Collision Behaviors of Two Successive Compound Droplets in an Abrupt Expansion Microchannel

In the present paper, merging of two successive compound droplets in an abrupt expansion microchannel using direct numerical simulations is presented. The compound droplets undergo deformation and velocity decreases when entering the expansion region. Their interaction behaviors are divided into two modes of merging and non-merging. These two modes are dominated, and influenced by fluid dynamic parameters, compound droplets’ center distance, the expansion ratio of microchannels and the size of compound droplets, which are analyzed through the results of numerical simulation. The capillary number, the fluid viscosity, and the droplets’ distance increase lead to the merging time of the droplets increases. Although increasing the inner interfacial tension does not significantly affect the merging time of two outer droplets, it significantly reduces the merging time of two inner droplets. Meanwhile, varying the expansion ratio and the droplet size results in the transition between the two interaction modes. Two diagrams for the mode transition, based on the capillary number, the droplet center separation, and the droplet size are also given.