The effect of a surfactant monolayer on the temperature field of a water surface undergoing evaporation

The surface temperature field of a body of water undergoing evaporation was measured using infrared imaging techniques, demonstrating for the first time the effect of surfactant monolayers on the spatial structure of this field. Measurements were obtained from a water surface which was covered with a monolayer of the surfactant oleyl alcohol, and also from a surface which was free of surfactants. The oleyl alcohol and surfactant-free experiments were compared at equivalent heat fluxes. The presence of surfactants increased the characteristic length scale of the surface temperature field. This conclusion is supported by both visual observation of the infrared imagery and spatial Fourier transforms of the temperature fields. The presence of the surfactant monolayer had a small effect on the root mean square of the temperature field but significantly affected the skewness, creating a more positively skewed probability density function for the surfactant covered field. These observations were found to hold when comparison between the clean and surfactant case was made at heat fluxes varying by a factor of ∼11.