Water Condensation and Frosting on Multiscale Surface Textures

Yao, Yuehan

doi:https://doi.org/10.21985/n2-q0x8-3y31

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Water Condensation and Frosting on Multiscale Surface Textures

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Water vapor condenses into liquid water when it encounters a cold surface. If the surface temperature is sufficiently low, freezing follows the condensation step, and the process is holistically referred to as condensation frosting. Both phase change processes are fundamental to many industries ranging from water harvesting, thermal management, solar desalination, and ice protection systems. Developing high performance functional surfaces to precisely control the kinetics of water condensation and frosting is highly desired in these applications. In the past decades, an increasing attention has been paid to surfaces with extreme wettabilities inspired by a variety of biological examples including lotus leaves, dessert beetles, and pitcher plants. The surface chemistry and textures of such materials are sophisticatedly modified on the length scale from molecular level to millimeters, which significantly influence the nucleation, growth, and transport stages of the condensation and condensation frosting processes. However, the micro/nanoscale textures and chemical coatings lack durability when the surfaces are subject to harsh working conditions such as mechanical degradations and high humidity environments, while the role of the millimetric surface features in the phase change kinetics has yet been being fully understood. Based on these considerations, we explore new designs for two important applications, water condensers and anti-frosting surfaces, by synergistically integrating the millimetric surface textures with attributes on a smaller scale such as chemical coating and micro/nano-textures. The first part of the thesis is focused on understanding the fundamental physics of water condensation on surfaces with millimetric features. By quantifying the droplet growth, we showed that condensation is facilitated on a convex surface, and is suppressed on a concave surface. This correlation is further demonstrated by numerical simulations of the mass transport of water vapor near the surface features, which provides a universal mechanism for precisely tuning the phase change kinetics using millimetric surface textures. The second part of the thesis dives further to develop a counterintuitive water condenser based on superhydrophilic aluminum. In addition to droplet growth, we showed that the millimetric convex features also benefit the water transport, potentially making condensation more thermal- efficient. We enhanced water mobility by integrating a wavy pattern with curvature gradient in the surface design. Compared with the conventionally-used hydrophobic flat surfaces, the superhydrophilic wavy condenser showed enhanced kinetics in all stages of water condensation including nucleation, growth, and transport. The third part of the thesis naturally extends the millimetric surface designs to a more complicated water phase change phenomenon, condensation frosting. We investigated the frosting process on aluminum surfaces with millimetric serrated structures. Dropwise condensation, during the first stage of frosting, is found to be enhanced on the peaks and suppressed in the valleys, causing frost to initiate from the peaks. The condensed droplets in the valley are then evaporated due to the lower vapor pressure of ice compared with water, resulting in a frost-free zone in the valley. The dependence of the frost-free areal fraction on the geometric parameters and the ambient conditions is elucidated by both numerical simulations and an analytical method based on steady state diffusion independent of surface wettability, including the hydrophobicity and superhydrophilicity explored in the first and second parts, respectively. In summary, by studying the kinetics of condensation and condensation frosting on surfaces with millimetric features, we have been able to utilize the unique mass transport mechanism of diffusion for developing the next generation of water condensers and anti-frosting technologies. For future work, we have briefly discussed potential means to further understand and improve the performance of the surface designs, and possibilities to scale up the production of the functional textured surfaces for commercialization.

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