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At least 595 records · Page 33Linked to original sources
PubMed2001

Static friction between elastic solids due to random asperities.

Several workers have established that the Larkin domains for two three-dimensional nonmetallic elastic solids in contact with each other at a disordered but atomically flat interface are enormously large, implying that there should be negligible static friction per unit area in the macroscopic solid limit. In contrast, the present Letter argues that when the Larkin domains are calculated for disorder on the multiasperity scale, they are much smaller than the interface size. This can account for the virtual universal occurrence of static friction.

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PubMed2001

Dynamical turbulent flow on the Galton board with friction.

We study numerically and analytically the dynamics of charged particles on the Galton board, a regular lattice of disk scatters, in the presence of constant external force, magnetic field, and friction. It is shown that under certain conditions friction leads to the appearance of a strange chaotic attractor. In this regime the average velocity and direction of particle flow can be effectively affected by electric and magnetic fields. We discuss the applications of these results to the charge transport in antidot superlattices and the stream of suspended particles in a viscous flow through scatters.

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PubMed2002

Kinetics of capillary condensation in nanoscopic sliding friction.

The velocity and humidity dependence of nanoscopic sliding friction has been studied on CrN and diamondlike carbon surfaces with an atomic force microscope. The surface wettability is found to be decisive. Partially hydrophilic surfaces show a logarithmic decrease of friction with increasing velocity, the slope of which varies drastically with humidity, whereas on partially hydrophobic surfaces we confirm the formerly reported logarithmic increase. A model for the thermally activated nucleation of water bridges between tip and sample asperities fully reproduces the experimental data.

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PubMed2002

Effect of internal friction on biofilament dynamics.

We consider biofilaments-flexible multimolecule structures common in cell biology-and show how "internal" friction associated with either conformational fluctuations or with fluid flow through narrow pores inside the filaments can dominate the external hydrodynamic friction usually considered to be the main energy dissipation process. The signature of this is wave-number-independent relaxation time of bending fluctuations. Preliminary experimental data for bending fluctuations of single folded (mitotic) chromosomes display these dynamics.

Chromosomes↗
PubMed2002

Contrasting friction and diffusion in molecularly thin confined films.

We study, using fluorescence correlation spectroscopy, translational diffusion in molecularly thin liquids confined within a surface forces apparatus. The diffusion coefficient decreases exponentially from the edges towards the center of the Hertzian contact and further suggests the presence of a small number of distinct diffusion processes. This holds alike a crystallizable fluid (OMCTS) and a glass-former (1,2-propane diol), both of which displayed static friction. We conclude that friction, the average of an ensemble of molecules, masked massively heterogeneous molecular mobility.

Journal Article↗
PubMed2002

Nature of mechanical instabilities and their effect on kinetic friction.

It has long been recognized that kinetic friction F(k) between two solids must be due to instabilities, sudden "pops" of certain degrees of freedom. Here, such pops are studied with a focus on boundary lubrication. The pops' characteristics and consequently the friction-velocity relationship depend qualitatively on dimensionality, commensurability, and details of the lubricant wall interaction. It is found that F(k) should be small between commensurate surfaces. F(k) is large for incommensurate surfaces, unless the lubricant's motion is confined to 1D. The effects of thermal noise are discussed and computer simulations are employed to show the relevance of the predictions to less idealized models.

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PubMed2003

Dry friction due to adsorbed molecules.

Using an adiabatic approximation method, which searches for Tomlinson-model-like instabilities for a simple but still realistic model for two crystalline surfaces, with mobile molecules present at the interface, sliding relative to each other, we are able to account for the virtually universal occurrence of "dry friction." The model makes important predictions for the dependence of friction on the strength of the interaction of each surface with the mobile molecules.

Journal Article↗
PubMed2003

Frictional coupling between sliding and spinning motion.

The tangential motion at the contact of two solid objects is studied. It consists of a sliding and a spinning degree of freedom (no rolling). We show that the friction force and torque are inherently coupled. As a simple test system, a sliding and spinning disk on a horizontal flat surface is considered. We calculate, and also measure, how the disk slows down and find that it always stops its sliding and spinning motion at the same moment. We discuss the impact of this coupling between friction force and torque on the physics of granular materials.

Journal Article↗
PubMed2003

Interaction potential and hopping dynamics governing sliding friction.

The friction force on a nanometer-sized tip sliding on a surface is related to the thermally activated hopping of the contact atoms on an effective atomic interaction potential. A general analytical expression relates the height of this potential and the hopping attempt frequency to measurements of the velocity dependence of the friction force performed with an atomic force microscope. While the height of the potential is roughly proportional to the normal load, the attempt frequency falls in the range of mechanical eigenfrequencies of the probing tip in contact with the surface.

Journal Article↗
PubMed2003

Sliding friction with polymer brushes.

