Hu Yuan-Zhong

Numerical distortion and effects of thermostat in molecular dynamics simulations of single-walled carbon nanotubes

In this paper, single-walled carbon nanotubes (SWCNTs) are studied through molecular dynamics (MD) simulation. The simulations are performed at temperatures of 1 and 300 K separately, with atomic interactions characterized by the second Reactive Empirical Bond Order (REBO) potential, and temperature controlled by a certain thermostat, i.e. by separately using the velocity scaling, the Berendsen scheme, the Nose–Hoover scheme, and the generalized Langevin scheme. Results for a (5,5) SWCNT with a length of 24.5 nm show apparent distortions in nanotube configuration, which can further enter into periodic vibrations, except in simulations using the generalized Langevin thermostat, which is ascribed to periodic boundary conditions used in simulation. The periodic boundary conditions may implicitly be applied in the form of an inconsistent constraint along the axis of the nanotube. The combination of the inconsistent constraint with the cumulative errors in calculation causes the distortions of nanotubes. When the generalized Langevin thermostat is applied, inconsistently distributed errors are dispersed by the random forces, and so the distortions and vibrations disappear. This speculation is confirmed by simulation in the case without periodic boundary conditions, where no apparent distortion and vibration occur. It is also revealed that numerically induced distortions and vibrations occur only in simulation of nanotubes with a small diameter and a large length-to-diameter ratio. When MD simulation is applied to a system with a particular geometry, attention should be paid to avoiding the numerical distortion and the result infidelity.

Simulation of temperature distribution of point contacts in mixed lubrication

The numerical simulation of temperature distribution of point contacts in mixed lubrication is presented. The calculating includes two steps. First, temperature rises on two surfaces are obtained by a temperature integration method of transient point heat source. Second, the partition coefficients of heat flux are determined by matching the temperature of two surfaces. Similar to the calculation of elastic deformation, double linear interpolation function is used to get a better accuracy, and moving grid method is used to increase the efficiency of the computation. Due to the symmetry of influence coefficient matrix in the direction perpendicular to the velocity, storage and computational work are further reduced by 50%. Numerical samples validate the algorithm and program. The calculating results of the cases of smooth surface and isotropic sinusoidal surface are presented.

A full numerical solution for transient elastohydrodynamic contacts with an eyring fluid

Abstract A non-steady Reynolds equation for an Eyring fluid was derived and solved simultaneously with the elastic equation and other basic equations. The numerical algorithm employed in this paper is a relaxation iteration method. The results show that the non-newtonian effect slightly reduces the lubricant film thickness with an increase in the slide/roll ratio and a decrease in the Eyring stress τ 0 . Furthermore, as the slide/roll ratio increases, the pressure spikes diminish. The full numerical solution of the Reynolds-Eyring equation has provided more precise pressure distributions and film profiles for traction and fatigue calculations.

The spreading behaviour of perfluoropolyether droplets on solid surfaces

The spread of perfluoropolyether (PFPE) droplets on solid surfaces has been measured from the top-down view through a microscope system. Effects of substrates, molecular weight and end-group functionality on spreading of the PFPE droplets have been studied experimentally and the results were compared with those by molecular dynamics (MD) simulations. Silicon wafer and diamond-like carbon (DLC) substrates were used to study the effect of substrates on spreading. Two types of PFPE, Z-dol and Z-tetraol, with the same chain structure and various molecular weights (2000 and 4000 g/mol) were employed in experiments. Effect of molecular weight has been investigated through comparing the spreading of Z-dol 2000 and Z-dol 4000, and it is found that the increase of molecular weight will decrease the mobility of PFPE. Comparison between spreading of Z-dol and Z-tetraol of the same molecular weight proved that functional end group plays a significant role on the spreading of PFPE, which confirmed the MD simulation results.

Role of atomic transverse migration in growth of diamond-like carbon films

The growth of diamond-like carbon (DLC) films is studied using molecular dynamics simulations. The effect of impact angle on film structure is carefully studied, which shows that the transverse migration of the incident atoms is the main channel of film relaxation. A transverse-migration-induced film relaxation model is presented to elucidate the process of film relaxation which advances the original model of subplantation. The process of DLC film growth on a rough surface is also investigated, as well as the evolution of microstructure and surface morphology of the film. A preferential-to-homogeneous growth mode and a smoothing of the film are observed, which are due to the transverse migration of the incident atoms.

Molecular dynamic simulation of lubricant spreading: effect from the substrate and endbead

Molecular dynamic simulations based on a coarse-grained, bead-spring model are adopted to investigate the spreading of both nonfunctional and functional perfluoropolyether (PFPE) on solid substrates. For nonfunctional PFPE, the spreading generally exhibits a smooth profile with a precursor film. The spreading profiles on different substrates are compared, which indicate that the bead-substrate interaction has a significant effect on the spreading behaviour, especially on the formation of the precursor film. For functional PFPE, the spreading generally exhibits a complicated terraced profile. The spreading profiles with different endbeads are compared, which indicate that the endbead-substrate interaction and the endbead–endbead interaction, especially the latter, have a significant effect on the spreading behaviour.

Molecular Dynamics Study on Carbon Nanotubes Sandwiched between Si Surface

Distortion and friction of bundle of single-walled carbon nanotubes sandwiched between two hydrogen-terminated Si(1,1,0) surfaces are investigated by molecular dynamics simulations. The atomic forces are determined by the second REBO (reactive empirical bond-order) for hydrocarbons, Tersoff for Si-Si and Si-H bonds, and Lennard-Jones potential for non-bond interactions. After reaching equilibrium state, a compressive force is applied to the carbon nanotubes until structural destruction appears on carbon nanotubes or substrates. Friction of the system is then investigated when the upper substrate slides along X direction under no load and high load conditions. Distortion of carbon nanotubes can be observed during loading process and no structural destruction occurs ever under the pressure as high as 3.8GPa because of their flexibility. Bundles of SWCNT (10,10) roll randomly and slightly under no pressure condition, but exhibit slide-and-roll combined motion under 3.8GPa pressure. The results also show relatively low lateral forces in both cases. The low friction is attributed to the ewlatively weak Lennard-Jones interaction between substrate and carbon nanotubes with no hanging bond. Excellent performance is therefore expected when carbon nanotubes without defects are used as lubricant or addictives.

Modelling of spreading process: effect from hydrogen bonds

Lubricant spreading on solid substrates has drawn considerable attention not only for the microscopic wetting theory but also for the dramatic application in head-disk interface of magnetic storage drive systems. Molecular dynamic simulation based on a coarse-grained bead-spring model has been used to study such a spreading process. The spreading profiles indicate that the hydrogen bonds among lubricant molecules and the hydrogen bonds between lubricant molecules and polar atoms of solid substrates will complicate the spreading process in a tremendous degree. The hydrogen bonds among lubricant molecules will strengthen the lubricant combination intensity, which may hinder most molecules from flowing down to the substrates and diffusing along the substrates. And the hydrogen bonds between lubricant molecules and polar atoms of solid substrates will confine the lubricant molecules around polar atoms, which may hinder the molecules from diffusing along the substrates and cause precursor film to vanish.