Development of an energy-based framework to determine the surface free energy of asphalt binder: theoretical models

The surface free energy of asphalt binder is crucial to quantify the adhesion between asphalt and aggregates, especially when an asphalt mixture is exposed to moisture. Most of the current methods use contact angles to determine the surface free energy of asphalt binder. However, the conventional contact angle test methods have limitations in either experimental design or theoretical derivation, which compromise the accuracy of the determined surface free energy. In order to fill this research gap, this paper develops an energy-based framework on the basis of energy balance principles, which can be adopted as an alternative to determine the surface free energy of asphalt binder. In this framework, three energy balance models are derived for the three commonly-used contact angle test methods, including the Wilhelmy Plate method, the Sessile Drop method, and the Tilting Plate method. Based on the developed models, the contact angle between asphalt-liquid can be calculated from the Wilhelmy Plate model or measured directly from the Sessile Drop test and the Tilting Plate test. Using the calculated or measured contact angles, an equation set can be established based on the Young-Dupre equation to solve for the surface free energy of asphalt binder.