  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.
