In present work, planar laser-induced fluorescence (PLIF) technique was employed to visualize the gas-liquid mass transfer accompanied by an irreversible chemical reaction inside a single droplet hanging out of a capillary. The dynamic change of fluorescence signal intensity in the droplet emitted by the fluorescence dye, Rhodamine B, was quantitatively captured by a 12-bit high-speed CCD camera (IMPERX, IPX-VGA210-L), where the concentration of the dye varies in time and space due to the liquid micro-mixing coupled with an oxidation reaction by ozone environment in the gas phase. The reaction process inside the droplet can then be recorded quantitatively, integrated the process of gas-liquid mass transfer and liquid-phase reaction.
The experiments were conducted under different conditions, i.e., a nearly stationary gas flow around the droplet with little disturbance, an uneven gas flow across the droplet with certain disturbance on the droplet, and the droplet growing process. The results implied that the mass transfer was almost entirely achieved by molecular diffusion under nearly stationary gas flow condition. The fluorescence dye in the liquid phase was oxidized by ozone gradually from the surface to the interior region of the droplet. However, as an uneven gas flow was directly exerted on the droplet, the reaction no longer proceeded from outside to inside. In order to investigate the effect of environment gas flow conditions on the reaction and mass transfer process between the gas and liquid droplet, particle image velocimetry (PIV) was employed to visualize the flow patterns inside the droplet under the different conditions mentioned above. The results showed that the gas flow which induced certain disturbance on the droplet had a significant impact on the internal flow pattern of the droplet and thus intensified the mass transfer and reaction rate. Besides, the reaction and mass transfer as well as the velocity field was measured during the droplet growing process with different growing rates. The straightforward visualization measurement illustrated a rich variety of micro-mixing behaviors with the improved understanding on the mechanisms of gas-liquid mass transfer and micro-mixing inside a droplet.