Molecular dynamics and energy analysis of ultrasonic cavitation effects on aged asphalt

The physicochemical degradation of asphalt during long-term service leads to pronounced aging, characterized by increased molecular aggregation, elevated viscosity, and degraded mechanical performance. Ultrasonic treatment has shown potential for reducing the viscosity of heavy oils and asphaltic materials; however, the underlying rejuvenation mechanism of aged asphalt at the molecular scale remains insufficiently understood. In this study, the interaction between ultrasonic cavitation and aged asphalt is investigated from an energy-based and molecular-level perspective using molecular dynamics (MD) simulations combined with cavitation energy analysis. A representative aged asphalt model consisting of twelve molecular species was constructed based on the SARA fraction composition of JingBo 70# asphalt, incorporating oxidation-induced functional groups (CO and SO). Intermolecular cohesion within the aged asphalt system was quantified, revealing that π-π stacking and hydrogen bonding dominate the electrostatic interaction energy that stabilizes molecular aggregates. An order-of-magnitude comparison indicates that the energy released during the collapse of a single ultrasonic cavitation bubble is sufficient to overcome the characteristic intermolecular cohesive energy within the affected volume. To examine the molecular response under ultrasonic excitation, a time-dependent ultrasonic-like external force was introduced in MD simulations at 20 kHz and 453 K. The results show that ultrasonic excitation induces periodic energy and temperature oscillations and leads to rapid bond scission at the early stage of loading. Bond rupture occurs preferentially at weaker C-C and C-S single bonds, particularly at side-chain positions and C-C bonds adjacent to carbonyl groups, resulting in molecular fragmentation and reduced aggregation stability. Complementary experimental results, including viscosity measurements and SARA fraction analysis, exhibit trends consistent with the simulated molecular disruption behavior. These findings provide an energy-based and molecular-scale mechanistic interpretation of ultrasonic treatment of aged asphalt and offer theoretical support for its potential application in asphalt rejuvenation and recycling.