Binary collision of dissimilar viscosity drops

The head-on collision of identical drops culminates in coalescence or reflexive separation depending on the relative magnitude of inertia, viscosity, and surface tension forces. We use direct numerical simulation to study the head-on collision of drops having non-identical viscosity. The volume-of-fluid method-based open-source solver "Gerris" is used to perform the numerical simulations. For miscible drops, the average viscosity of the two liquids is anticipated to replicate the transition boundaries of coalescence and reflexive separation for a single fluid. However, numerical simulations reveal that this is true only for low-viscosity ratios. A high-viscosity ratio creates asymmetric flow; hence, the average viscosity does not accurately represent the local viscous effect. The asymmetric flow also facilitates the pinch-off of a thread without the separation of a satellite. The present investigation reveals that viscosity contrast leads to two additional outcomes of the head-on collision of drops: encapsulation and crossing separation. During encapsulation, the low-viscosity drop completely encapsulates the high-viscosity drop, and during crossing separation, the low-viscosity drop stretches beyond the high-viscosity drop and separates out one or more drops. Based on the numerical results, we identify the different regimes on the viscosity ratio−Weber number phase diagram.