Droplet dynamics on superheated surfaces with circular micropillars

Abstract

In this study, we investigated droplet (water and ethanol) dynamic behaviors on superheated surfaces with micro-textures. The textures have a circular pillar shape (∼9.0 μm diameter and ∼22 μm height) and the pitch between the pillars varied from 15 to 120 μm. Quantitative data on parameters governing the heat transfer process at droplet impact, including sequential droplet impact behavior, droplet residence time tR, spreading dynamics, and dynamic Leidenfrost temperature TDLT, were obtained experimentally and analyzed. Through the experimental observations, the droplet behaviors could be categorized into contact boiling, partial boiling, and film boiling regimes, and boiling regime maps for various fluids and surfaces were generated. tR remained almost constant under the same fluid conditions regardless of the fluid types, and the surface textures did not significantly affect the time scale. The spreading dynamics of water and ethanol droplets also demonstrated universal behaviors on smooth and textured surfaces. These results may indicate that no direct interaction between the droplet and surface occurs at the Leidenfrost point. However, TDLT was strongly affected by the surface textures. For water droplets, TDLT on the textured surfaces with pillar pitches of 60–120 μm increased by ∼60% compared with that on the smooth surface. However, for ethanol droplets, especially at relatively higher Weber number, TDLT was lower on all textured surfaces than on the smooth surface. To provide reasonable explanations for these observations, we performed approximate scaling analysis considering downward and upward pressures acting on a droplet at the impact and related parameters, such as vapor permeability and effective thermal conductivity, on the textured surface. © 2019 Elsevier Ltd