Investigation of the Rheological Behaviors and Microstructural Evolution of Lubricants at Different Gravitational...

Understanding the rheological properties and dynamic behaviors of lubricants under different gravitational intensities is crucial for ensuring long-term lubrication in aerospace lubrication systems. This study utilizes molecular dynamics simulations to investigate the effects of gravitational intensity on the rheological properties and dynamic behaviors of the polyalphaolefin (PAO) lubricant and elucidates its underlying microscopic mechanisms from the perspective of molecular structural evolution. The results indicate that at low shear rates, an increase in gravitational intensity leads to higher shear viscosity, induces uneven velocity distributions, and creates localized high-temperature zones, thereby intensifying energy dissipation. However, these phenomena gradually weaken as the shear rate increases. Furthermore, gravitational intensity induces significant spatial heterogeneity in the dynamic behavior of the PAO lubricant at low shear rates. Specifically, a strong dissipation region emerges in the lower region, while a weaker dissipation region forms in the upper region. At the interface between these two regions, stress concentration occurs, coinciding with the transition point in the velocity gradient, leading to an increase in viscosity. Additionally, the conformational evolution of PAO molecules exhibits significant differences with respect to variations in spatial positions: in the upper region, molecules exhibit stretched and unstable conformations, whereas in the lower region, they adopt bulk-like, stable configurations. These molecular conformations have distinct effects on the dynamic behaviors and rheological properties. In the upper region, the stretched and unstable conformations reduce the momentum transfer and attenuate energy dissipation. Conversely, the stable conformations in the lower region enhance intermolecular interactions, thereby increasing the dissipation. These findings clarify the influence of gravitational intensity on the rheological properties and dynamic behavior of lubricant and reveal the underlying mechanisms of microscopic structural evolution, providing essential guidance for the design and optimization of advanced lubrication technologies tailored to aerospace environments.