**2. Asphalt under the AFM – historic overview**

In an effort to enhance the understanding and characterization of bitumen microstructure, atomic force microscopy (AFM) techniques have been employed to bituminous materials over the last 15 years. Typical image obtained from AFM scanning is depicted in Fig. 1, where one can easily observe the existing of microstructures in the bitumen matrix.

Loeber et al. (1996, 1998) was the first research group who published a inclusive investigation of bitumen using AFM. In these studies, bitumen samples were prepared by a heat-casting method to preserve the solid-state morphology. They revealed ripple microstructures with several micrometers in diameter and tens of nanometers in height. The authors named those rippled yellow and black strips microstructures ''bumble bees'' and attributed them to the asphaltenes (Redelius, 2009). Other shapes and textures including networks and spherical clusters were also observed in the study.

Based on Loeber's findings, Pauli et al. (2001) wanted to find the correlation between the ''bumble bee'' shaped microstructures and the amount of asphaltenes in the bitumen. In this study, they investigated solvent-cast films of bitumen under AFM using both friction-force and tapping mode. From these, they found the same Loeber's bee-shaped microstructures. To confirm that the bee-shaped structures were asphaltenes, they doped bitumen with asphaltenes and observed an increase in the density of bee-shaped microstructures. Their group also observed that the microstructure disappeared after repetitive scanning over the same area (Pauli & Grimes, 2003) and assumed the reason for this phenomenon was due to the red laser light of the AFM which produces heat and softening of the surface.

A research group from Vienna University of Technology (Jäger et al., 2004), investigated five different types of bitumen and also reported randomly distributed bee-shaped structures, which, just like the previous researchers they related to the asphaltenes in the bitumen.

the material and says very little about the chemo-mechanical healing propensity, nor helps

Bitumen is a complex mixture of molecules of different size and polarity, for which microstructural knowledge is still rather incomplete. As physical properties of bitumen are largely dependent on this microstructure, the prediction of the performance of asphalt pavements is also directly related to this. A detailed knowledge of microstructure is needed to understand the physico-chemistry of bitumen, which can serve as the direct link between the molecular structure and the rheological behaviour (Lesueur et al., 1996; Loeber et al., 1998). Optical microscopy techniques have been employed for more than three centuries to study the microstructure of materials (e.g. Baker, 1742). Researchers (Lu & Redelius 2005; Hesp et al., 2007) used optical microscopy in asphalt field to have a better understanding and visualization of bitumen microstructures. However, because of the opacity and adhesive properties of bitumen, optical microscopy has not received much attention from the asphalt industry. To overcome some of the limitations of optical microscopy, researcher in the asphalt field have chosen to use scanning probe microscopy such as the atomic force microscopy (AFM). AFM is capable of measuring topographic features at atomic and molecular resolutions as compared to the resolution limit of optical microscopy of about 200nm. Moreover, the AFM has the advantage of imaging almost any type of surface which opens the window for investigating

In an effort to enhance the understanding and characterization of bitumen microstructure, atomic force microscopy (AFM) techniques have been employed to bituminous materials over the last 15 years. Typical image obtained from AFM scanning is depicted in Fig. 1,

Loeber et al. (1996, 1998) was the first research group who published a inclusive investigation of bitumen using AFM. In these studies, bitumen samples were prepared by a heat-casting method to preserve the solid-state morphology. They revealed ripple microstructures with several micrometers in diameter and tens of nanometers in height. The authors named those rippled yellow and black strips microstructures ''bumble bees'' and attributed them to the asphaltenes (Redelius, 2009). Other shapes and textures including

Based on Loeber's findings, Pauli et al. (2001) wanted to find the correlation between the ''bumble bee'' shaped microstructures and the amount of asphaltenes in the bitumen. In this study, they investigated solvent-cast films of bitumen under AFM using both friction-force and tapping mode. From these, they found the same Loeber's bee-shaped microstructures. To confirm that the bee-shaped structures were asphaltenes, they doped bitumen with asphaltenes and observed an increase in the density of bee-shaped microstructures. Their group also observed that the microstructure disappeared after repetitive scanning over the same area (Pauli & Grimes, 2003) and assumed the reason for this phenomenon was due to

A research group from Vienna University of Technology (Jäger et al., 2004), investigated five different types of bitumen and also reported randomly distributed bee-shaped structures, which, just like the previous researchers they related to the asphaltenes in the bitumen.

the red laser light of the AFM which produces heat and softening of the surface.

where one can easily observe the existing of microstructures in the bitumen matrix.

in understanding of the controlling parameters.

microstructures of different polymers and wax modified bitumen.

networks and spherical clusters were also observed in the study.

**2. Asphalt under the AFM – historic overview** 

Fig. 1. Topographic 2D (left) and 3D (right) AFM image of bitumen indicating evidence of microstructures

Based on the obtained AFM images from non-contact mode and pulsed-force mode, four different material phases (i.e., hard-bee, soft-bee, hard-matrix and soft-matrix phase) were identified. However, they could not distinguish between soft-bee and soft-matrix phase as the relative stiffness for the two phases were more or less the same. After investigating the bee-shaped structures in the 5μm scale, the authors reported that the distance between the higher parts of the bees was approximately 550nm and appeared to be independent of the source of bitumen.

