Influence of the substrate permeability on Leidenfrost temperature

Leidenfrost state of droplets compromises heat dissipation in cooling applications because of the formation of a thin insulating vapor layer over which the coolant droplet levitates. Here we report on the dependence of the Leidenfrost temperature (LFT) of a deionized water droplet on the surface morphology and propose a pressure-based model to explain the associated droplet dynamics. We observe that the LFT increases with the height of micropillars and spacing between them. The LFT increases by similar to 270 degrees C compared to a smooth surface and reaches similar to 507 degrees C for a micropillar array with an interpillar spacing of 100 mu m and height of 63 mu m. The wall heat flux at the Leidenfrost state of the droplet varies between similar to 15 W cm(-2) and similar to 52 W cm(-2) for different microtextured surfaces. We show that irrespective of the height of the textures, the effect of surface roughness diminishes beyond a certain critical interpillar spacing. We develop a semi-analytical model to show that the excess vapor gap between the top of the pillars and the base of the droplet is dependent on the permeability of the substrate and influences the vapor pressure under the droplet. The excess vapor gap is shown to play a crucial role in determining the range of temperatures that sustain transition boiling and affects the rate of evaporation of the droplet in Leidenfrost state. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

The Leidenfrost state of droplets can compromise heat dissipation in cooling applications, as a thin insulating vapor layer forms over which the coolant droplet levitates. The Leidenfrost temperature of a deionized water droplet was found to depend on surface morphology, with an increase in LFT observed with the height of micropillars and spacing between them. A pressure-based model was proposed to explain the droplet dynamics, demonstrating the crucial role of excess vapor gap in sustaining transition boiling and affecting droplet evaporation rate in the Leidenfrost state.