Coalescence is a significant fundamental issue in chemical engineering process, which appeals to scientists for a long time. Random collision usually made it hard to investigate the basic rules of droplet/bubble coalescence in flows for the uncontrolled collision results. In recent years, microfluidics shows its potential in droplet and bubble manipulation. So that the investigations of droplet/bubble coalescence under laminar condition can be well implemented in microchannels . Cross-shaped microchannel was effective for the research on head-on collision ; Y-junction microchannel can be used to study the effect of collision angle [3,4], and the expansion microchannel was useful for the study of coalescence in chasing flow . Experimental results showed that the regular collision results can be divided to absolute coalescence, probabilistic coalescence and non-coalescence. Critical capillary number (Ca*) was important for distinguishing coalescence and non-coalescence. For the water/alcohol (with alkyl chains varying between 5 and 10 carbon atoms in molecule) systems, Ca* is about 0.005 in head-on collision processes. Investigations showed that the critical capillary number varied with droplet size, collision angle and physical properties . Furthermore, the microfluidic platform was fit for the investigation on the liquid-liquid systems containing nanoparticles . Model equations were proposed to predict the critical capillary numbers based on the droplet contact time and liquid film drainage time, which may provide more valuable information for understanding the basic laws of coalescence phenomenon.
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