Logarithm Of Velocity Research Articles

Generalized master curve procedure for elastomer friction taking into account dependencies on velocity, temperature and normal force

Abstract The friction between an elastomer and a rigid substrate may depend on a large number of system and loading parameters. Based on a general structure of the law of friction, we propose a generalized master curve procedure for elastomer friction where the significant governing parameter – indentation depth (or normal force) was taken into account. Unlike the generation of the classical master curve by horizontal shifting of dependence “friction - logarithm of velocity” for different temperatures, in the case of various indentation depth the shifting in both horizontal and vertical direction is required. We experimentally investigated coefficient of friction of elastomer on sliding velocity for different indentation depths and temperatures, and generated a ‘master curve’ according to this hypothesis.

Velocity dependence of nanoscale wear in atomic force microscopy

The velocity dependence of nanoscale wear is studied over a wide range of velocities up to 100mm∕s. High sliding velocities are achieved by modifying an existing setup with a custom calibrated piezostage. Pt-coated atomic force microscope probes are scanned against uncoated diamondlike carbon-coated Si wafers and with an overcoat of a lubricant film. The wear rate increases with logarithm of velocity up to a certain value and then levels off. A wear model is developed to explain the velocity dependence of nanoscale wear based on thermally activated atomic-scale stick slip and tribochemical wear.

Boundary Lubrication and Surface Mobility of Mixed Alkylsilane Self-Assembled Monolayers

We use a combination of experiment and theory to study the boundary lubrication characteristics of mixed self-assembled monolayers (SAMs) formed by tethering alkylsilanes with different chain lengths to planar substrates. The structure of mixed SAMs is characterized using atomic force microscopy, infrared spectroscopy, and hexadecane contact angle measurements. Lateral force microscopy is used to quantify the load and velocity dependence of friction force for one-component (pure) and two-component (mixed) SAMs. We find that if two-component SAMs and one-component SAMs are created with comparable packing density, the two-component systems possess lower friction coefficients. The friction force measured on silicon surfaces grafted with mixed SAMs increases linearly with the logarithm of sliding velocity over a broad range, whereas for pure SAMs, the friction force first increases linearly with the logarithm of velocity and then reaches a plateau. This plateau is believed to arise from the viscoelasticity of...

Direct measurement of drug–enzyme interactions by atomic force microscopy; dihydrofolate reductase and methotrexate

There is a wealth of structural information on the drug–enzyme complex, dihydrofolate reductase and methotrexate. However, the dissociation dynamics of the complex remain relatively poorly understood. This paper describes the application of the atomic force microscope to quantify the rupture forces upon the forced dissociation of the ligand from the enzyme. The intermolecular forces experienced between dihydrofolate reductase and methotrexate as a binary and ternary complex, and the effect of pH upon these forces were studied. The rate at which forced dissociation occurs is known to have a significant effect upon the forces experienced between receptor–ligand complexes. This dependency of rupture force upon retract velocity is investigated. A linear relationship between force and the logarithm of velocity is demonstrated for the forced dissociation of the binary complex, inferring the presence of a barrier in the energy landscape positioned approximately 3 Å away from the bound state. The influence of the cofactor, NADPH, upon these rupture forces was negligible, suggesting that the barrier probed does not facilitate the cooperativity of the ternary complex. However, protonation of the Asp26 residue situated deep within the methotrexate binding site results in a decrease in rupture force.

Dependence of craze velocity on stress and on the absorption and diffusion of environmental gases

Abstract A comprehensive equation for the velocity of a craze in an environmental gas was obtained in terms of the stress, temperature, and partial pressure of the gas. The physical parameters of the gas that enter the equation are boiling point, heat of vaporization and diffusion coefficient. The equation is based on the viewpoint that a steady-state velocity is determined by a thermally activated deformation process wherein the plasticization of the polymer by the gas reduces the activation energy. At low velocities the process is not diffusion-limited, and therefore the logarithm of the velocity is a linear function of pressure and stress. At higher velocities the logarithm of velocity increases less rapidly with stress and pressure because the penetration of gas into the polymer is decreased by the velocity as it constantly generates a fresh gas-polymer interface. The agreement between the equation and the experimental data is demonstrated.

Thermally Activated Domain Wall Movement

AbstractMagnetization reversal by slow domain wall motion under constant applied field has been observed by the Faraday effect in permalloy films. A quantitative study (Craik and Wood 1965) of the dependence of wall velocity on temperature and field has been made in the temperature range −47 to +22°C. The linear relationship between the field H and the logarithm of velocity is consistent with previous work, but in addition the variation of the slope of this line with temperature is noted. The analysis of the results is based on Stacey's theory for uniform potential barriers, and an activation volume deduced which is approximately equal to the cube of the film thickness. An estimate is also made of the frequency factor incorporated in the theory, which is consistent with theoretical values. The validity of these calculations is discussed.