Spreading rate and dispersion behavior of evaporation-suppressant monolayer on open water surfaces: Part 2 - Under wind stress

Wind causes migration and eventual removal (dispersal or beaching) of evaporation-suppressing monolayer on open-water storages. Hence, an autonomous system capable of adaptive re-application of monolayer according to the prevailing wind conditions is highly desirable. Key to the design and functioning of such a system is a fundamental understanding of the spatial movement/distribution characteristics of the monolayer material. To 'bridge' between centimeter-scale, clean room laboratory experimentation (e.g. Petri dish-scale in a wind tunnel) and field conditions (i.e. hectare-scale open-water storages), the drift velocity and spreading behavior of C18OH monolayer (in water-emulsion), applied continuously during constant wind stress, were investigated on a 6 m-diameter indoor water tank for wind speeds in the range 4-8 m/s. Monolayer was found to spread in a teardrop shape initially, which evolved into a wedge shape whose close-to-straight edges were detectable visually due to the wave-damping effect of the monolayer. The internal angle of the wedge decreased with increasing wind speed, consistent with the force equilibrium between the lateral force of the monolayer spreading outwards and the increasing shear imposed with increasing wind speed. The relationship between internal angle of the wedge and wind velocity was a power law. The widely-accepted spreading kinetics formula was used to derive an empirical relationship for the drift velocity that is a power law with respect to the wind speed. This model was compared with the experimental data, with a modest degree of agreement.