Numerical study of collision modes of multi-core compound droplets in simple shear flow

Collisions of multi-core compound droplets have generated substantial interest in recent years because of their applications in the industry and academia. This study uses the front-tracking method to simulate the transition between the two collision modes of multi-core compound droplets in a simple shear flow. Compound droplets initially assumed identical have two sub-droplets of equal size. Given the shear flow, the droplets collide with one another and behave in two main modes, namely, passing-over and reversing. In the passing-over mode, the droplets pass over one another after coming into contact. The reversing mode appears with two compound droplets returning to their initial sides after the collision. During collision, the sub-droplets circulate approximately at the center of their enclosing outer droplets. Some parameters, including capillary number Ca, viscosity ratios mu(io) and mu(mo), radius ratio R-io of the sub-droplets to the outer droplets, and sub-droplet angle a(0), are investigated to determine their impact on these modes of collisions. We find that the transition from a reversing to passing-over mode occurs when we increase the value of Ca from 0.01 to 0.63}, {tiR(io) from 0.20 to 0.475, and mu(io) and mu(mo) in the range of 0.16-6.3. However, an increase in the value of a(0) between -75 degrees and 90 degrees leads to a change from a passing-over to reversing mode. Diagrams of the colliding modes are also presented in this research.