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.     

Molecular origin of model membrane bending rigidity

The behavior of the bending modulus kappa of bilayers in lamellar phases was studied by Small Angle X-ray Scattering technique for various nonionic C(i)E(j) surfactants. The bilayers are either unswollen and dispersed in water or swollen by water and dispersed in dodecane. For unswollen bilayers, the values of kappa decrease with both an increase in the area per surfactant molecule and in the polar head length. They increase when the aliphatic chain length increases at constant area per surfactant molecule. Whereas for water-swollen membranes, the values of kappa decrease as the content of water increases converging to the value of the single monolayer bending modulus. Such a behavior results from the decoupling of the fluctuations of the two surfactant membrane monolayers. Our results emphasize the determinant contribution of the surfactant conformation to kappa.     

Molecular friction between an ionic lattice and a dielectric medium

The tangential component of the interaction force between an ionic lattice and a moving dielectric medium is calculated in this paper, the dependence of the friction on the velocity is discussed, and the anisotropy of the friction is examined.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 67–73, October, 1977.     

掺硅类金刚石薄膜摩擦过程的分子动力学模拟

掺硅类金刚石(Si-DLC) 薄膜表现出优异的摩擦学性能, 在潮湿空气和高温中显示出极低的摩擦系数和很好的耐磨性, 但是许多实验表明Si-DLC膜的摩擦性能受其硅含量的影响很大. 因此, 本文采用分子动力学模拟的方法分别研究干摩擦和油润滑两种情况下不同硅含量的Si-DLC膜的摩擦过程. 滑移结果表明干摩擦时DLC膜和掺硅DLC膜之间生成了一层转移膜, 而油润滑时则为边界膜. 因此干摩擦时的摩擦力明显大于油润滑时的摩擦力. 少量添加硅确实能降低DLC膜的摩擦力, 但是硅含量大于20%后对DLC膜的摩擦行为几乎无影响. 干摩擦时硅含量对转移膜内键的数量影响很大, 转移膜内CC键和CSi键都先增加后减少, 滑移结束时几乎不含CSi键.     

Molecular origin of auxetic behavior in tetrahedral framework silicates

Recent analytical models for the Poisson's ratios (nu(ij)) of tetrahedral frameworks are applied to alpha-cristobalite and alpha-quartz for the first time. Rotation and dilation of the SiO4 tetrahedral subunits are considered. Each mechanism leads to negative nu(31) values, whereas negative and positive values are possible when they act concurrently. The concurrent model is in excellent agreement with experiment and explains the dichotomy between negative and positive nu(31) values in alpha-cristobalite and alpha-quartz, respectively. The predicted strain-dependent trends confirm those from molecular modeling.     

Molecular dynamics simulation of nano-scale interfacial friction characteristic for different tribopair systems

3D non-equilibrium molecular dynamics (NEMD) simulations using embedded atom potentials method (EAM) are performed to identify the dynamics processes of atomic-scale interfacial friction taking places in metal tribopairs. A block-block sliding simulation model for soft-to-hard (Cu/Fe) and soft-to-soft (Cu/Ag) tribopairs with is built. The microstructural evolution and temperature variation of the two tribopairs are analyzed at different sliding speeds. The results show that the average temperature of the two different tribopairs both increases rapidly during the transient sliding period. The different microstructural changes for the two tribopairs, including extensive plastic deformation, mechanical mixing and material transfer are observed when the temperature rapidly increases. The characteristics of the friction effects for the two tribopairs are also revealed by analyzing the friction force evolution as a function of time and velocity.     

Molecular origin of thermal diffusion in benzene + cyclohexane mixtures

The isotope effect in thermal diffusion (Soret effect) of benzene+cyclohexane mixtures has been investigated by a holographic grating technique. The Soret coefficient can be split into additive contributions. One contribution, the isotope effect, stems from the differences of both mass and moment of inertia, and is independent of composition. An additional "chemical" contribution depends on concentration and even changes its sign at a benzene mole fraction x(benz) approximately 0.7. The mass effect is in agreement with molecular dynamics calculations: the heavier component migrates to the cold side.     

