(296g) Electrostatic Suppression of the Leidenfrost State

Authors: 
Das, S., University of Texas Austin
Bonnecaze, R. T., The University of Texas at Austin
It is a common sight in the kitchen to observe droplets of water dancing on a hot frying pan. The evaporating liquid droplet floats above the hot surface, its weight supported by the slightly higher pressure in the thin layer of its vapor between the droplet and the surface. This homely observation known as the Leidenfrost effect has serious ramifications for process phenomena like steam generation, thermal desalination and quenching of metals1. Here, we investigate the fundamental mechanism and identify the competing forces underlying the limits of electrostatic suppression of the Leidenfrost state. We consider liquids with finite electrical conductivity, wherein the fluid remains equipotential and the entire potential difference falls across the vapor gap. We postulate and analyze several competing forces on the perturbed interface of the droplet as the voltage approaches its threshold value. The attraction (interface destabilizing) force is due to the electrostatic attraction between the charged fingers of a disturbance on the interface and the electrically grounded surface. There are two repulsive or stabilizing effects acting on these fingers. Firstly, there are capillary forces that smooth out disturbances to the interface, especially the high wave number disturbances. Secondly, as a finger gets closer to the interface, the rate of vaporization of the liquid increases locally and creates an additional stabilizing pressure to suppress the finger. If the applied voltage is large enough, the electrostatic forces dominate and the fingers will grow and bridge the vapor gap completely, enabling liquid-solid contact. We observe and analyze the electrohydrodynamic instabilities occurring above the threshold voltage that result in a wavy liquid-vapor pattern with a characteristic wavelength. The critical voltage required for Leidenfrost suppression is estimated and is found to increase with both the droplet volume and the surface temperature2. There is very good agreement between theoretical predictions and experimental measurements.

[1] D. Quere, Leidenfrost dynamics, Annu. Rev. Fluid Mech. 45, 197 (2013).

[2] Shahriari, A., Das, S., Bahadur, V., & Bonnecaze, R. T. (2017). Analysis of the instability underlying electrostatic suppression of the Leidenfrost state. Physical Review Fluids, 2(3), 034001.