Microscopic Friction Research Articles

Nanomechanical properties of polymer composite nanoclusters

The paper contains results of quantum mechanical and molecular dynamics simulation of the structure, energy characteristics, and nanomechanical properties of nanoclusters of a “polymer matrix - carbon filler” composite medium. Quantum mechanical simulation is carried out in the framework of the cluster approach, in the approximation of microscopic friction and deformation (tension) for a model polymer - filler boundary layer. Molecular dynamics simulation is performed in NPT- and NVT-ensembles with periodic boundary conditions under normal conditions. Interaction parameters are determined according to the GROMACS force field, the potential energy of rotation about a bond is found from the quantum mechanical simulation results. The simulation findings are used to estimate strength properties of the medium. Based on the data of direct quantum mechanical calculation, we derive approximating dependences for potentials that describe interaction energy in the system “organic polymer matrix - carbon filler particles”. The molecular dynamics simulation data are used to calculate the compressibility factor.

Strong-weak network anisotropy switching and hysteresis in three-dimensional granular materials

We address hysteresis of three-dimensional polydisperse granular packs, comparing macro- and microscopic viewpoints, to reveal their elastic/inelastic mechanics and force network anisotropy. During the uniaxial loading-unloading cycle of an appropriately prepared pack, one can decompose the force network into weak and strong subnetworks. The first stages of loading exhibit arching, where all the fabric displays negative anisotropy. For later stages, the strong (weak) network shows positive (negative) anisotropy. On unloading, the force network progresses to a fabric wide hydrostatic point, where the anisotropies of the weak and strong subnetworks switch signs. During the loading stage, a Mohr circle analysis permits the identification of a well-defined macroscopic internal friction angle, whose value is larger than that of grain-grain interactions. To analyze unloading, a generalized local Coulomb-friction argument predicts a continuously changing friction angle, that vanishes at the hydrostatic point. A suggestive interplay between microscopic friction and fabric structure, at different loding stages, is proposed as the mechanism for the emergence of a macro internal friction angle.

Diffusion of Probe Molecule in Small Liquid n-Alkanes: A Molecular Dynamics Simulation Study

The probe diffusion and friction constants of methyl yellow (MY) in liquid n-alkanes of increasing chain length were calculated by equilibrium molecular dynamics (MD) simulations at temperatures of 318, 418, 518 and 618 K. Lennard-Jones particles with masses of 225 and 114 g/mol are modeled for MY. We observed that the diffusion constant of the probe molecule follows a power law dependence on the molecular weight of nalkanes, DMY ~M −γ well. As the molecular weight of n-alkanes increases, the exponent γ shows sharp transitions near n-dotriacontane (C32) for the large probe molecule (MY2) at low temperatures of 318 and 418 K. For the small probe molecule (MY1) DMY1 in C12 to C80 at all the temperatures are always larger than Dself of n-alkanes and longer chain n-alkanes offer a reduced friction relative to the shorter chain n-alkanes, but this reduction in the microscopic friction for MY1 is not large enough to cause a transition in the power law exponent in the log-log plot of DMY1 vs M of n-alkane. For the large probe molecule (MY2) at high temperatures, the situation is very similar to that for MY1. At low temperatures and at low molecular weights of n-alkanes, DMY2 are smaller than Dself of n-alkanes due to the relatively large molecular size of MY2, and MY2 experiences the full shear viscosity of the medium. As the molecular weight of n-alkane increases, Dself of n-alkanes decreases much faster than DMY2 and at the higher molecular weights of n-alkane, MY2 diffuses faster than the solvent fluctuations. Therefore there is a large reduction of friction in longer chains compared to the shorter chains, which enhances the diffusion of MY2. The calculated friction constants of MY1 and MY2 in liquid n-alkanes supported these observations. We deem that this is the origin of the so-called “solventoligomer” transition.

