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Title:Condensation driven solid-liquid interfacial phenomenon on functional surfaces
Author(s):Cha, Hyeongyun
Director of Research:Miljkovic, Nenad
Doctoral Committee Chair(s):Miljkovic, Nenad
Doctoral Committee Member(s):Jacobi, Anthony M; King, William; Braun, Paul V; Takata, Yasuyuki
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Heat Transfer
Jumping Droplet
Heterogeneous Nucleation
Volatile Organic Compounds
Abstract:Vapor condensation is routinely observed in nature and has a large influence on the performance of a wide range of industrial systems. This doctoral thesis focuses on the fundamental understanding of solid-liquid interactions on micro/nanoengineered surfaces, as well as the evaluation of long-term durability for such surfaces under industrial condensation conditions. We first studied droplet-surface interactions during condensation on superhydrophobic surfaces to understand jumping droplet dynamics. Droplets are of particular interest due to their large surface-to-volume ratios, and hence enhanced transport properties. For example, coalescence induced droplet jumping on superhydrophobic surfaces has recently received much attention for its potential to enhance heat transfer, anti-icing, and self-cleaning performance by passively shedding microscale water droplets. To study droplet jumping, we developed a single-camera technique capable of providing three-dimensional (3D) information through the use of focal plane manipulation. Termed ‘focal plane shift imaging’ (FPSI), we used FPSI to study the jumping process on superhydrophobic surfaces having a wide range of structure length scales (10 nm < l < 1 µm) and droplet radii (3 µm < R < 160 µm). We benchmarked the FPSI technique and studied the effects of droplet mismatch, multi-droplet coalescence, and multi-hop coalescence on droplet jumping speed. Furthermore, we were able to resolve the full 3D trajectory of multiple jumping events, to show that unlike previously theorized, angular deviation arises due to in-plane motion post-coalescence governed by droplet pinning. The outcomes of this work both elucidate key fundamental aspects governing droplet jumping, and provide a powerful imaging platform for the study of dynamic droplet processes which result in out-of-plane motion such as sliding, coalescence, or impact. Using our understanding of the importance of droplet pinning and defects on coalescence induced droplet jumping, we experimentally investigated the origins of pinning by studying the unexpected nucleation of water droplets on hydrophobic surfaces at low supersaturations (≈ 1), well below the critical supersaturation for low surface energy hydrophobic coatings (≈ 3). We demonstrate the formation of high surface energy nanoscale agglomerates on hydrophobic coatings after condensation/evaporation cycles in ambient conditions. To investigate the deposition dynamics, we studied the nanoscale agglomerates as a function of condensation/evaporation cycles via optical and field emission scanning electron microscopy (FESEM), contact angle measurements, nucleation statistics, and energy dispersive X-ray spectroscopy (EDS). The FESEM and EDS results indicated that the nanoscale agglomerates stem from absorption of aerosol particles inside the droplet and adsorption of volatile organic compounds on the liquid-vapor interface during water vapor condensation, which act as preferential sites for heterogeneous nucleation. The insights gained from this study elucidate fundamental aspects governing the behavior of both short- and long-term heterogeneous nucleation on hydrophobic surfaces, suggest previously unexplored microfabrication and air purification techniques, and present insights into the challenges facing the development of durable dropwise condensing surfaces. In addition to understanding of fundamental mechanisms of droplet pinning and nucleation, a simple but powerful contact angle measurement technique was developed to detect local defect sites. Although powerful, state-of-the-art goniometric techniques have difficulty characterizing microdroplets, can be cumbersome and expensive, and have trouble handling surfaces with local wetting heterogeneity and deformed non-circular contact lines. Furthermore, past methods are incapable of measuring contact angle in situ during experiments (e.g. condensation). Here, we develop simple yet powerful contact angle measurement techniques using conventional optical microscopy that utilizes focal plane shift imaging, ray optics, and wave interference. We used our techniques to study the wetting characteristics for a wide range of water droplet diameters (3 µm < D < 600 µm) and apparent contact angles (0° ≤ θ^app ≤ 180°). The outcomes of this work establish a powerful tool to more easily and rapidly characterize microscale droplet advancing and receding contact angles. Finally, to enable a step forward in the widespread commercial and industrial adoption of functional surfaces, we experimentally investigated coating longevity during steam condensation. We rationally selected various promising hydrophobic promoters on different substrates and evaluated their long-term (~year) durability under industrial condensation conditions in a custom-built experimental chamber. The results offer insights on how to achieve long lasting functional coatings.
Issue Date:2020-04-15
Rights Information:Copyright 2020 Hyeongyun Cha
Date Available in IDEALS:2020-08-27
Date Deposited:2020-05

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