Experimental Study on Double Emulsion Breakup Characteristics at a Microfluidic Y-Junction.

The dynamic mechanisms of double emulsions transporting through a microfluidic Y-junction are experimentally studied. Typical flow patterns are categorized according to the interfacial characteristics. Transition rules under the influence of different parameters, including the normalized droplet lengths and the capillary number, are discussed. The evolutions of the interfacial parameters are analyzed to reveal the dynamics. More influencing parameters and transition rules are confirmed for double emulsions, compared to conventional single emulsions. Furthermore, the coupling interaction between interfaces is found to affect the breakup threshold. Flow pattern maps are built using the two droplet lengths to indicate the occurrence domain of the coupling interaction. To explain the evolution tendency of the upper transition line of the coupling interaction with influencing parameters, the breakup time is explored. Expressions of the breakup time in different ranges of droplet lengths are built, and the relation of the transition line is further obtained. It is found that the relation perfectly separates the two different regimes, with or without coupling interaction, at all tested capillary numbers, indicating the generality of the built relations. With the aid of the relation, evolutions of the transition line with the droplet lengths and the capillary number are explained.