Ferrofluid Leidenfrost droplets

We experimentally investigate the behavior of ferrofluid Leidenfrost droplets subject to a static magnetic field gradient. The droplets are deposited on a hot substrate and trapped over the vertical axis of a permanent magnet placed at a distance d above the substrate. Several effects are observed. Firstly, the droplet evaporation rate is strongly influenced by the distance d. Secondly, the droplet takes off from the substrate when its radius decreases to a critical value. The introduction of an effective gravity, which accounts for the magnetic force, allows a successful description of these effects. Finally, we observe an instability for which the droplet starts bouncing with irregular amplitudes. This behavior is qualitatively interpreted by introducing the synchronization of the free fall time between successive bounces with the period of the fundamental vibration mode of the droplet.