Molecular dynamics simulation of the lubricant conformation changes and energy transfer of the confined thin lubricant film

Dynamic Behavior of Lubricant Molecules Under Oscillating Motion: Insight from Molecular Dynamics Simulations

Understanding the dynamic behavior of lubricant molecules under oscillation lubrication is important for the development of advanced film lubrication technology. Herein, the dynamic behaviors of lubricant molecules under oscillating motion are studied by using molecular dynamics simulations. The effects of oscillation period on the film temperature, film velocity distribution, film stress and strain, and molecular orientation are investigated. The results show that when the oscillation period becomes longer, the film temperature distribution changes from asymmetrical to symmetrical. Under short oscillation periods, there is a massive dissipation of kinetic energy in the upper region of the film, while the rest of the film moves in a solid-like manner, which leads to an unsymmetrical distribution of film temperature. However, when the oscillation period is longer, the momentum transfer among molecules becomes more adequate, and the kinetic energy converted to heat is more adequately transferred; thus, the temperature maps gradually become symmetrical. In addition, the hysteresis time between stress and strain gradually increases in longer oscillation periods, which means that the viscous component increases and the film fluidity is enhanced. Finally, the evolution of the molecular orientation during different oscillation periods is discussed by calculating the chain orientation in different layers and analyzing snapshots of the molecular conformation. Results show that the film fluidity and homogeneity properties increase in longer oscillation periods. When the oscillation period is 10 ps, the upper molecules in the box are in a small elastic deformation state, and the lower molecules in the box are in a solid-like state; when the oscillation period is 33.33 ps, the film is in an orientation lagging state; and when the oscillation period is 100 ps, the film is in a quasiviscous flow state. These findings can help in gaining a deeper understanding of the dynamic behavior of films in oscillation lubrication.

Investigation of Tribological Behavior and Lubrication Mechanisms of Zinc Oxide under Poly α-olefin Lubrication Enhanced by the Electric Field

The electric field induces complex effects on the tribological properties of zinc oxide (ZnO) under lubricated conditions, particularly at the nanoscale, where the friction process and mechanism remain unclear. In this paper, the tribological behaviors of ZnO under the lubrication of poly α-olefins (PAO) were investigated by molecular dynamics (MD) simulations with reactive force field (ReaxFF). The results reveal a significant enhancement in the tribological performances of ZnO with the application of the electric field, resulting in a 58.6% reduction in the coefficient of friction (COF) from 0.193 at 0 V/Å to 0.080 at 0.1 V/Å. This improvement can be attributed to the weakening of interfacial interaction, evidenced by a reduction in the number of C-O covalent bonds under the influence of the electric field, along with the formation of an adsorption film due to applied load and shear effects. Notably, the effect of the electric field and applied load extends the impact of interface slip on the tribological performance of ZnO. Overall, this study provides a comprehensive understanding of the impact of the electric field on reducing the friction of ZnO-based structured models, shedding light on explaining their tribological properties and lubrication mechanisms.

The Simulation of Ester Lubricants and Their Application in Weak Gel Drilling Fluids

To enhance the performance and reduce the amount of ester-based lubricants used in weak gel drilling fluids, a shear dynamics simulation under extreme pressure conditions was employed to refine the formulation of the base oil and pressure additives. The simulation results were validated using fatty acid methyl, ethyl, and butyl esters. Fatty acid methyl ester demonstrated the lowest temperature increase and the highest load-bearing capacity post-shear. The four-ball friction test revealed that methyl oleate had a coefficient of friction of 0.0018, approximately a third of that for butyl oleate, confirming the simulation's accuracy. By using methyl oleate as the base oil and oleamide as the pressure-resistant component, the optimal shear stress was achieved with a 10% addition of oleamide. A lubricant composed of 90% methyl oleate and 10% oleamide was tested and showed a coefficient of friction of 0.03 when 0.5% was added to bentonite slurry, indicating a strong lubricating film. Adding 1% of this lubricant to a low gel drilling fluid system did not affect its rheological properties, and the gel structure remained stable after seven days of aging. Field tests at the Fu86-3 well in the Jiangsu Oilfield of Sinopec confirmed that adding 1% of the ester-based lubricant to the drilling fluid significantly improved drilling efficiency, reduced drag by an average of 33%, and increased the drilling rate to 22.12 m/h. This innovation effectively prevents drilling complications and successfully achieves the objectives of enhancing efficiency.