Mass transfer from a bubble to the surrounding liquid plays an important role in chemical engineering processes. To improve the efficiency and safety of the processes, a deep understanding of the mass transfer mechanism from bubbles to the surrounding liquid is essential. In the present study, we examined a CO2 single bubble of 2∼3 mm in equivalent diameter that ascended zigzag in purified water and contaminated water (500ppm 1-pentanol solution). We used a high speed video camera systems with high spatial and temporal resolution, for visualization of the bubble wake and bubble-induced surrounding liquid motion.

The dissolution process of CO2 from the bubble to the surrounding liquid was visualized via LIF/HPTS (Laser Induced Fluorescence) method. HPTS, which is a fluorescent substance, was excited by Ar ion laser with a wavelength of 458 nm, then emitted with a wavelength of 513 nm. A pH level of CO2 solution decreased with increase in CO2 concentration; hence the emission intensity of HPTS was reduced. As a result, dark regions observed below the bubble rear accorded with the bubble wakes; from visualization of this bubble wakes through the high speed video cameras, dynamic CO2 dissolution process was obtained. In the purified water, the bubble shape was oblate ellipsoid, and horse-shoe-like vortices were formed in the rear of the bubble. On the other hand, in the contaminated water, the bubble was nearly spherical. Furthermore, behavior of the vortices changed. These different results in two conditions were caused by the decrease in the surface tension owing to the bubble surface contamination. While the bubble was rising, the non-uniform distribution of the surfactant on the bubble surface occurred. Hence, a gradient of the surface tension was formed on the bubble surface, furthermore, it caused the Marangoni convection.

Meanwhile, in order to consider the relationship between dissolution process and the surrounding liquid motion, we measured the liquid phase velocities via PIV.

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