Molecular dynamics simulation study of the effect of molecular structure of warm mix additive on lubricating properties

Based on tribological theory, molecular dynamics (MD) simulation is used to characterize the lubrication behavior of warm mix additive at the friction interface between asphalt and aggregate from the molecular scale. The base asphalt lubricating film model and the warm mix asphalt lubricating film model were established respectively. The asphalt-mineral interfacial shear friction system was constructed using two types of lubricating film models and SiO2 (0 0 1) friction surface. The effects of shear rate, system temperature and the hydrophobic end carbon chain length of the warm mix additive on the lubricating property of the simulated system were investigated, and the mechanism of warm mix additive was further revealed. The results show that the higher the shear rate, the greater the shear stress on the surface of the lubricating film. The shear stress on the surface of the warm mix asphalt lubricating film is significantly smaller than the base asphalt lubricating film. The system temperature has no noticeable effect on the shear stress. Additionally, with increasing hydrophobic end carbon chain length of the warm mix additive, the shear stress of the warm mix asphalt lubricating film decreases. The temperature of the lubricating film gradually rises from the interface to the inside, reaching a maximum value at the film's centre. The temperature inside the base asphalt lubricating film rises with an increasing shear rate. In contrast, the temperature distribution inside the warm mix asphalt lubricating film is relatively constant within a specific range. The cohesive energy of asphalt lubricating film and its adsorption energy on the SiO2 surface both decrease with an increasing shear rate, and the temperature has little effect on the cohesive energy and adsorption energy. With the increase of the hydrophobic end carbon chain length of the warm mix additive, the adsorption energy of warm mix asphalt lubricating film increases and then decreases, the self-diffusion coefficient decreases first and then increases, and the cohesive energy increases gradually. The number density distribution shows a stronger affinity between warm mix additive and SiO2. In conclusion, this paper reveals the lubrication mechanism of surfactant warm mix additive, which can provide reference and guidance for subsequent scholars to further study the warm mixing mechanism by MD method, as well as for the study of asphalt-aggregate interfacial behavior.