Influence of the substrate permeability on Leidenfrost temperature

Abstract

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 ~ 270 °C compared to a smooth surface and reaches ~ 507 °C for a micropillar array with an interpillar spacing of 100 µm and height of 63 µm. The wall heat flux at the Leidenfrost state of the droplet varies between ~ 15 W cm−2 and ~ 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.