Laser-Induced Fluorescence Studies of Oxygen Transfer Across Unsheared Flat and Wavy Air−Water Interfaces

Oxygen transfer across unsheared flat and wavy air−water interfaces is studied using a laser-induced fluorescence (LIF) technique. The LIF technique measures the dissolved oxygen (DO) concentration through oxygen quenching of the fluorescence from dissolved pyrene butyric acid (PBA). Fluorescence intensities in a two-dimensional plane in the water near the interface are imaged (instantaneously) with a high-resolution CCD camera. A series of image-processing and image-analysis steps corrects the intensities for variations in the fluorescence attributed to factors other than oxygen quenching and transforms the corrected fluorescence intensities into spatial measurements of the DO concentration. Near-interface vertical oxygen concentration profiles and two-dimensional concentration fields result. Estimates were obtained for the concentration boundary layer thickness, δc, and the mass transfer coefficient, k l, along flat and wavy interfaces. A severalfold increase in mass transfer rates is observed for an unsheared wavy air−water interface. The concentration field measurements suggest that this is attributed to a thinner δc and the presence of detached concentration layers.