A systematic AFM-based method to measure adhesion differences between micron-sized domains in asphalt binders

The adhesion strength of asphalt binders is one of the fundamental properties that affect asphalt pavement's performance. Atomic force microscopy (AFM) is one of the few methods that are capable of measuring adhesion of asphalt binders at the micro scale. Due to the stickiness and viscoelastic nature of the binder, challenges such as tip contamination are present in the measurement of AFM-based adhesion and the effects of operational parameters (e.g., the maximum compressive force and scanner drive speed) on adhesion measurements are often ignored in published studies. In this paper, a systematic AFM-based adhesion testing procedure was developed to minimize the above limitations, and used to characterize the adhesive force of four different asphalt binders. Results indicated that topographic images of solution-cast binder films displayed two micron-sized domains, with a height difference of a few nanometers between them. The adhesion difference between the raised and recessed areas of the binder films was considered to be statistically significant from t-tests, which might be attributed to the different chemical compositions between these two different domains. Such a rigorous AFM-based testing procedure can benefit adhesion measurements of other sticky polymeric materials. The adhesion difference between different domains should be helpful for building the link between chemical and mechanical properties of asphalt binders.