Interaction between two falling droplets in the liquid–liquid two-phase flow

The interaction between two falling droplets in the liquid–liquid two-phase flow has been investigated in detail with the aids of the index matching technique and PIV. It has been almost impossible to measure or even visualize the multiple phases simultaneously. This is because of the difference in the refractive indices of each phase. In this study, the refractive index of the water phase has been equalized to that of the oil phase by adjusting the concentration of the aqueous solution of glycerol and thus the simultaneous visualization of both phases and also the PIV measurement have been carried out. Presently, the interaction between two falling droplets has been intensively investigated. The interaction between two solid particles has been well known by the name of drafting–kissing–tumbling (DKT) motion. Similar motion to DKT is also observed for the droplets, which are found not to really kiss. It is found that the flows inside of the droplets are also affected by the interaction between two droplets while one droplet overtakes the other. The conditions for the initial relative positions of the two droplets to overtake have been summarized. It has been made clear that the chasing droplet is pulled by the wake of the preceding droplet.