Molecular origin and dynamic behavior of slip in sheared polymer films

The behavior of the slip length in thin polymer films subject to planar shear is investigated using molecular dynamics simulations. At low shear rates, the slip length extracted from the velocity profiles correlates well with that computed from a Green-Kubo analysis. Beyond chain lengths of about N=10, the molecular weight dependence of the slip length is dominated strongly by the bulk viscosity. The dynamical response of the slip length with increasing shear rate is well captured by a power law up to a critical value where the momentum transfer between wall and fluid reaches its maximum.     

Molecular hydrodynamics of inhomogeneous systems: The origin of slip boundary conditions

Molecular hydrodynamic equations derived by linear response theory for inhomogeneous systems are investigated in two extreme cases: a simple fluid in a gravitational field (an example of a long-range inhomogeneity), and a simple fluid in the presence of a rigid flat wall described by an infinite potential step (an example of a short-range inhomogeneity). In both cases the phenomenological equations result when the molecular equations are smoothed over a length which is large compared to the correlation lenght. For these systems the local equilibrium assumption is shown to be valid. In addition it is shown how the usual slip boundary conditions arise as a consequence of the interaction which causes the short-range inhomogeneity.     

Molecular solvent-dynamical effects on activated electron-transfer kinetics: How important is solvent friction?

The likely extent of retardation exerted by solvent friction upon the rates of activated electron-transfer (ET) processes is explored and evaluated with reference to some extant solvent-dependent data for metallocene ET self exchanges. Barrier-crossing frequencies extracted from the experimental kinetic and barrier data for suitably adiabatic reactions are compared with solvent inertial frequencies (i.e. the zero-friction limit) as estimated from currently available analytic expressions. The additional extent of rate retardation seen in passing from low-friction to ostensibly strongly overdamped solvents, as deduced in this manner, is seen to be muted substantially in comparison with the predictions of conventional Debye-continuum approaches, although following a solvent functionality that is nevertheless roughly in accord with the latter. The likely importance of more rapid dynamics associated with short-range (molecular) solvation is addressed in the light of these findings.     

On the origin of why static or breakloose friction is larger than kinetic friction, and how to reduce it: the role of aging, elasticity and sequential interfacial slip

We discuss the origin of static friction and show how it can be reduced towards kinetic friction by the appropriate design of the sliding system. The basic idea is to use elastically soft solids and apply the external forces in such a way that different parts of the contacting interface start to slip at different times during the (tangential) loading process. In addition, the local slip must be large enough in order to result in a strong drop in the static friction force. We illustrate the theoretical predictions with the results of a simple model experiment.     

Molecular origin of shear thickening in transient polymer networks: A molecular dynamics study

This paper proposes a simple model of transient networks of telechelic associating polymers for molecular simulations and reports the main results obtained by molecular dynamics on the rheological properties of the transient networks. The steady shear viscosity obtained by the non-equilibrium molecular dynamics simulation exhibits shear thickening at moderate shear rates and shear thinning at larger shear rates. The behavior is similar to that observed in experiments of telechelic associating polymers. By analyzing the distribution function of the end-to-end vector of bridge chains as a function of the shear rate, we find that shear thickening is mainly caused by the stress from the bridge chains highly stretched by shear flow. We also find that fracture of the transient network occurs in the shear-thinning regime at high shear rates.     

Quantum friction

We investigate the van der Waals friction between graphene and an amorphous SiO(2) substrate. We find that due to this friction the electric current is saturated at a high electric field, in agreement with experiment. The saturation current depends weakly on the temperature, which we attribute to the quantum friction between the graphene carriers and the substrate optical phonons. We calculate also the frictional drag between two graphene sheets caused by van der Waals friction, and find that this drag can induce a voltage high enough to be easily measured experimentally.