Introduction
Bubble formation and coalescence are widely encountered in the chemical process industry, petrochemical industry and mineral processing1-3. As the bubble size decreases, its surface area for a given gas volume increases and its residence time in liquid becomes longer. Thus, the use of microbubbles has attracted the attention of many researchers and engineers in chemical engineering fields covering absorption and chemisorption, bio-reaction and electrochemical systems4,5. The most common way to generate millimeter-sized bubbles is to inject gas direct into a liquid through a submerged nozzle, but microbubbles cannot be generated by that way because the diameter of produced bubbles is considerably larger than that of the injection orifice5. To decrease the bubble size, many additional techniques are used, such as adding an outer liquid stream or using acoustic fields, electric fields, and porous media6-11. Partial coalescence is a special coalescence process with the generation of smaller satellites12-16. This process not only affects the bubble size distribution in the bubble column but also provides a potential approach for generation of microbubbles. In droplet coalescence, these satellites are roughly half the original size, while in bubble coalescence they are only 10% approximately15.
The partial coalescence of bubbles was reported by Ohnishi et al.17 as early as 1999. Then, Zhang and Thoroddsen13 studied the coalescence of a sessile and a rising bubble in more detail. They found that the satellite bubble of partial coalescence was generated by capillary waves which converged at the interfacial apex and pinched off the smaller bubble. Experimental investigations13,15 further showed that the partial coalescence was affected by the ratio of size of parent bubbles, viscosity and gas density obviously. The viscosity dampened the capillary waves and prevented the pinch-off. The gas density and relative size of parent bubbles determined the speed of drainage of the coalescing bubbles, further affecting the satellite size. Partial coalescence in three-component emulsions was investigated by Li et al.18 in experiments of a bubble passage through an interface between immiscible liquids. Except the bubble size, the triple line where the three phases meet was also found to influence the coalescence dynamics considerably. In previous studies, the periodic formation of microbubbles was not seen by the partial coalescence because it was difficult to generate the periodic coalesced bubble in their experimental apparatus.
The periodic coalescence of bubbles can be achieved when injecting gas into a liquid through a capillary nozzle. Different from the nozzle of millimeter scale, a special coalescence bubbling regime was observed for the micron-sized nozzle, where some subsequent small bubbles (trailing bubbles) grew rapidly and merged into the detached larger bubble (leading bubble). This phenomenon was firstly reported by Xie et al.19 and also found by Ata et al.20, Quinn and Finch21 in experiments. Qu et al.22 and Zhang et al.23 investigated the effect of nozzle diameter and gas flow rate on this coalescence behavior with upward-pointing capillary nozzles. Pei et al.24 found that adding the microorganism in the liquid could decrease the number of the trailing bubbles coalesced into the departed bubble. Actually, the periodic generation of microbubbles through the partial coalescence was also not seen in previous studies that injected gas through an upward-pointing capillary nozzle.
In this study, experiments of gas injection in liquid through a submerged downward-pointing capillary nozzle were performed. Through comparison of visualizations, we found that the bubble formation process for downward-pointing capillary nozzle is very different from that for upward-pointing nozzle. Continuous microbubbles can be formed by means of partial coalescence in experiments with the downward-pointing capillary nozzle. However, few studies deal with the bubble dynamics for the downward-pointing capillary nozzle. Thus, the focus of the present work is mainly made on the bubbling regimes for downward-pointing capillary nozzles and mechanism for the formation of microbubbles. The effects of gas flow rate and nozzle diameter on bubble behaviors and microbubble size are also discussed with an aid of a high-speed video camera.