The effect of a surfactant monolayer on oxygen transfer across an air/water interface during mixed convection

A set of laboratory experiments are presented that reveal the effect of a surfactant monolayer on the transport of oxygen across an air/water interface during mixed convection conditions. The experiments were conducted in a wind/water tunnel where forced convection was imposed via the air flow, and natural convection by heating the water bulk above the air temperature, resulting in mixed convection conditions. The data acquired during these experiments were used to develop a parameterization between the Sherwood number for oxygen transport Sh and the Reynolds Re and Rayleigh Ra numbers. This parameterization was obtained for the case of a clean water surface and for a water surface covered with a surfactant monolayer. The data reveal that, at a given Ra and Re, the presence of a surfactant monolayer reduces Sh by approximately one order of magnitude. The elasticity of the air/water interface which results from the presence of a surfactant is used to explain these results. The data also show that Sh was not sensitive to Re. For both the clean and surfactant cases, Sh increases with Ra.