<p>In this paper, we report a thermal circuit model developed by us to analyze the evaporation dynamics of a water droplet dwelling on the hot micro-structured superhydrophobic surfaces. The thermal domain of a droplet evaporating on a micro-structured substrate is divided into two main parts: evaporation at the droplet spherical cap with the liquid-vapor interface and evaporation at the droplet base. Evaporation at the droplet base in direct contact with the tips of micropillars leads to the decrease of the average temperature at the droplet base, which accounts for the delayed onset of nucleate boiling of water droplet on the hot microstructured substrate over 100°C. According to our observation, the evaporation process of the water droplet can be described by the constant contact radius (CCR) mode, the constant contact angle (CCA) mode and the hybrid mode of CCR and CCA, respectively. Thermal resistances from the substrate to the droplet spherical cap are calculated by integrating the thermal resistances of the substrate, the micropillars, and the water droplet itself. Conduction thermal resistance of the water droplet is calculated by dividing the water droplet into infinite thin layers and then integrating all the thermal resistance of each layer from the base to the cap. Temperature difference between the droplet cap and base causes the surface tension gradient and density gradient of water, which lead to the internal convection inside the droplet. Convection thermal resistance inside the water droplet is taken into account by employing the effective conductivity coefficient, which relies on the droplet’s Peclet number. Temperature distribution from the substrate to the water droplet cap is obtained and depicted in the form of contour plot based on the thermal circuit model. Substrates with different pillar configurations are used for the parametric study of evaporation and the substrates are heated for a large range of temperature (from 20 °C to the onset boiling temperature around 140 °C). The ratio of evaporation flux at the droplet cap surface to that of the base surface on different microstructured substrates and with different substrate temperatures are predicted by our model. In particular, the variations of the spherical cap temperature of the droplet on different microstructured substrates and with different substrate temperatures are evaluated by our analytical approach and match pretty well with the experimental results. Our model indicates that water droplet temperature increases during the evaporation process on a certain hot substrate and the large periodicity distance of the substrate micropillar arrays leads to a higher onset boiling temperature of the water droplet. The evaporation flux ratio between the droplet top and bottom surfaces decreases with the rise of the substrate temperature and increases during the evaporation process on a constant substrate temperature. This work combines thermal circuit analysis, theoretical study and experimental validation, providing a new perspective on sessile droplet evaporation on a microstructured surface. &nbsp;</p>

Thermal Circuit Analysis of Droplet Evaporation on Hot Microstructured Superhydrophobic Surfaces

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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