Citation: Lakew, E.; Sarchami, A.; Giustini, G.; Kim, H.; Bellur, K. Thin Film Evaporation Modeling of the Liquid Microlayer Region in a Dewetting Water Bubble. Fluids 2023, 8, 126. https://doi.org/10.3390/ fluids8040126 Academic Editors: Alireza Mohammad Karim and D. Andrew S. Rees Received: 7 March 2023 Revised: 27 March 2023 Accepted: 31 March 2023 Published: 4 April 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). fluids Article Thin Film Evaporation Modeling of the Liquid Microlayer Region in a Dewetting Water Bubble Ermiyas Lakew 1 , Amirhosein Sarchami 1 , Giovanni Giustini 2 , Hyungdae Kim 3 and Kishan Bellur 1, * 1 Department of Mechanical and Materials Engineering, University of Cincinnati, P.O. Box 210072, Cincinnati, OH 45221, USA 2 Department of Mechanical, Aerospace & Civil Engineering, School of Engineering, University of Manchester, Manchester M13 9PL, UK 3 Department of Nuclear Engineering, Kyung Hee University, Yongin 17104, Republic of Korea * Correspondence: bellurkn@ucmail.uc.edu Abstract: Understanding the mechanism of bubble growth is crucial to modeling boiling heat transfer and enabling the development of technological applications, such as energy systems and thermal management processes, which rely on boiling to achieve the high heat fluxes required for their operation. This paper presents analyses of the evaporation of “microlayers”, i.e., ultra-thin layers of liquid present beneath steam bubbles growing at the heated surface in the atmospheric pressure nucleate of boiling water. Evaporation of the microlayer is believed to be a major contributor to the phase change heat transfer, but its evolution, spatio-temporal stability, and impact on macroscale bubble dynamics are still poorly understood. Mass, momentum, and energy transfer in the microlayer are modeled with a lubrication theory approach that accounts for capillary and intermolecular forces and interfacial mass transfer. The model is embodied in a third-order nonlinear film evolution equation, which is solved numerically. Variable wall-temperature boundary conditions are applied at the solid–liquid interface to account for conjugate heat transfer due to evaporative heat loss at the liquid–vapor interface. Predictions obtained with the current approach compare favorably with experimental measurements of microlayer evaporation. By comparing film profiles at a sequence of times into the ebullition cycle of a single bubble, likely values of evaporative heat transfer coefficients were inferred and found to fall within the range of previously reported estimates. The result suggests that the coefficients may not be a constant, as previously assumed, but instead something that varies with time during the ebullition cycle. Keywords: evaporation; thin film; contact line; microlayer; boiling 1. Introduction The phase change flux from a thin film is several orders of magnitude greater than the bulk liquid [1]. Hence, researchers have recently increased the use of thin films in high-performance phase change heat transfer devices such as spray coolers, heat pipes, capillary pumped loops, and grooved evaporators [2]. Even in a nucleate boiling scenario, a thin film is momentarily formed underneath a bubble just prior to departure and is generally referred to as the “microlayer” (Figure 1). The superheated thin film forms a conduction path between the solid–liquid interface (heated wall) and the liquid–vapor interface, where evaporation takes place. This results in thermal resistance and is critical to overall heat transfer and fluid flow. The evaporation from this superheated liquid film causes the bubble to grow while diminishing film thickness at the same time. As the bubble grows in size, a net upward force develops due to the density gradient and vapor recoil. The upward forces finally overcome surface tension when the bubble becomes large enough to detach itself from the heated surface. As seen in Figure 1, the liquid region can be delineated into three main parts based on the dominant pressure component. In the bulk region, the surface/capillary force is generally dominant and influences the macroscale Fluids 2023, 8, 126. https://doi.org/10.3390/fluids8040126 https://www.mdpi.com/journal/fluids