The critical Weber number for vortex and jet formation for drops impinging on a liquid pool

When a liquid drop impacts on a pool, the drop will either coalesce into the host liquid, with the creation of a vortex ring below the surface and little splashing above, or will splash, producing a cavity in the host liquid that collapses inward, producing an upward jet of fluid. It is found that there is a critical Weber number, Wec=U(ρD/T)1/2≂8, below which vortex rings are formed and above which a jet is produced for drops falling into the identical fluid. Here, U is the drop speed at impact, D is the drop diameter, and ρ and T are density and surface tension, respectively. The Weber number criterion is compared with experiments using water and mercury.