Visualisation of Oxygen Concentration Profiles in the Aqueous Boundary Layer

TitleVisualisation of Oxygen Concentration Profiles in the Aqueous Boundary Layer
Publication TypePhD Thesis
Year of Publication2007
AuthorsFalkenroth, A
UniversityFakultät für Chemie und Geowissenschaften, Univ.\ Heidelberg

In environment studies as well as for technical application, the study of air-water gas exchange is crucial. For process studies, a novel visualisation technique of oxygen concentrations in water was realised with high spatial resolution. To resolve turbulent processes in water, also the temporal resolution was pushed to the limit of a imaging frame rate of 185 Hz. For this purpose, the well-established method of laser-induced fluorescence (LIF) was extended introducing in this type of studies a new phosphorescent ruthenium dye that is more than 15 times more sensitive to oxygen than the previously used indicator dye. The chemical synthesis of this metal-ligand complex MLC was adapted to a preparation without intermediate steps. The challenge of this imaging technique for small-scale interactions was to resolve a very thin boundary layer extending less than a millimetre below the water surface. An image processing algorithm was developed that allow the automatic detection of the exact location of the air-water phase boundary within the resolution of 25 um/pixel. Only by this step, an accurate direct determination of an important parameter for gas-exchange studies, the boundary-layer thickness, is feasible. The developed methods were applied to systematic gas-transfer measurements mostly with surfactants, conducted in a range of wind speeds between 0.8-7 m/s in a circular wind-wave facility. The measured gas-transfer velocities compared extremely well to exchange rates derived from mass-balance methods. The novel visualisation technique drastically increased the poor signal quality inherent to standard LIF techniques. This enabled accurate measurements of gas-transfer velocities from aqueous concentration profiles for the first time.

Citation Keyfalkenroth2007