Molecalar Dynamics Simulation Studies On Hydrocarbon Lubrication And Rheological Behavior At Nano Level

Tribological and lubricating behaviors on material surface widely exist in machinery, automobile, electronics and many other industries, so the control of rheological and tribological phenomena is particularly important to prevent wear and save energy. From 1980s, with the rocketing development of Nano machine, Micro-electronic mechanic system (MEMS) and magnetic storage device from nanotechnology, microtribological problems gradually emerged and proposed require to lubrication. As the microtribological surface distance is at nanometer scale, theory of traditional macroscopical lubrication and continuum mechanics is no longer applicable and research of ultra-thin film lubrication at nano level is gradually paid more attention to. To investigate the structure of the ultra-thin film and rheology under shear by computer simulation method can not only reach high shear rate and temperature that cannot be realized in experiment, but also provide a academic guide in predicting experiment results and lubrication molecular design. In this paper we studied structure and rheology of hydrocarbon ultra-thin film confined between two solid surface using nonequilibrium molecular dynamics method. The primary contents and results are as follows:1. The behavior of dodecane ultra-thin film confined between two iron wall under shear was investigated using nonequilibrium molecular dynamics simulations and shear is imparted by moving the walls to drive the fluid flow. The dynamics processes and simulation results indicated that the structure and dynamics are different from bulk behavior. The lubrication molecular tend to organize in strata Parallel to the solid walls by the strong wall affinities and interlayer slip develops inside the lubrication film. The shear in the flow can change the conformation of the chains, which have a tendency to stretch in the shear direction. There is a temperature profile between the walls and it is influenced by the shear rate. The ultra-thin film between the walls exhibit structural and dynamical inhomogeneities at nano level across the film. 2.To discuss the structural and dynamical inhomogeneities of the film, we investigated the lubricating and rheological properties of the tridecane ultra-thin film with different molecular layers, shear rate and temperature. The simulation results indicated that the close proximity of the surfaces causes the interfacial effect to strongly influence the structure of the film and the influence fell off with the thickness of the film. As the film layers increased, the configuration transformed from completely ordered structure to disordered structure and emerged continuum mechanics behavior. The interlayer slip was found to be influence by the shear rate and thickness of film. Interlayer slip occurred in the density valley and increased with increasing shear rate, but decreased with the film thickness increased. A temperature profile formed between the walls and increased with increasing the shear rate and the film thickness. The shear stress near the wall was founded to be periodical vibration and increased with increasing the shear rate. We also investigated the dynamical behavior of the film by the mean square displacement and diffusion coefficient. It showed that chain mobility was mainly in the x-axis and was restrainted in the y-axis and z-axis. At the same time, the diffusion coefficient increased with increasing the shear rate obviously.