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    Droplet Evaporation on Hot Micro-Structured Superhydrophobic Surfaces: Analysis of Evaporation from Droplet Cap and Base Surfaces

    Year: 2022

    Journal: Int. J. Heat Mass Transf., Volume 185, APR

    Authors: Huang, Wenge; He, Xukun; Liu, Cong; Li, Xiaojie; Liu, Yahua; Collier, C. Patrick; Srijanto, Bernadeta R.; Liu, Jiansheng; Cheng, Jiangtao

    Organizations: NSF CBET [2133017, 1550299]; NSF ECCS [1808931]; NSFC [52075071]; Directorate For Engineering; Div Of Electrical, Commun & Cyber Sys [1808931] Funding Source: National Science Foundation; Div Of Chem, Bioeng, Env, & Transp Sys; Directorate For Engineering [2133017] Funding Source: National Science Foundation

    Keywords: droplet evaporation; micro-structured surfaces; superhydrophobic surfaces; thermal circuit analysis; depressed boiling; evaporative cooling

    In this study, evaporation of sessile water droplets on hot micro-structured superhydrophobic surfaces is experimentally and theoretically investigated. Water droplets of 4 mu L are placed on micro-pillared silicon substrates with the substrate temperature heated up to 120 degrees C. A comprehensive thermal circuit model is developed to analyze the effects of substrate roughness and substrate temperature on the sessile droplet evaporation. For the first time, two components of heat and mass transfer, i.e., one from the droplet cap surface and the other from the droplet base surface, during droplet evaporation are distinguished and systematically studied. As such, the evaporation heat transfer rates from both the droplet cap surface and the interstitial liquid-vapor interface between micropillars at the droplet base are calculated in various conditions. For droplet evaporation on the heated substrates in the range of 40 degrees C - 80 degrees C, the predicted droplet cap temperature matches well with the experimental results. During the constant contact radius mode of droplet evaporation, the decrease of evaporation rate from the droplet base contributes most to the continuously decreasing overall evaporation heat transfer rate, whereas the decrease of evaporation rate from the droplet cap surface is dominant in the constant contact angle mode. The influence of internal fluid flow is considered for droplet evaporation on substrates heated above 100 degrees C, and an effective thermal conductivity is adopted as a correction factor to account for the effect of convection heat transfer inside the droplet. Temperature differences between the droplet base and the substrate base are estimated to be about 2 degrees C, 5 degrees C, 8 degrees C, 13 degrees C and 18 degrees C for droplet evaporation on substrates heated at 40 degrees C, 60 degrees C, 80 degrees C, 100 degrees C, and 120 degrees C, respectively, elucidating the delayed or depressed boiling of water droplets on a heated rough surface due to evaporative cooling. (c) 2021 Elsevier Ltd. All rights reserved.
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