Numerical study of head-on collision of two equal-sized compound droplets

Although on-axis collisions between compound droplets are involved in numerous technological applications, no detailed investigation of such collisions is yet available. To address this problem, the present work uses an axisymmetric front-tracking method to numerically explore the dynamics of on-axis collisions of compound droplets that contain one or more inner droplets. Two identical droplets are placed symmetrically on the midplane of a computational domain and made to make contact with an initial colliding velocity. Various parameters such as the Reynolds number Re, the Weber number We, the size of the inner droplets, the interfacial tension ratio, and the eccentricity are considered. Three primary outcomes are observed: complete coalescence (CC), outer coalescence (OC), and rebound (R) for Re = 4–256 and We = 1–128. CC is when both the inner and outer droplets coalesce, whereas OC is when only the outer droplets coalesce. R is when the droplets come into contact and then bounce back. Increasing Re or decreasing We enhances the CC pattern, as does increasing the size of the inner droplets or the interfacial tension ratio. The influence of the initial distance between the droplets is also investigated. Finally, regime diagrams related to these patterns of collision are also presented.