Levitation of a nonboiling droplet over hot liquid bath

Abstract

We study the behavior of a droplet placed over a liquid bath. We examine water, glycerol, and silicone oil for bath liquids and the same liquids and alcohols for droplets. To extend the study beyond the classical Leidenfrost effect, we test the temperature of the bath below the boiling temperature of the droplet. Depending on liquids, the droplet sinks either immediately after deposition or after staying over the bath surface up to a few tens of seconds without significant change in the size or levitates during a few tens or hundreds of seconds. A levitating droplet gradually evaporates, while its temperature does not significantly change. The convective flow in the bath creates a shear flow in the air that pulls the droplet toward the center of the convective cell and holds air under the droplet, producing the supporting force. We propose a theoretical model for the levitation. The driving force of the process is the convective flow in the bath. The convection determines the liquid velocity and the heat flux from the bath. The droplet finds the position for thermal equilibrium with the surrounding air; the evaporation balances the incoming heat. Unlike the classical Leidenfrost effect, the vapor leaves the droplet not from the lower part but from the whole surface and moves away by diffusion and with the upflowing air. For small droplets, we find shapes of free surfaces and estimate the thickness of the gap between the droplet and the bath surface to have the order from 1 to 10 μm.