VORTEX RING BELOW A SPREADING ALCOHOL FILM ON WATER

We study the expansion of a vortex ring generated due to the spreading of ethanol-water droplets, with ethanol concentration range of 20%6Ce6100%, on the surface of a 50 mm deep water layer. Once deposited on the water layer, the surface tension difference leads to some part of the lighter ethanol droplet spreading as a thin film over the water layer. We observe an expanding vortex ring below the radially spreading film front. We visualize the film spreading from top using aluminum particles, while the vortex is visualized from the side using polyamide particles with LIF from the dyed drop used to distinguish the alcohol from the water. PIV is used to obtain the velocity and the vorticity fields below the spreading film. Vortex regions and their centers, identified by the λ2 method from the velocity fields, are tracked to determine the vortex expansion. We show that the vortex ring expands with the same velocity of expansion as that of the spreading ethanol film at the free surface, possibly since the vortex ring is created due to the surface tension difference across the film front. Using dimensional arguments, we also propose a scaling for the upward velocity, induced by this expanding vortex ring, uΓ and show that uΓ ∼ t−1/2.