Early Interfacial Vortex when Drop Coalesces with a Curved Interface

The coalescence of a drop with a pool is widely observed in daily life, such as raindrops merging with a puddle and drops of liquid condiments being added into soup. Due to capillary-driven motions along the large curvature of the free interface, there is usually an axisymmetric vortex ring generated which travels into the pool. However, the early strength of the original interfacial vertical layer remains an open question. Using high-speed imaging with a long-distance microscope we catch the early contact and coalescence followed by the separation of the interfacial vorticity from the curved free surface in the neck between the drop and pool. Seeding the liquid with micron-size particles we apply particle image velocimetry with a thin laser sheeting to characterize the vorticity evolution. The neck between the pool and drop is pulled up above the initial pool surface and we find the early progenitor of the vortex layer separating from the free surface inside the drop, less than 100 μs after the first contact. The strength of the initial vorticity is measured at least 30000 1/s, which is larger than the peak observations in previous literature, after the ring starts traveling into the pool. These findings support the origin of the vorticity in the vortex ring.