Marangoni vortex rings in miscible spreading

We present the observation of an expanding vortex ring below a fast-spreading ethanol-water film over a deep substrate of water. The radius of the vortex ring ($r_v$) is shown to scale with time as $t^{1/2}$. The outer radius of the disturbed region ($r_o$), which comprises the expanding film of length $r_f$ and the plume of length $l_p$ at its periphery, scales in the same way as $r_v$. Based on this observation, contrary to the earlier proposal of the origin of such vortex rings to be due to evaporative cooling, we hypothesize that such vortex rings occur due to the separation of the shear layer below the plumes. Based on this hypothesis, we obtain scaling laws for the radius of the vortex ring ($r_v$) and its radial expansion velocity ($U_v$). These scaling laws are shown to match our measurements, thereby validating the phenomenology of such vortex rings.