Simulations of the untying of molecular friction knots between individual polymer strands

The dynamics of molecular knots is implicated in a broad range of phenomena, from DNA replication to relaxation of polymer melts. Motivated by the recent experiments, in which biopolymer knots have been observed and manipulated at a single-molecule level, we have used computer simulations to study the dynamics of "friction knots" joining individual polymer strands. A friction knot splicing two ropes becomes jammed when the ropes are pulled apart. In contrast, molecular friction knots eventually become undone by thermal motion. We show that depending on the knot type and on the polymer structure, a microscopic friction knot can be strong (the time tau the knot stays tied increases with the force F applied to separate the strands) or weak (tau decreases with increasing F). The strong knot behavior is a microscopic analog of macroscopic knot jamming. We further describe a simple model explaining these behaviors.

Conformational Relaxation ofp-Phenylenevinylene Trimers in Solution Studied by Picosecond Time-Resolved Fluorescence

Two p-phenylenevinylene (PV) trimers, containing 3'-methylbutyloxyl (in MBOPV3) and 2'-ethylhexyloxyl (in EHOPV3) side chains, are used as model compounds of PV-based conjugated polymers (PPV) with the purpose of clarifying the origin of fast (picosecond time) components observed in the fluorescence decays of poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV). The fluorescence decays of MBOPV3 and EHOPV3 reveal the presence of similar fast components, which are assigned to excited-state conformational relaxation of the initial population of non-planar trimer conformers to lower-energy, more planar conformers. The rate constant of conformational relaxation k(CR) is dependent on solvent viscosity and temperature, according to the empirical relationship k(CR)=aeta(o) (-alpha)exp(-alphaE(eta)/RT), where aeta(o) (-alpha) is the frequency factor, eta(o) is the pre-exponential coefficient of viscosity, E(eta) is the activation energy of viscous flow. The empirical parameter alpha, relating the solvent microscopic friction involved in the conformational change to the macroscopic solvent friction (alpha=1), depends on the side chain. The fast component in the fluorescence decays of MEH-PPV polymers (PPVs), is assigned to resonance energy transfer from short to longer polymer segments. The present results call for revising this assignment/interpretation to account for the occurrence of conformational relaxation, concurrently with energy transfer, in PPVs.

Study of friction using driven vortices of superconductor as a model system

Dynamics of driven vortices in high-Tc superconductors was investigated in terms of the elementary process of the motion and the microscopic friction. The I-V characteristics and the transient response of driven vortices were measured in La2-xSrxCuO4 thin films and in Bi2Sr2CaCu2Oy bulk crystals. First, with an aid of a recently proposed scaling relation between the driving force and the velocity, we found that the non-Arrhenius process did exist for interacting vortices at low temperatures. With increasing magnetic field, Arrhenius process revived suddenly at the vortex-glass vs Bragg-glass transition, which shows that this approach is suitable as a new method to investigate the equilibrium phase diagram of vortices. After constructing an explicit formula connecting the motion of driven vortices to the physics of friction, the dependence of the kinetic friction force on the sliding velocity was investigated. Based on the experimental results, we propose that the non-Amontons-Coulomb behavior is regarded as the consequence of the broadened dynamic phase transition. We also found the remarkable dependence of the maximum static friction force on the waiting time at low temperatures. Such a strong dependence changed into a weak logarithmic dependence at higher temperatures. The strong time dependence suggests that a characteristic time scale to stabilize vortices exists at low temperatures. This relaxation phenomenon is very similar to the so-called boundary lubrication. These results imply that the dynamics of vortices can be used for a model not only of the dry friction but also of the lubricated friction.

