Molecular origin of friction

The wearless friction originating from molecular interactions has been discussed in this paper. We find that the frictional properties are closely related to the structural match of two surfaces in relative motion. For the surfaces with incommensurate structure and week inter-surface interaction, zero static and kinetic friction can be achieved. In a sliding considered as in a quasi-static state, the energy dissipation initiates when interfacial particles move in a discontinuous fashion, which gives rise to a finite kinetic friction. The state of superlubricity is a result of computer simulations, but the prediction will encourage people to look for a technical approach to realizing the state of super low friction.     

Dry friction in the Frenkel-Kontorova-Tomlinson model: dynamical properties

Wearless friction is investigated in a simple mechanical model called Frenkel-Kontorova-Tomlinson model. We have introduced this model in [Phys. Rev. B, 53, 7539 (1996)] where the static friction has already been considered. Here the model is treated for constant sliding speed. The motion of the internal degrees of freedom is regular for small sliding velocities or weak interaction between the sliding surfaces. The regular motion for large velocities is strongly determined by normal and superharmonic resonance of phonons excited by the so-called “washboard wave”. The kinetic friction has maxima near these resonances. For increasing interaction strength the regular motion becomes unstable due to parametric resonance leading to quasistatic and chaotic motion. For sliding velocities beyond first-order parametric resonance bistability occurs between the strongly chaotic motion (fluid sliding state), where friction is large and a regular motion (solid sliding state), where friction is weak. The fluid sliding state is mainly determined by the density of decay channels of m washboard waves into n phonons. This density describes qualitatively the effectiveness of the energy transfer from the uniform sliding motion into the microscopic, irregular motion of the degrees of freedom at the sliding interface. For a narrow interval of the sliding velocities we also found enhanced friction due to coherent motion. In the regime of coherent motion nondestructive interactions of dark envelope solitons occur.     

Modelling of a two-dimensional Coulomb friction oscillator

Xia proposes a model for investigating the stick-slip motion caused by dry friction of a two-dimensional oscillator under arbitrary excitations. Instead of the harmonic balance method used by most investigators, a numerical approach to investigate the system is provided. The concept of the friction direction angle is introduced to determine the components of the static and kinetic friction force vector and the hyperbolic secant function is introduced to deal with the transition of the friction force from the static to the kinetic state. The friction direction angle is determined by either relative velocities or input forces. With this method the switch conditions for stick state, slip state and stick-slip state can be easily derived. The orbits of the responses, which are either straight line segments, circular or elliptic are obtained. In the general case, the orbit of the response is a complex planar curve. Zero-stop, one-stop, two-stops and more than two-stops per cycle are also found.     

MODES OF CONTACT AND UNIQUENESS OF SOLUTIONS FOR SYSTEMS WITH FRICTION-AFFECTED SLIDERS

The kinetic equations of planar multi-body systems with friction-affected sliding joints are reformulated for the computation of closed-form solutions for the kinetic parameters. The state of such systems is characterized not only by the position parameters and velocities, but in addition, the modes of contact in the sliding joints must be specified. Then the cases with one or several sets of solutions, obtained for the same position parameters, velocities, active forces and friction parameters, can be related to positions of the system with different modes of contact between sliders and guiding surfaces. They are physical unequivocal states and can be interpreted as unique solutions for the kinetic problem with specified configuration of the system. If no solutions exist, then the friction parameters considered are too large and exceed limiting values, for which friction locking occurs.     

Stick-slip melting of a boundary lubricant between two rigid surfaces with nanoscale asperities

With a simple mechanical analog of the elastic tribological system, the friction of two rough surfaces is studied using the model of first-order phase transitions. The surfaces rub under boundary friction conditions in the presence of a lubricant layer in between. Stick-slip motion is considered, which is due to periodic phase transitions arising between kinetic friction conditions. It is shown that when rubbing surfaces are rough, a time-varying domain structure with a spatially distributed order parameter occurs in the plane of friction during motion.     

Friction and normal forces of model friction modifier additives in simulations of boundary lubrication

ABSTRACT

Interaction forces between solid surfaces are often mitigated by adsorbed molecules that control normal and friction forces at nanoscale separations. Molecular dynamics simulations were conducted of opposing semi-ordered monolayers of united-atom chains on sliding surfaces to relate friction and normal forces to imposed sliding velocity and inter-surface separation. Practical examples include adsorbed friction-modifier molecules in automatic transmission fluids. Friction scenarios in the simulations had zero, one, or two fluid layers trapped between adsorbed monolayers. Sliding friction forces increased with sliding velocity at each stable separation. Lower normal forces were obtained than in most previous nanotribology molecular simulations and were relatively independent of sliding speed. Distinguishing average frictional force from its fluctuations showed the importance of system size. Uniform velocities were obtained in the sliding direction across each adsorbed film, with a gradient across the gap containing trapped fluid. The calculated friction stress was consistent with measurements reported using a surface forces apparatus, indicating that drag between an adsorbed layer and trapped fluid can account sufficiently for sliding friction in friction modifier systems. An example is shown in which changes in molecular organisation parallel to the surface led to a large change in normal force but no change in friction force.     

