Morphology and kinetics of asphalt binder microstructure at gas, liquid, and solid interfaces

We combined optical and atomic force microscopy to observe morphology and kinetics of microstructures that formed at free surfaces of unmodified pavement-grade 64-22 asphalt binders upon cooling from 150 $^{\circ}$C to room temperature (RT) at 5 $^{\circ}$C/min, and changes in these microstructures when the surface was terminated with a transparent solid (glass) or liquid (glycerol) over-layer. The main findings are: (1) At free binder surfaces, wrinkled microstructures started to form near the wax crystallization temperature ($\sim$45 $^{\circ}$C), then grew to $\sim$5 $\mu$m diameter, $\sim$25 nm wrinkle amplitude and 10-30$\%$ surface area coverage upon cooling to RT, where they persisted indefinitely without observable change in shape or density. (2) Glycerol coverage of the binder surface during cooling reduced wrinkled area and wrinkle amplitude three-fold compared to free binder surfaces upon initial cooling to RT; continued glycerol coverage at RT eliminated most surface microstructures within $\sim$4 hours. (3) No surface microstructures were observed to form at binder surfaces covered with glass. (4) Sub-micron bulk microstructures were observed by near-infrared microscopy beneath the surfaces of all binder samples, with size, shape and density independent of surface coverage. No tendency of such structures to float to the top or sink to the bottom of mm-thick samples was observed. (5) We attribute the dependence of surface wrinkling on surface coverage to variation in interface tension, based on a thin-film continuum mechanics model.