Breakup of compound jets with inner droplets in a capillary flow-focusing device

The coupling between the multilayer interfaces in compound jets has notable effects on the structure and generation sequence of the formed double emulsions. These effects are important for the performance of double emulsions, such as the capacity, release rate, and controlled release threshold in medical and chemical applications. In this work, the influence of the inner droplet on the necking of compound jets is investigated in a horizontally placed capillary flow-focusing device based on microfluidics. Three types of interface coupling modes are explored. Scaling laws that describe the time evolution of the neck radius for these different coupling modes are analyzed, and the reasons for transitions between such scaling laws are discussed. The results show that the motion and deformation of the droplet have a large impact on the neck breakup in the inertial regime, causing the scaling law to change, but only a slight effect in the viscous regime. Moreover, the inner droplet can prevent the jet from breaking up owing to interface coupling. These findings could help us to understand the role of interface coupling in compound jets and provide a reference for controlling the generation of compound droplets.