After this, Masson et al. (2006) conducted extensive AFM studies on the microstructure phases in bitumen by using 12 different types of Strategic Highway Research Program (SHRP) bitumen. In this study, bitumen samples were prepared by heat casting films onto glass plates. By using AFM phase detection microscopy (PDM), the detected four phases were defined as catanaphase (bee-shaped), periphase (around catanaphase), paraphrase (solvent regions) and salphase (high phase contrast spots), which were similar to those reported by Jäger et al. (2004). Interestingly, the researchers, for the first time, found very poor correlation between the asphaltene content and the bee-shaped structures. Furthermore, no correlation was found between the PDM results and the SARA fractions (Ssatutares, A-aromatics, R-resins, A-asphaltenes) or acid-base contents of the observed bitumen. In addition, the area ratio of the catanaphase appeared to correlate with the mass parent of Vanadium and Nickel metals for several samples. The authors also reported different microstructures of the same bitumen samples prepared by heat casting and solvent casting. Continuing their study to bitumen stiffness at low temperatures under cryogenic AFM (Masson et al., 2007), at temperatures -10°C to -30°C, they showed that not all the bitumen phases contracted equally. The reason for this different contraction could be that the catana, peri and para phases were related to the existence of domains with different glass transition temperatures. One of the key findings of this low temperature AFM study was that bitumen with multiple phases at room temperature are never entirely rigid, even when well below the glass transition temperature.

De Moraes et al. (2009), studied the high temperature behaviour of bitumen using AFM at different temperatures followed by different rest periods. The authors observed that phase

Atomic Force Microscopy to Characterize the Healing Potential of Asphaltic Materials 213

depends on the thermal or mechanical energy input. Recently, based on this phase separation phenomena under certain kinetic conditions, Kringos et al. (2009a) developed a healing model by assuming the bitumen matrix has two types of phases (i.e., phase α and

parameters

From mechanical considerations, it is known that the interfaces between two materials with different stiffness properties serve as natural stress inducers. This means that when the material is exposed to mechanical and or environmental loading, these interfaces will attract high stresses and are prone to cracking. On this scale, this would result in a crazing pattern, which can be detected on a higher (macro) scale by a degradation of the mechanical properties of the material, such as the stiffness or fracture strength. If this process would continue, these micro-cracks (or crazes) would continue developing, start merging and

A finite element simulation done by Kringos et al. (2012) demonstrated the concept of diminished response and the introduction of high stresses in an inhomogeneous material, as shown in Fig. 3, for a constant displacement imposed on a homogeneous and an inhomogeneous bitumen. The bituminous matrix is hereby simulated as a visco-elastic material and the inclusions as stiff elastic. From the deformation pattern it can be seen that the inhomogeneous material acts not only stiffer, but is no longer deforming in a smooth,

From the bitumen AFM scans it was shown that many bitumen sapmles, under certain circumstances, will form such inhomogeneities. If then, by changing the thermodynamic conditions of the material by inputting thermal or mechanical energy, these inclusions rearrange themselves or disappear; restoration of the mechanical properties would appear on a macro-scale. Since the phase-separation is occurring on the nano-to-micro scale, the interfaces between the clusters and the matrix could start crazing when exposed to (thermo) mechanical loading. A change of these clusters, either by rearranging themselves or by merging into the main matrix, would then lead to a memory loss of these micro-crazes and

uniform, manner and high stresses appear from the corners of the stiffer particles.

C

C

phase β), as shown in Fig. 2.

AFM evidence

**3.1 Postulated healing mechanism** 

finally form visible cracks.

Fig. 2. AFM evidence of phase separation in bitumen (Pauli et al., 2011)

contrast and topography images were highly dependent on storage time and temperature. This research group also observed the bee phase, a soft matrix phase surrounding the bee phase and a hard matrix dispersed on the soft phase, similar to findings reported by Jäger et al. (2004) and Masson et al. (2006). The authors also observed that the bee-shaped structure completely disappeared for samples at temperatures higher than 70°C and upon cooling to 66°C they began nucleating.

Concurrently, the changing microstructure after aging of bitumen was studied using AFM by Wu et al. (2009). In these studies, both neat and SBS polymer modified bitumen was aged using a pressure aging vessel (PAV). Comparing the obtained images before and after aging, the authors reported that the bee-shaped structure significantly increased after aging. To support the observed phenomena, the researchers related this to the production of asphaltene micelle structures during the aging process. These findings are consistent with the study by Zhang et al. (2011).

Tarefder et al. (2010a, 2010b) studied nanoscale characterization of asphalt materials for moisture damage and the effect of polymer modification on adhesion force using AFM. They observed wet samples always showed higher adhesive force compared with dry one. The authors concluded that the adhesion behaviour of bitumen can vary with the chemistry of the tip and functional groups on it.

Recently, Pauli et al. (2009, 2011) reported that all of these interpretations, including their previous findings were at least partially wrong and came up with a new hypothesis in which they stated that the ''bees are mainly wax". To prove their hypothesis, they scanned different fractions of bitumen and found bee-shaped structures even in the maltenes which contain no asphaltenes, while the de-waxed bitumen fractions did not show any microstructures. The authors concluded that the interaction between the crystallizing paraffin waxes and the remaining bitumen are responsible for much of the microstructuring, including the well-known bee-shaped structures. They also reported sample variables such as film thickness, the solvent spin cast form, and the fact that solution concentration could also strongly influence the corresponding AFM images.

Even though different research groups concluded significantly different reasons for the structures to appear, the extensive atomic force microscopy (AFM) studies showed that bitumen has the tendency to phase separate under certain kinetic conditions and is highly dependent on its temperature history. Besides focusing on the reasons behind the microstructure growth, it is also important to relate these microstructures to the performance of bitumen.
