The underside of a Leidenfrost drop on a bath

The Leidenfrost effect can also be observed on a liquid substrate, i.e volatile drops may levitate on their own vapor when placed on a hot bath of non-volatile liquid. Compared to the classical Leidenfrost effect on solid substrates, the liquid bath presents three major differences: the substrate is atomically smooth, deformable, and fluid. As a consequence of such fluidity, heat transfer through the bath to the drops is most certainly dominated by convection and not by just conduction as in the solids. Here, we examine experimentally and numerically the flow motion in the bath of silicone oil V20 under the action of a Leidenfrost drop. We highlight the development of a toroidal vortex under the drop. Interestingly, the sense of circulation in this vortex is found to depend on the nature of the liquid that makes the drop. We show that this is due to a shift in a complex and delicate interplay between three mechanisms pulling in different directions: the local cooling of the bath by the drop gives rise to both (i) a buoyancy action and (ii) Marangoni stresses, whereas the vapor escaping from the gap between the drop and the bath exerts (iii) a shear action on the bath surface.