Skin Formation During the Drying of a Bitumen Emulsion

Abstract

ABSTRACT This article presents a study of skin formation at the free surface of a drying bitumen emulsion. The presence of a skin slows down the evaporation process and may even stop it. High initial bitumen content in the emulsion and fast drying rates are shown to favor skin formation. The effect of mineral fillers is also studied and it is shown that depending on the amount of cations released, the process of skin formation may be greatly accelerated. This may explain former results of very slow drying of slurry seals made with limestone aggregates. A simple theoretical argument is presented to explain skin formation as a consequence of the competition between an advancing drying front that accumulates particles at the surface, and Brownian diffusion that tends to homogenize particle concentration throughout the drying emulsion. Therefore, particle size and drying rates are the governing parameters. Finally, practical consequences are discussed, and it is concluded that too fast drying rates can be detrimental in applications such as surface dressings.