Using high-resolution shear force measurements, we examine in detail the frictional drag between rubbing surfaces bearing end-tethered polymeric surfactants (brushes). The drag attains a maximum on initial motion, attributed to elastic stretching of the chains, which falls by a cascade of relaxations to a value characteristic of kinetic friction. This has a very weak velocity dependence, attributed to chain moieties dragging within a self-regulating, mutual interpenetration zone. When sliding stops, the shear stress across the polymer layers decays logarithmically with time, consistent with the relaxation of a network of dangling ends.

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PubMed2004

Nonmonotonic velocity dependence of atomic friction.

We propose a theoretical model for friction force microscopy experiments with special emphasis on the realistic description of dissipation and inertia effects. Its main prediction is a nonmonotonic dependence of the friction force upon the sliding velocity of the atomic force microscope tip relative to an atomically flat surface. The region around the force maximum can be approximately described by a universal scaling law and should be observable under experimentally realistic conditions.

Journal Article↗
PubMed2005

Quantum tunneling at zero temperature in the strong friction regime.

In the large damping limit we derive a Fokker-Planck equation in configuration space (the so-called Smoluchowski equation) describing a Brownian particle immersed into a thermal environment and subjected to a nonlinear external force. We quantize this stochastic system and survey the problem of escape over a double-well potential barrier. Our finding is that the quantum Kramers rate does not depend on the friction coefficient at low temperatures; i.e., we predict a superfluidity phenomenon in overdamped open systems. Moreover, at zero temperature we show that the quantum escape rate does not vanish in the strong friction regime. This result, therefore, is in contrast with the work by Ankerhold et al. [Phys. Rev. Lett. 87, 086802 (2001)]] in which no quantum tunneling is predicted at zero temperature.

Journal Article↗
PubMed2005

Internal friction and vulnerability of mixed alkali glasses.

Based on a hopping model we show how the mixed alkali effect in glasses can be understood if only a small fraction c(V) of the available sites for the mobile ions is vacant. In particular, we reproduce the peculiar behavior of the internal friction and the steep fall ("vulnerability") of the mobility of the majority ion upon small replacements by the minority ion. The single and mixed alkali internal friction peaks are caused by ion-vacancy and ion-ion exchange processes. If c(V) is small, they can become comparable in height even at small mixing ratios. The large vulnerability is explained by a trapping of vacancies induced by the minority ions. Reasonable choices of model parameters yield typical behaviors found in experiments.

Journal Article↗
PubMed2006

Capillary condensation in atomic scale friction: how water acts like a glue.

We present atomic-scale friction force measurements that strongly suggest that the capillary condensation of water between a tungsten tip and a graphite surface leads to the formation of ice at room temperature. This phenomenon increases the friction force, introduces a short-term memory in the form of an elastic response against shearing, and allows us to "write" a temporary line of ice on a hydrophobic surface. Rearrangements of the condensate are shown to take place on a surprisingly slow time scale of seconds.

Journal Article↗
PubMed2006

Velocity dependence of friction and hydrogen bonding effects.

We show that the friction force varies with the sliding velocity in a manner that depends on the chemical nature of the interface. Surfaces terminated with the hydrogen acceptor and donor moieties capable of forming H-bond networks exhibit a friction that decreases with sliding velocity, a behavior that is opposite to that of surfaces where no such networks can form. We explain the results with a model where the domains of glassy H-bond networks are disrupted at a critical applied stress leading to slippage.

Journal Article↗
PubMed2006

Rotational dynamics and friction in double-walled carbon nanotubes.

We report a study of the rotational dynamics in double-walled nanotubes using molecular dynamics simulations and a simple analytical model that reproduces the observations very well. We show that the dynamic friction is linear in the angular velocity for a wide range of values. The molecular dynamics simulations show that for large enough systems the relaxation time takes a constant value depending only on the interlayer spacing and temperature. Moreover, the friction force increases linearly with contact area and the relaxation time decreases with the temperature with a power law of exponent -1.53+/-0.04.

Journal Article↗
PubMed2006

Nanoscale origins of dynamic friction in an asymmetric contact geometry.

Friction, adhesion, and bounce dynamics are found to be asymmetric with respect to sliding direction for a tilted pad sliding at high speed. The differences originate from the nanoscale effects of the mechanical action against the lubricant layer forming an asymmetric meniscus around the pad and from a self-excited vibration induced by friction hysteresis coupled with adhesion hysteresis.

Journal Article↗
PubMed1975

Surface adaptations of the vertebrate epidermis to friction.

Epidermal surfaces in representative vertebrates specialized for lowered or increased friction were studied with the scanning electron microscope. Microvillous and microridged patterns predominate in aquatic vertebrates. In squamate reptiles, the complex and varied ornamentation of the Oberhäutchen functions both in adhesive modifications and in modulating surface reflectivity. Frictional surfaces in birds and mammals are characterized more by anatomical than by cytologic specializations, the detailing of surface cells being mostly a function of turnover rate.

Adaptation, Physiological↗