## 3 Results and Discussion
### Morphological Properties  
Within the existing literature, AFM has been found to be a useful technique that can be used to study bitumen's inner structure as it allows the topographic characterization of surfaces at resolutions not available with light microscopy. It is also more practical and straightforward than scanning electron microscopy (SEM) because AFM samples require minimal preparation. Using the AFM technique for the purpose of studying bitumen's  
Figure 1: Complex modulus G* vs. temperature of reference samples. Figure 2: Complex modulus G* vs. temperature of R1- and R2-modified samples. inner structure, four phases are usually observed in the images: catana phase (consisting of bee structure), peri phase, para phase, and sal phase [2]. 1. The catana phase is made up of so-called bee structures, which resemble undulated wavy structures having a morphology of alternating swellings and depressions. 2. The peri phase is the domain surrounding the bee structures. 3. The para phase is dominant in bitumen's matrix, flat in nature, and surrounds the peri phase. 4. The sal phase in combination with the para phase consists of aromatics and saturates, and these constitute the smooth matrix. Evidence from a literature review of AFM studies carried out on bitumen suggests that several interpretations of AFM images exist among researchers. The bee structures are speculated to be connected to bitumen's asphaltenic composition, with resins being the regions surrounding the bee structures. Aromatics and saturates are said to correspond to the smooth matrix (para and sal phases) observed in AFM images [18; 19; 20; 21]. Regardless of differences in opinion regarding interpretation, AFM studies provide a wealth of useful information, and are instrumental in understanding the intricacies of bitumen's inner structure. As can be seen from Figure 3, the aging of the bitumen brought about an enlargement and roughening of the catana phase. This can be explained by the incorporation of oxygen by the asphaltenes during aging, and this translates to an increase in stiffness and rigidity of the bitumen, since the bee structures grow larger in size [22]. The para phase, which probably represents the maltenic fraction of bitumen, also diminished with aging. The addition of the R1 recycling agent is shown to reduce the size of the asphaltenes, and also restore the maltenic fraction, thus making it similar to non-aged bitumen. These aforementioned changes in bitumen's morphology were repeatedly observed with the subsequent aging and addition of R1 recycling agent. The bee structures increased in size due to their incorporation of oxides via oxidative aging, and also formed clusters. These clusters usually occur due to asphaltene aggregation, making the aged bitumen stiff and fragile, and thus susceptible to cracking [23]. The first addition of the R1 recycling agent reversed this increase in bee structure size, and then the second aging cycle increased the bee structure size again. The second cycle of R1 modification of the aged bitumen brought about, once again, a reduction in the size of the bee structures; the third aging cycle slightly increased the bee structure size, even though the effect of the third cycle of aging on the recycled bitumen was not highly pronounced, likely due to the significant amount of the R1 recycling agent present in the sample. This shows the effectiveness of R1 in restoring the balance of the ratio of the different fractions of aged bitumen, reversing the effects of multiple aging cycles. In Figure 4, on the other hand, it can be observed that upon the first addition of the R2 recycling agent to the aged bitumen, the agglomeration of bee structures observed in the aged sample was de-clustered in such a way that a few clusters still exist, but these small clusters seem to be dissolved clusters resulting from a larger cluster. When this recycled sample was aged for the second time, the peri phase surrounding the de-clustered bee structures became more pronounced.