Molecular Dynamics Study of the Wetting Behavior of Biodiesel Combustion Particles under Exhaust-Plume Conditions

The hygroscopic growth of engine-emitted particulate matter in exhaust plumes is strongly influenced by surface wettability. In this study, molecular dynamics simulations were performed on biodiesel- and diesel-derived combustion-particle models constructed on a unified defective carbon framework to investigate wetting behavior under representative exhaust-plume temperature and humidity conditions. Under the reference condition of 333 K and a saturation ratio of 1.2, the equilibrium contact angles on smooth biodiesel, rough biodiesel, and rough diesel surfaces were 45.4°, 63.5°, and 95.5°, respectively. The trends in work of adhesion and interfacial hydrogen-bond statistics were consistent with the contact-angle results. Temperature primarily modulates interfacial water exchange and liquid-phase rearrangement, whereas the saturation ratio affects the availability of vapor-phase water and its contribution to rearrangement near the contact line and stable solid-liquid bonding. Ultimately, particle wetting behavior is governed by the competition among four interdependent processes: vapor-phase water supply, liquid-phase rearrangement, interfacial bonding, and contact-line motion. Radial distribution function (RDF) analysis, interfacial hydrogen-bond statistics, and spatially resolved profiles of interfacial density and local pressure show that, relative to diesel combustion particles, biodiesel combustion particles—with a higher density of surface oxygen functionalities—exhibit stronger interfacial water enrichment, a denser hydration layer, and more stable interfacial bonding. These differences are consistent with their smaller equilibrium contact angle and greater wetting propensity. These results provide molecular-scale insight into particle-water interactions relevant to early hygroscopic growth in exhaust-plume environments.