Deformation characteristics of compound droplets with different morphologies during transport in a microchannel

A numerical investigation of the deformation of compound microdroplets transported inside a circular microchannel is described in this article. Two droplet morphologies are considered (shell-core and Janus), which correspond to nonequilibrium and equilibrium states, respectively, based on the balancing of the three interfacial tensions at the triple line. Numerical simulations coupled with a three-phase volume-of-fluid method are performed on axisymmetric models to consider both the absence and presence of a triple line. In addition to adaptive mesh refinement on the interfaces, topology-oriented refinement is used to resolve thin films between the shell and core droplets. After experimental validation, the effects of flow rates, physical properties, and confinement conditions are considered. In the reference frame of the droplets, there are five inner vortexes inside the shell-core droplet, while only three are present inside the Janus droplet, the same as single-phase droplets. For shell-core droplets, the aspect ratio of the shell droplet decreases with the capillary number of the continuous phase and droplet sizes, while sudden jumps are identified when the thin film forms between the shell and core interfaces. Conversely, the aspect ratio of the core droplet increases and then decreases when the shape of the core droplets is influenced by the flow and space confinements. With Janus droplets, the aspect ratio decreases with the capillary number. The axial length of the front portion decreases with the capillary number and then reaches a plateau with small variations, while that of the rear portion increases nearly linearly.