Electric field makes Leidenfrost droplets take a leap

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.

Weakly charged droplets fundamentally change impact dynamics on flat surfaces

Electric charges are often found in naturally or artificially formed droplets, such as raindrops and those generated by Kelvin's water dropper. In contrast to the impact of neutral droplets on a flat solid surface upon which a thin convex lens shape layer of the gas film is typically formed, we show that the delicate gas thin film can be fundamentally altered for even weakly charged droplets. As the charge level is raised above a critical level of ∼1% of the Rayleigh limit for representative impact conditions, the Maxwell stress overcomes the gas pressure buildup to deform the droplet bottom surface. A conical liquid tip forms and pierces through the gas film, leading to a circular contact line moving outwards that does not trap any gas. The critical charge level only depends on the capillary number based on the gas viscosity. This finding applies to common liquids and molten alloy droplets, providing new insights into a range of applications such as mitigating pinhole defects in additive manufacturing.