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.