## 4 Results and Discussion
### Atomic Force Microscopy (AFM)  
AFM has been found to be a useful technique in bitumen microscopic analysis. In this work, we aimed to analyze the AFM images regardless of the correlations between the bitumen structures at different ageing processes in AFM images.  
AFM characterization was performed in tapping mode at room temperature in air on a Multimode 8. The AFM system equipped with a Nanoscope V controller (Bruker) provided simultaneous topography and phase imaging of the sample. The measurements of bitumen were performed using probes with a conical tip of nominal end radius 10 nm and a resonance frequency of 150 kHz.  
Phase images can show the sample's viscoelastic properties and are useful for bitumen microscopic analysis because materials with different viscoelasticity are clearly distinguishable: soft domains appear dark, while the hard ones appear bright.  
The AFM measurements in Figure 7 show that with increasing exposure time (RTFOT), the morphological structure of asphaltenes changes. More specifically, in the aged samples, we note the arising of progressively more oscillations in the AFM signal within each single cluster, a feature which is known as \"bee structure\" [57] and which denotes structuring of asphaltene clusters at the micrometer-length scale [50]. Moreover, the asphaltene domain size increased with ageing time. For instance, at BRTFOT-225, the domains were about 10 microns in length and 5 microns in width;  
Figure 6: NMR self-diffusion coefficient characterization in the temperature range 90–130 \({}^{\circ}\)C for the different samples (PB, BRTFOT-75, BRTFOT-150, BRTFOT-225, and BPAV).  
the AFM image of PAV bitumen instead revealed asphaltene domains smaller than those observed in BRTFOT-225, homogeneously dispersed on the sample surface.  
It must be pointed out that asphaltene clusters as they are seen by optical microscopy (length scales of micrometers) are the overall, long-range result of asphaltene molecules' hierarchical aggregation occurring at different length scales [50], for which different kinds of intermolecular interactions, from strong and short-range to weak and long-range, are involved. Among these, however, polar-polar interactions are expected to play the major role, since amphiphilic molecules can effectively bind, through their polar moiety, the polar groups of the dispersed molecules or the interfacial polar groups if the molecules are dispersed as clusters, as in the case of asphaltenes in the bitumen. So, a change of the cluster structure can be expected if the polar interactions are strong enough to trigger competition between asphaltene-asphaltene interactions and asphaltene-additive interactions, competition which has been found to be typical at the nanoscale in complex systems [58]. In light of this observation, atomic force microscopy can be seen additionally as a technique to indirectly probe the interactions taking place at shorter length scales, allowing us to explain, in the present work, the trends observed by FT-NMR-SDC. AFM images show the structural morphology of the bitumen, and they allow us to understand where maltene molecular mobility occurs. Considering the RTFOT ageing, the decrease in self-diffusion coefficients at all temperatures is apparently due to an increase in the size of asphaltenes, while the BPAV image clearly shows a different structural morphology from that in the image of BRTFOT-225.