Different methods of selecting probe liquids to measure the surface free energy of asphalt binders

The present study evaluates two different methods for selection of probe liquids to determine the surface free energy (SFE) of asphalt binders: (i) Condition Number (CN) of probe liquids, and (ii) plot between cosine of total SFE (ϒL Cosθ) and total SFE of probe liquids (ϒL). Five different probe liquids, i.e. Water (W), Formamide (F), Ethylene Glycol (E), Diiodomethane (D), and Glycerol (G) were selected. Overall, ten different combinations of probe liquid triplets (W-G-F, W-G-E, W-G-D, W-F-E, W-F-D, W-D-E, G-F-E, G-F-D, F-E-D, and D-E-G) were formed to examine CN and plot of ϒL Cosθ versus ϒL. Three asphalt binders, namely unmodified (VG30), SBS polymer modified binder (PMB40) and crumb rubber modified (CRMB60) binders were selected for the present study. The contact angle of asphalt binders was measured using sessile drop method, and SFE of each of the selected asphalt binders was determined for all ten combinations of probe liquids. Thereafter, CN of probe liquid triplets was determined using Singular Value Decomposition (SVD) method. In addition, ϒL Cosθ versus ϒL plot was made for triplets using the measured contact angles of probe liquids. The CN approach and ϒL Cosθ versus ϒL plot were compared based on binder's total SFE to identify a better approach for selecting appropriate probe liquid triplets. CN approach was observed to be superior compared to the plot of ϒL Cosθ versus ϒL. In fact, it was observed that the plot approach may not be valid for asphalt binders.