Submerging and Floating-up Vortices in the Picture of Smooth Inflow of a Free-Falling Ethanol Drop into Water

The comparative videorecording of the coalescence pictures of free falling drops of 0.01% water solution of potassium permanganate and 95% ethanol solution of smaller density is performed. The kinetic energy of both fluids is smaller than the potential surface energy. The drop of the solution, whose density is greater than that of the target fluid, flows smoothly into the fluid thickness, while a cavity is formed with retardation. The submerging intrusion transforms into an annular vortex, which is pushed by a growing cavity. The ethanol drop also flows into the fluid thickness but forms a floating-up intrusion, which distorts the shape of the target cavity. When the conical cavity stops to sharpen and attains a maximum depth, a small vortex, which contains the light fluid, is thrown into the fluid thickness. The dimensions of the vortex forming the secondary intrusion remain almost the same during the motion. The light vortex stops gradually and forms the secondary intrusion. The central region of the secondary intrusion floats up and transforms into a vortex ring. The dimensions of the buoyancy-driven floating-up vortex increase with time. The time dependences of the dimensions of the basic structural components are presented.