Multiscale Investigation of Interfacial Heterogeneity in Asphalt Mixtures

Abstract The non-uniformity of internal interface adhesion within asphalt mixtures plays a crucial role in determining their durability. This study employed molecular dynamics (MD) simulations, Atomic Force Microscopy (AFM) experiments, Scanning Electron Microscopy–Energy-Dispersive Spectroscopy (SEM–EDS), and contact angle measurements to analyze the spatial heterogeneity of internal interfaces within asphalt mixtures across multiple scales─from molecular to microscopic, mesoscopic, and macroscopic─in terms of both molecular structure at the adhesion interface and the adhesion mechanism itself. The results of the calculation of the relative density distribution of MD indicate that, in the interface model established, the various asphalt components exhibit a distinct spatial inhomogeneity. The SEM–EDS results further show that the oxygen signal in the edge regions of the interface is lower than that in the interior regions, which is consistent to some extent with the spatial variation trends obtained from the simulation. The radial distribution function calculation results indicate that the interior regions of the interface exhibit more pronounced short-range molecular spatial correlations, whereas molecular correlations in the edge regions are relatively weaker. AFM results indicate that both the DMT modulus and the adhesion at the granite–asphalt and limestone–asphalt interfaces increase significantly from the edge toward the interior; the maximum increase in the DMT modulus at the limestone–asphalt interface reached 24.17%. The adhesion energy calculated based on contact angle and surface free energy also exhibited significant spatial variations, with values in the central regions of the granite–asphalt and limestone–asphalt interfaces increasing by 80.6% and 38.74%, respectively, compared to the edge regions. Taken together, these results indicate that the structure and adhesion behavior of the asphalt–aggregate interface exhibit significant spatial heterogeneity. These findings provide evidence of spatial heterogeneity in the structure and behavior of the bitumen–aggregate interface across multiple scales, offering theoretical support for deepening our understanding of the interface interaction mechanisms and optimizing the adhesion properties of bituminous mixtures.