Tribological system in the boundary friction mode under a periodic external action

A mechanical analog of a tribological system in the boundary friction mode is studied. A thermodynamic model is used to analyze the first-order phase transition between liquidlike and solidlike structures of a lubricant. The time dependences of the friction force, the relative velocity of the interacting surfaces, and the elastic component of the shear stresses appearing in the lubricant are obtained. It is shown that, in the liquidlike state, the shear modulus of the lubricant and the elastic stresses become zero. The intermittent (stick-slip) friction mode detected experimentally is described. It is shown that, as the lubricant temperature increases, the frequency of phase transitions between the lubricant structural states decreases and the total friction force and elastic stress amplitudes lower. When the temperature or the elastic strain exceeds the corresponding critical value, the lubricant melts and a kinetic slip mode in which the elastic component of the friction force is zero takes place.     

Towards a statistical theory of solid dry friction

Wearless dry friction of an elastic block of weight N, driven by an external force F over a rigid substrate, is investigated. The slider and substrate surfaces are both microscopically rough, interacting via a repulsive potential that depends on the local overlap. The model reproduces Amontons’s laws which state that the friction force is proportional to the normal loading force N and independent of the nominal surface area. In this model, the dynamic friction force decays for large velocities and approaches a finite static friction for small velocities if the surface profiles are self-affine on small length scales.     

Fabrication of durable hydrophobic surfaces through surface texturing

Low surface energy polymer thin-films can be applied to surfaces to increase hydrophobicity and reduce friction for a variety of applications. However, wear of these thin films, resulting from repetitive rubbing against another surface, is of great concern. In this study, we show that highly hydrophobic surfaces with persistent abrasion resistance can be fabricated by depositing fluorinated carbon thin films on sandblasted glass surfaces. In our study, fluorinated carbon thin films were deposited on sandblasted and as-received smooth glass using deep reactive ion etching equipment by only activating the passivation step. The surfaces of the samples were then rubbed with FibrMet abrasive papers in a reciprocating motion using an automatic friction abrasion analyzer. During the rubbing, the static and kinetic friction forces were also measured. The surface wetting properties were then characterized using a video-based contact angle measuring system to determine the changes in water contact angle as a result of rubbing. Assessment of the wear properties of the thin films was based on the changes in the water contact angles of the coated surfaces after repetitive rubbing. It was found that, for sandblasted glass coated with fluorinated carbon film, the water contact angle remained constant throughout the entire rubbing process, contrary to the smooth glass coated with fluorinated carbon film which showed a drastic decrease in water contact angle with the increasing number of rubbing cycles. In addition, the static and kinetic friction coefficients of the sandblasted glass were also much lower than those of the smooth glass.     

Nanofriction studies of reactively or non-reactively cluster-eroded single crystal diamond

Friction properties of cluster-eroded surfaces of synthetic single crystal diamond (Monodite) are compared after erosion with high-speed CO₂ cluster beams as well as with corresponding Ar cluster beams, the cluster impact kinetic energy being 100 keV in both cases. The respective friction values are determined by atomic force microscope measurements. Using CO₂ clusters, the reactive accelerated cluster erosion (RACE) of the single crystal diamond substrates leads to more than seven times higher friction values than those observed after erosion with non-reactive accelerated Ar clusters. Molecular dynamics calculations reveal related differences in the simulations of respective single cluster impacts already at 2 ps after impact.     

Heterogeneous catalysis in solution at high pressures lactose hydrolysis over an immobilized β-galactosidase

Abstract

The hydrolysis of lactose over an immobilized β-galactos-idase was studied under pressures up to 98 MPa. The data were analyzed on the basis of a Michaelis-Menten type kinetic model. A model for the transition state complex is proposed by analyzing kinetic parameters.     

Study on effect of dimples on friction of parallel surfaces under different sliding conditions

The influence of rectangle dimples with flat bottom on the friction of parallel surfaces at different sliding conditions is investigated based on lubrication equations. The elastic deformation of rough surfaces is evaluated using continuous convolution fast Fourier transform (CC-FFT). The friction coefficients for dimpled and non-dimpled parallel surfaces by simulation are compared with experimental results. Results show that this kind of dimples can reduce the friction coefficient for cases with the smaller ratio of film thickness to roughness (h/R_q), small roughness or large applied load. The friction force for the parallel surfaces can decrease due to the dimple effect over the range of the larger sliding speed, larger load or smoother surfaces.     