Dependence of maximum static friction on waiting time using dynamics of vortices in superconductors

Vortices in high-Tc superconductors were investigated in terms of the microscopic friction. The I-V characteristics and the transient response of driven vortices were measured in La2-xSrxCuO4 thin films with different Sr concentrations and samples with columnar defects. We found the remarkable dependence of the maximum static friction force on the waiting time at low temperatures. The result suggests that the competition between the flux creep by the thermal fluctuation and the pinning yields a characteristic time scale to stabilize vortices. This relaxation phenomenon is very similar to the so-called boundary lubrication, which occurs under the existence of a thin lubricant film between interfaces. Such a strong dependence changed into a weak logarithmic dependence at higher temperatures. These results imply that the dynamics of vortices can be used for the model not only of the dry friction but also of the lubricated friction. Also, the dependence of the kinetic friction force on the sliding velocity showed the non-Amontons-Coulomb-like behavior even for the lubricated friction. Thus, we confirmed that the Amontons-Coulomb's law does not realize when the thermal fluctuation is sufficiently large.

Rotational dynamics of UVITEX-OB in alkanes, alcohols and binary mixtures

Steady-state and time resolved fluorescence anisotropy measurements were carried out to study the rotational diffusion dynamics of UVITEX-OB (U-OB) in series of alcohols, alkanes and binary mixtures of toluene and butanol at room temperature. The experimentally measured rotational reorientation times were compared with those estimated by the hydrodynamic and molecular models developed for microscopic friction. The experimental results are in good agreement with theoretical slip hydrodynamics and a deviation towards subslip behavior is noted. Also a faster rotation of the probe in binary mixture of toluene and butanol is noted as compared to that in alcohols and alkanes.

Shear strength, dilatancy, energy and dissipation in quasi-static deformation of granularmaterials

The effect of the interparticle friction coefficientμ on macroscopic, continuum characteristics, such as shear strength and dilatancyrate, of granular materials in quasi-static deformation is investigated usingtwo-dimensional discrete element simulations of biaxial tests with various values forμ. Qualitatively, the macroscopic behaviour observed in the limit cases and (when the microscopic behaviour at the contacts is elastic) is the same as in the cases with ‘normal’ valuesfor μ, showing that interparticle friction does not form the exclusive microscopic origin of themacroscopic frictional behaviour of granular materials. Based on the results of the discreteelement simulations, a stress–dilatancy relation is formulated. Furthermore, the effect ofinterparticle friction on the evolution of the energy distribution is investigated. It is shownthat the ratio between the energy in the tangential springs and the energy in the normalsprings does not change appreciably with imposed strain. The investigation of theinfluence of interparticle friction on the dissipative characteristics shows thatmacroscopic dissipation is present even in the limit cases and and , i.e. when microscopic friction is effectively absent. A dissipation function is formulatedthat describes the observed dissipative characteristics, for the various values ofμ considered, reasonably well.

Breakdown of Time−Temperature Superposition Principle and Universality of Chain Dynamics in Polymers

Detailed comparison of temperature dependences of segmental (τα) and chain (τn) relaxation times shows that decoupling of these relaxation processes occurs (the ratio τn/τα starts to drop) at a similar range of τα ∼ 10-5−10-7 s regardless of a polymer and its molecular weight. The degree of decoupling depends on polymer and is stronger in more fragile systems. The most intriguing result is a rather universal temperature dependence of τn observed for the analyzed polymers when it is plotted vs Tg/T. These observations call for a revision of current models describing the microscopic friction coefficient in polymer melts.

Influence of particle elasticity in shear testers

Two dimensional simulations of non-cohesive granular matter in a biaxial shear tester are discussed. The effect of particle elasticity on the mechanical behavior is investigated using two complementary distinct element methods (DEM): Soft particle molecular dynamics simulations (Particle Flow Code, PFC) for elastic particles and contact dynamics simulations (CD) for the limit of perfectly rigid particles. As soon as the system dilates to form shear bands, it relaxes the elastic strains so that one finds the same stresses for rigid respectively elastic particles in steady state flow. The principal stresses in steady state flow are determined. They are proportional to each other, giving rise to an effective macroscopic friction coefficient which is about 10% smaller than the microscopic friction coefficient between the grains.

