Atomic Force Microscopy Reveals How Ageing Rewrites Asphalt at the Nanoscale

Beneath every smooth highway surface lies a material whose fate is decided at scales far smaller than any driver will ever see. Asphalt binder, the glue that holds roads together, carries within it a microscopic landscape of ridges and domains that shifts as the material ages. Now a research team from the Politecnico di Torino has shown that how scientists prepare and probe that landscape can change what they measure, a finding published in Case Studies in Construction Materials that helps explain why decades of atomic force microscopy studies on bitumen have produced conflicting results.

The study, led by Sadegh Yeganeh, Reza Salehfard, Davide Dalmazzo and Ezio Santagata, examined two asphalt binders of different penetration grades, 70–100 and 40–60, and subjected them to a spectrum of ageing treatments. Using the Rolling Thin Film Oven for 85, 170 and 340 minutes, plus the Pressure Ageing Vessel for long-term ageing, the team produced binders spanning a wide range of chemical and colloidal states. They then interrogated each binder with atomic force microscopy, or AFM, a technique that drags a nanoscale silicon tip across a surface to map its topography and, in force-distance mode, to measure how stiff and sticky that surface is at the nanoscale.

AFM studies of bitumen have long revealed a distinctive morphology known as bee structures, alternating ridges and valleys that form the catana phase, surrounded by a solid-like peri phase and embedded in a matrix called the para phase. But whether ageing enlarges, shrinks or leaves these structures untouched has been hotly disputed. Some researchers report that prolonged ageing increases the length and area of bee structures, others report the opposite, and still others see no effect at all. Similarly, the effect of ageing on surface adhesion has been reported to decrease, increase or fluctuate depending on the study. The new work suggests that much of this inconsistency may stem from differences in sample preparation and probe selection rather than from the binders themselves.

To test that hypothesis, the team adopted two protocols that differed simultaneously in sample geometry and probe stiffness. Protocol 1 poured roughly two grams of hot binder into a metal container 25 millimetres in diameter and three millimetres thick, producing a relatively thick sample probed with a soft cantilever of 2.8 newtons per metre spring constant. Protocol 2 spread just 0.04 grams on a glass plate, yielding a thinner specimen probed with a stiffer 6 newtons per metre cantilever. Both were quenched at three degrees Celsius and stored for 24 to 36 hours before testing. The team also characterised the bulk binders chemically using thin-layer chromatography with flame ionization detection to quantify saturates, aromatics, resins and asphaltenes, the SARA fractions, and Fourier transform infrared spectroscopy to track functional groups such as carbonyls and sulfoxides that form during oxidative ageing.

The chemical analysis alone produced striking results. The aromaticity index, which rises as nonpolar components convert to aromatic structures, barely changed during the standard 85-minute short-term ageing but climbed significantly with prolonged RTFO treatment, suggesting that aromatization is mainly a feature of extended ageing rather than its initial stages. Carbonyl and sulfoxide indices, markers of oxidation, increased with ageing severity, with the softer 70–100 binder showing higher carbonyl values across all conditions, attributed to better dispersibility of its asphaltenes. The long chain index, linked to crystalline wax phases, initially rose and then fell with increasing RTFO duration, peaking at a much higher value in the 70–100 binder than in the stiffer 40–60 grade.

Under the microscope, the two protocols produced visibly different landscapes. Protocol 1 samples showed rougher, less homogeneous surfaces with larger bee structures and occasional optical interference fringes caused by the probe sticking on high-roughness regions. Protocol 2 samples appeared smoother and more uniform, with terraced patterns around the bee structures absent. The team attributes the larger bee structures in protocol 1 to the greater sample thickness: during thermal conditioning, paraffin wax molecules diffuse from the bulk to the surface, and a thicker sample transports more crystalline material to the surface, enlarging the observed domains. Because the two protocols varied sample geometry, probe stiffness and cooling history simultaneously, the researchers caution that the observed differences represent the overall response of the adopted protocols rather than the isolated effect of any single experimental factor.

Perhaps the most compelling result came from correlating the AFM-derived bee structure parameters with the chemical indices. When data from both binders across all ageing states were pooled, protocol 2 consistently produced the strongest and most statistically significant correlations with SARA fractions and colloidal indices. Bee size correlated best with the Gaestel index, while bee coverage correlated best with the asphaltene index and aromatics content. These findings support the hypothesis that asphaltenes form the convex part of the bee structure while aromatics constitute its flatter segment, tying the microscopic morphology directly to the colloidal chemistry of the binder.

The team also derived a new quantity, the instability index, defined as the ratio of catana phase area to peri phase area from AFM phase images. This quantity correlated remarkably strongly with the Gaestel index calculated from bulk chemistry, with a Pearson coefficient of 0.94 across ten pooled samples. In other words, the balance between the asphaltene-rich catana phase and the peptizing peri phase, visible only under the AFM, mirrors the colloidal instability that chemists measure from SARA fractions. The researchers suggest this makes the instability index a promising tool for assessing colloidal stability, though they note the conclusion rests on only two binder sets and needs validation across a more diverse range of materials.

Micromechanical measurements added further nuance. Adhesion force, the pull-off force required to detach the tip from the surface, peaked at intermediate ageing severity for both binders, consistent with the formation of carbonyl groups that create hydrogen bonds with the silicon probe. At higher ageing intensities, large crystalline structures form in the binder, reducing adhesive force. The adhesion force obtained from both protocols correlated strongly and negatively with the aliphatic index, with coefficients around negative 0.86, confirming that higher polarity, indicated by fewer aliphatic compounds, produces better adhesion performance. The DMT modulus, a stiffness measure derived from the Derjaguin-Muller-Toporov model, generally increased with ageing intensity under protocol 1, tracking the rise in polar fractions and the macroscale storage modulus. But under protocol 2, several exceptions appeared at the most severe ageing conditions, where the modulus actually decreased, likely because the stiffer probe and the selected peak force of two nanonewtons produced insufficient indentation in the hardest binders.

The broader lesson is one that extends well beyond road engineering. When a measurement technique is sensitive to sample preparation and probe characteristics, the resulting literature can fragment into contradictory camps, each reporting genuine observations that reflect different experimental conditions rather than different materials. By systematically comparing two protocols and anchoring the AFM data to independent chemical characterisation, the Politecnico di Torino team has provided a roadmap for resolving those contradictions. Protocol 2, with its thinner samples and stiffer probe, emerges as the better window onto the inherent characteristics of the binders, producing stronger correlations with colloidal chemistry and avoiding the probe-sticking artefacts of protocol 1. The work also carries a practical warning for engineers: to achieve accurate modulus values, the target peak force should be adjusted based on probe type and binder stiffness, a detail that could reshape how nanomechanical testing of viscoelastic materials is conducted. As infrastructure ages worldwide, understanding how bitumen rewrites itself at the nanoscale, and how to measure that rewriting reliably, may prove as important as any macro-scale test ever devised.

Subject of Research: AFM characterization of aged asphalt binders

Article Title: Quantitative characterization of morphological and micromechanical properties of aged asphalt binders using atomic force microscopy: Insights into different testing protocols

Article References: Salehfard, R., Yeganeh, S., Dalmazzo, D., & Santagata, E. (2026). Quantitative characterization of morphological and micromechanical properties of aged asphalt binders using atomic force microscopy: Insights into different testing protocols. Case Studies in Construction Materials, 25, Article e06603. https://doi.org/10.1016/j.cscm.2026.e06603

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06603

Keywords: Quantitative, characterization, morphological, micromechanical, properties, aged, asphalt, binders, atomic, force, microscopy, Insights