Dynamics below an ethanol film spreading on a water layer.

We study the flows in the bulk of a water layer, which develop as a consequence of an ethanol film spreading on its surface. Ethanol-water surface tension difference gives rise to surface tension driven spreading of a lighter, miscible droplet of ethanol on the surface of water layer in form of a film. The film spreads radially outwards with vortices underneath the film front. PIV technique was used to acquire the velocity field below the surface. The velocity field shows two counter rotating vortices moving outwards and away from the point of drop deposition. We obtained the vortex core positions for various time instances (t) to find the vortex translation velocity (U_v) which scales as t^-0.75. The scaling shows similarity to the film spreading velocity u_f~ t^-0.75 proposed by Dandekar et al.(2017) . From the velocity field, we observe the development of an upward flow in the bulk during the course of drop spreading . The upward flow is directed toward the region in between the vortices at any instant. The upward flow velocity (u_y) is found by averaging the vertical components of velocity over space in between the vortex cores. This velocity increases once the drop touches the surface, maintains a constant value and decrease to zero as the drop spreading comes to an end.