Phenomenological theory for the melting of a thin lubricant film between two atomically smooth solid surfaces

A thermodynamic model is developed for the melting of an ultrathin lubricant film squeezed between two atomically smooth solid surfaces. To describe the state of lubricant, an excess volume parameter is introduced; it appears due to the chaos in the structure of a solid body induced by melting. This parameter increases with the total internal energy upon melting. Thermodynamic melting and shear melting are described. The dependences of the friction force on the lubricant temperature and the shear rate of friction surfaces are analyzed. The calculated results are compared to the experimental data.     

One- and two-body dissipation in heavy-ion collisions

A microscopic theory has been formulated for one- and two-body dissipation in collisions between two heavy nuclei. With a nucleon-nucleon interaction as the basic perturbation in a density matrix approach with “linear response” approximations, the one- and two-body nuclear friction coefficients for the ⁴⁰Ca + ⁴⁰Ca system have been calculated and their dependence on relative kinetic energy and smearing of nuclear single-particle states was obtained. The results of our calculation show that: (a) the combined one- and two-body friction coefficients compare favourably with phenomenological values, (b) the one-body dissipation is more effective than two-body in kinetic energy damping, while both the mechanisms are comparable for the damping of relative angular momentum, (c) the importance of the two-body friction compared to one-body increases at higher relative kinetic energies and (d) the effect of introducing a smearing in nuclear levels appears as a lowering of nuclear friction.     

A fluctuation-dissipation theorem approach to mechanical relaxation in solid polymers

Abstract

Kubo theory formalism has been used to obtain expressions for shear and dilatational stress relaxation functions in terms of statistical mechanical time-dependent correlation functions. This is equivalent to obtaining expressions for the complex modulus or the complex viscosity for all frequencies. These results provide a basis for calculating the macroscopic consequences of molecular models presently used to provide qualitative understanding of relaxation peaks for solid polymers.

The shear and dilatational stress relaxation functions are quite different formally. For a particularly simple model it will be shown that the former is related to the frequency distribution of the kinetic energy and is also closely related to the dielectric relaxation function. The familiar results of the Rouse model are recovered in the results but no friction constant need be assumed in the present approach.     

Wetting modifications of uhmwpe surfaces induced by ion implantation

Ultra-high-molecular-weight-polyethylene (UHMWPE) surfaces are characterized in terms of roughness and wetting. Changes in the surface morphology of the polymer were induced macroscopically by mechanical friction and microscopically by ion implantation. The ion irradiation was obtained by using 300?keV Xe⁺ beams with doses ranging between 10¹⁴ and 10¹⁵?ions/cm².

Roughness and wetting measurements were performed in order to investigate the UHMWPE surface properties before and after the surface treatments. The wetting angle of the polymeric surface increases with the decrease of the roughness and with the increase of the absorbed dose. Results are discussed from the point of view of the biological reactions that could degrade the UHMWPE biocompatible surfaces employed in different mobile prostheses.     

Hydrostatic-Pressure-Induced Reentrance of the Metallic State in the κ-(ET)2Hg(SCN)2Cl Quasi-Two-Dimensional Organic Conductor

On cooling below 30 K, the κ-(ET)₂Hg(SCN)₂Cl quasi-two-dimensional organic metal, which is in the quantum spin liquid state at liquid helium temperatures, undergoes a transition to the Mott insulator state. The application of a hydrostatic pressure p = 0.7 kbar stabilizes the metallic state and makes it possible to study the behavior of the interlayer magnetoresistance at liquid helium temperatures. The field dependence of the magnetoresistance exhibits an unlimited power-law growth, which indicates that the polaron mechanism contributes to the interlayer transport. The spectrum of observed magnetoresistance oscillations corresponds to the Fermi surfaces characteristic of conducting layers with the κ-type structure.

     

Kinetics of atoms in an elliptically polarized standing wave

We examine the kinetics of atoms with their ground and excited states being degenerate in the projection of angular momentum. The atoms are located in a standing wave with uniform elliptical polarization. Using the j _g =1/2→j _e =1/2 transition as an example, we show that the friction and diffusion of atoms strongly depend on the ellipticity of the field. For instance, in the low saturation limit the frictional force contains, in addition to the ordinary Doppler friction term, a term that can be interpreted as Sisyphean friction. Under certain conditions, the contributions reflecting the degeneracy of the ground state are dominant, with the result that the values of the friction and diffusion coefficients (and hence the rate of kinetic processes) may differ from the values predicted by the two-level atomic model by several orders of magnitude. Zh. éksp. Teor. Fiz. 115, 791–804 (March 1999)