular blocks (16.8% vs 9.4%, P greater than 0.001) and complete bundle branch block (14.5% vs 7.1%, P greater than 0.001). In spite of this, the development of a pericardial rub did not increase the in-hospital mortality (10.8% in patients with pericardial rub; 11.3% in those without).
A patient is presented who manifested two previously undescribed complications of chlorpromazine therapy: a transient pericardial friction rub and "pseudo-giant P waves". After slowing of the heart rate, the "giant P-waves" proved to be superimposed U and P-waves. His history demonstrates the protean and at times unusual complications of chlorpromazine therapy and illustrates the dramatic therapeutic effects diphenhydramine may have in reversing some of this toxicity.
We present numerical and experimental evidence which demonstrates that under certain conditions friction can be reduced by spatial disorder and/or thermal noise. We discuss possible mechanisms for this behavior.
We propose a simple phenomenological model to describe the motion of a plate on granular layers pushed at a constant speed. The model contains the order parameter characterizing the transition from a solidlike state to a liquidlike state. The model reproduces the hysteresis in frictional force and the universal profile of the slip velocity, which are observed in experiment. Our model predicts that the area of hysteresis loop depends on the pushing speed.
We present a systematic investigation of the distribution of normal forces at the boundaries of static packings of spheres. A method for the efficient construction of large hexagonal-close-packed crystals is introduced and used to study the effect of spatial ordering on the distribution of forces. Under uniaxial compression we find that the form for the probability distribution of normal forces between particles does not depend strongly on crystallinity or interparticle friction. In all cases the distribution decays exponentially at large forces and shows a plateau or possibly a small peak near the average force but does not tend to zero at small forces.
The sliding friction of hard, micron-sized single asperities sliding on soft polyester films was studied. Transitions from steady sliding to so-called "stick-slip" or nonstationary motion occur for decreasing driving speed, decreasing driving spring stiffness, increasing normal load, decreasing tip radius, and decreasing crosslink density. Normal displacements of the tip during sliding were studied in some detail. It is argued these play an important role in the dynamics of the system, being the dominant factor in determining the contact area between asperity and substrate. A rather simple model is proposed that is related to rate-and-state descriptions of stick-slip phenomena. In this particular description the normal displacement plays a part analogous to that of the state parameter. In a limited comparison of experiment and numerical results we find qualitative agreement on all measured trends.