Tribological Properties of Alkylsilane Self-Assembled Monolayers

In this study, we perform molecular dynamics simulations of adhesive contact and friction between alkylsilane Si(OH)(3)(CX(2))(10)CX(3) and alkoxylsilane Si(OH)(2)(CX(2))(10)CX(3) (where X = H or F) self-assembled monolayers (SAMs) on an amorphous silica substrate. The alkylsilane SAMs are primarily hydrogen-bonded or physisorbed to the surface. The alkoxylsilane SAMs are covalently bonded or chemisorbed to the surface. Previously, we studied the chemisorbed systems. In this work, we study the physisorbed systems and compare the tribological properties with the chemisorbed systems. Furthermore, we examine how water at the interface of the SAMs and substrate affects the tribological properties of the physisorbed systems. When less than a third of a monolayer is present, very little difference in the microscopic friction coefficient mu or shear stresses is observed. For increasing amounts of water, the values of mu and the shear stresses decrease; this effect is somewhat more pronounced for fluorocarbon alkylsilane SAMs than for the hydrocarbon SAMs. The observed decrease in friction is a consequence of a slip plane that occurs in the water as the amount of water is increased. We studied the frictional behavior using relative shear velocities ranging from v = 2 cm/s to 2 m/s. Similar to previously reported results for alkoxylsilane SAMs, the values of the measured stress and mu for the alkylsilane SAM systems decrease monotonically with v.

Anisotropy in cohesive, frictional granular media

The modelling of cohesive, frictional granular materials with a discrete particle moleculardynamics is reviewed. From the structure of the quasi-static granular solid, the fabric,stress, and stiffness tensors are determined, including both normal and tangential forces.The influence of the material properties on the flow behaviour is also reported, includingrelations between the microscopic attractive force and the macroscopic cohesion as well asthe dependence of the macroscopic friction on the microscopic contact friction coefficient.Related to the dynamics, the anisotropy of both structure and stress are exponentiallyapproaching the maximum.

Frictional dynamics of perfluorinated self-assembled monolayers on amorphous SiO2

Silicon micromachines in microelectromechanical systems (MEMS) are coated with self-assembled monolayers (SAMs) in order to reduce the wear and stiction that are commonplace during operation. Recently, perfluorinated SAMs have been the focus of attention because they have better processing properties than hydrocarbon SAMs. In this study, we perform molecular dynamics simulations that model adhesive contact and friction for perfluorinated alkylsilane (Si(OH)3(CF2)10CF3) self-assembled monolayers (SAMs), which are commonly used in MEMS devices. Amorphous silica is used as the substrate for the SAMs in the simulations. The frictional behavior is investigated as a function of applied pressure (50 MPa–1 GPa) for a shear velocity of 2 m/s and compared to recent simulation results of hydrocarbon alkylsilane SAMs. The microscopic friction coefficient for the perfluorinated SAMs is the same as was measured for the hydrocarbon SAMs, but the shear stress is slightly larger than in the case of the hydrocarbon SAMs on amorphous silica.

Simple model for ac ionic conduction in solids

We present a model for the ac conduction in ionically conducting solids that takes into account, in a simple way, the interaction between carriers. The Coulomb force forms an "ionic atmosphere" that exerts a restoring force on a central ion, whose motion corresponds to an overdamped oscillator. We consider the effect of the relaxation of the ionic atmosphere by introducing an additional equation for the displacement of the potential toward the particle position. The general behavior of the ac conductivity can be understood in terms of two types of motions: motion of the bound ion at high frequencies determined by microscopic friction, and a much slower motion coupled to the surrounding carriers relaxation at low frequencies.

Microtribological Studies of Different Nanocomposite TiC/a-C:H Coatings Using a Modified Nanoindentation Setup

The microtribological behavior of different nanocomposite TiC/a-C:H coatings against 100Cr6 (AISI 52100) balls with 250 μm radius has been studied using a modified nanoindentation setup and was compared to the results of macroscopic pin-on-disc (POD) experiments. First results reveal significant differences between macroscopic friction coefficients μPOD determined using POD tests and microscopic friction coefficients μmicro. On the macroscopic scale low friction coefficients can be obtained for high hardness coatings. On the microscopic scale the high hardness samples induce considerable wear on the steel counterbody leading to high microscopic friction coefficients of around 0.3. For samples with lower hardness no wear has been observed and low microscopic friction coefficients (< 0.2) can be acheieved.

