Transportation behaviors of double emulsion droplets under the influence of varied interfacial tensions

The transportation behaviors and dynamic characteristics of double emulsion droplets in a Y-junction are experimentally investigated to explore the influence of the interfacial tension. Based on the breakup behaviors, three flow patterns are identified: non-breakup (NB), once breakup of the outer droplet (OB), and twice breakup of the outer droplet (TB). Compared to the single emulsions, the inner droplet leads to new flow patterns for the double emulsions due to the coupling effect between interfaces. The flow pattern map can be built using the normalized droplet length and the capillary number. With the aid of the quantitative expression of the transition thresholds using a power law relation, the transitions are further found to rely on the junction structure and the physical properties of the double emulsion. It is indicated that the dynamic characteristics of double emulsions are shaped jointly by multiple controlling parameters, such as the droplet length, the capillary number, and the junction structure. A force model is proposed to estimate the quantitative evolutions of the working forces in different flow patterns. The evolution of the droplet morphology is consistent with the forces acting on it, and the prerequisite condition for the droplet breakup is confirmed to depend on the competition between the sum of the driving forces and the sum of the interfacial tensions. The variation of the transition thresholds in different liquid systems is further analyzed, which may provide useful methods to manipulate the double emulsions by changing the physical properties.