As a disc moves through an oil-water interface, it entrains part of the oil into the water phase through a funnel-shaped volume. The shape of this volume is determined by a combination of potential flow and a starting vortex, but surprisingly, its shape remains independent of the disc velocity.

Read more about this in Physical Review Fluids: Volume entrained in the wake of a disc intruding into an oil-water interface, Ivo R. Peters, Matteo Madonia, Detlef Lohse, and Devaraj van der Meer Phys. Rev. Fluids 1, 033901 (2016)