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.