Flow structure of compound droplets moving in microchannels

Compound droplets can be used in substance encapsulation and material compartmentalization to achieve a precise control over the relevant processes in many applications, such as bioanalysis, pharmaceutical manufacturing, and material synthesis. The flow fields in compound droplets directly affect the performance of these applications, but it is challenging to measure them experimentally. In this study, the flow in compound droplets in axisymmetric microchannels is simulated using the finite volume method, and the interface is captured using the level set method with surface tension accounted for via the ghost fluid method. The combination of the level set method and the ghost fluid method reduces spurious currents that are produced unphysically near the interface and achieves a precise simulation of the complex flow field within compound droplets. The shape of compound droplets, the vortical patterns, the velocity fields, and the eccentricity are investigated, and the effects of the key dimensionless parameters, including the size of the compound droplet, the size of the core droplet, the capillary number, and the viscosity ratio, are analyzed. The flow structures in multi-layered compound droplets are also studied. This study not only unveils the complex flow structure within compound droplets moving in microchannels but can also be used to achieve a precise control over the relevant processes in a wide range of applications of compound droplets.