Scanning Probe Microscopy Measurements of Friction

AbstractThis article describes the details of scanning probe microscopy measurements of interfacial friction from an experimental perspective. In such studies, the probe tip is taken as a model of a single asperity within a tribological contact, and interfacial forces are measured as a function of the sliding contact of the probe tip with the surface. With appropriate detection schemes, friction and load forces can be monitored simultaneously and used together to describe the frictional properties of the microscopic contact. This article provides a detailed description of the procedures and protocols of friction measurements performed with scanning probe microscopy, the relevant properties of probe tips, and the influence of environment on microscopic friction measurements. In addition, the article provides a brief overview of several categories of friction studies performed with scanning probe microscopy, highlighting the type of materials characterized in these studies as well as the importance and impact of the microscopic measurements.

Atomic scale friction of nanoscale clusters

We study both the activation energy for diffusion and dynamics of nanoscale clusters of a Frenkel–Kontrova chain subject to the microscopic friction, in connection with atomic friction phenomena. We clarify the atomistic mechanism for the presence of a magic size for diffusion and show how the magic size effects appear in the friction. When a cluster is pulled by a constant driving force, the critical force strength of depinning depends crucially on the cluster size, and it is smallest at the magic size. When a cluster is pulled by a spring at a constant velocity below the critical value, clusters show a stick-and-slip motion, and the maximum force of a spring also becomes smallest at the magic size.

Modelling study of microscopic sliding on irregular substrates

In connection with microscopic solid friction phenomena, we study the underdamped dynamics of a driven Frenkel– Kontorova chain subject to a substrate potential defined by the sum of two sinusoidal function with different periodicity. We simulate microscopic sliding over quasiperiodic and multiple-well (periodic) substrates. We comment on the nature of the particle dynamics in the vicinity of the pinning–depinning transition point and consider the role played by the coverage variable on the depinning mechanism. We also investigate on the different nonlinear excitations forming during sliding and characterizing the dynamical states observed at different strengths of the imposed driving. The dependence of the static friction on the ratio of the model interaction strengths is analyzed. � 2004 Elsevier B.V. All rights reserved.

Temperature Dependence of Polymer Diffusion in Poly(vinyl acetate-co-dibutyl maleate) Latex Films

We describe polymer diffusion and its temperature dependence in poly(vinyl acetate-co-dibutyl maleate) [P(VAc−DBM)] latex films prepared from 4:1 w/w ratio of VAc:DBM. Two sets of polymers were investigated: one set containing 50% gel (high-M); the other set, with Mw ≈ 250 000 (M250K), free of a measurable gel content. Despite their similar chemical compositions, as determined by 1H NMR, these two sets of samples exhibited different glass transition temperatures (Tg). Latex particles were labeled with 9-methacryloxymethylphenanthrene as the donor dye and 2‘-acryloxy-4‘-methyl-4-(N,N-dimethylamino)benzophenone as the acceptor. Polymer diffusion was monitored by nonradiative energy transfer (ET), and apparent diffusion coefficients (Dapp) were calculated from the ET data using a simple diffusion model. These values increased with temperature and were characterized by an apparent activation energy (Ea) of 37 ± 2 kcal/mol for the high-M polymer and 45 ± 2 kcal/mol for the M250K sample. Rheology measurements at different fixed temperatures were carried out to follow the response of the dynamic moduli (G‘, G‘‘) with respect to frequency (ω). A master curve based on the Williams−Landel−Ferry (WLF) equation could be constructed as a plot of shift factors vs 1/T, and shift factors for Dapp for both sets of polymers as well as for the G‘, G‘‘ values fell on the same curve. Thus, the difference in Ea values for the polymer diffusion can be ascribed to changes in the microscopic friction coefficient and the differences in Tg of the two sets of samples.