Visualization of oxygen concentration fields and measurement of concentration gradients at bubble surfaces in surfactant-contaminated water

Abstract

A laser-induced fluorescence (LIF) technique is employed for visualizing a thin two-dimensional (2D) dissolved oxygen concentration field and measuring local oxygen concentration gradients near the surface of an oxygen bubble in water containing surfactant (Triton X-100, SigmaAldrich, St Louis, MO, USA)). The fluorescence of pyrene butyric acid (PBA) is induced by a planar pulse of nitrogen laser light. Oxygen transferring from the bubble to the deoxygenated water quenches the fluorescence of the PBA. Images of the fluorescence fields are captured by a UV-intensified CCD camera. The intensity of fluorescence quenching at each image pixel is used to measure dissolved oxygen concentration in a 2D field. Images of bubbles are obtained at 200 ppm, 100 ppm, and 50 ppm Triton X-100-containing water and in ultra clean deionized water. Higher surfactant concentrations decrease local and average concentration gradients of oxygen at the bubble surface. The ensemble means of dissolved oxygen concentration boundary layer thicknesses of 0.160 mm, 0.130 mm, and 0.072 mm, for the images of bubbles obtained at 200 ppm, 100 ppm, and 50 ppm Triton X-100-containing water, respectively. Local concentration boundary layer thickness increases from the top to the bottom along the bubble surface. A series of images of the bubble flow fields are analyzed to measure the oxygen concentration gradients in water in the presence of surfactant. The images captured in clean water are not fully resolvable because of their poor resolution. The formation of the attached wake in the fluorescence field images at the bottom of the bubbles in clean water tends to be promoted by increasing oblateness owing to the presence of surfactant at the surface.