Wettability, water droplet dynamics and freezing on irregularly roughened stainless-steel surfaces for ice protection application

Shi, Kewei (2023) Wettability, water droplet dynamics and freezing on irregularly roughened stainless-steel surfaces for ice protection application. Doctoral (PhD) thesis, Memorial University of Newfoundland.

[English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (6MB)

Abstract

In the Arctic and other cold environments, ice can jeopardize local infrastructure, hinder field operations, damage buildings, offshore and ship superstructures and threaten life and property. Ice protection techniques are essential for equipment, structures, and personnel in these environments. Passive techniques rely on the physical properties of the target surface to dispose of ice or prevent ice build-up without any required external energy source. As a passive ice protection technology, (super)hydrophobic metallic surfaces can significantly delay the water droplet impact and freezing process. However, hydrophobic surfaces are not always icephobic. The droplet impact dynamics and freezing process on irregularly roughened (super)hydrophobic metal surfaces have not been fully understood. Previous experimental studies have limited the impact and freezing of water droplets on cold surfaces at room temperature. Most experimental and modelling investigations have also been limited to separately studying droplet impingement and freezing processes. Hence, this study aims to investigate dynamic wetting behaviours and freezing processes coupled with the impact dynamics of a water droplet on irregularly roughened metal surfaces experimentally and analytically. To achieve this, irregularly roughened stainless-steel surfaces are fabricated by applying sandblasting, Zinc electrodeposition, stearic acid coating, and their combinations to receive various water wettability. The combination of electrodeposition and sandblasting can significantly increase the static contact angle from 91° ± 6° to 151° ± 2°, and these techniques can be applied on an industrial scale. Surface dynamic wetting characterization illustrates the challenges of understanding the wetting dynamics on irregularly roughened surfaces, including dynamic contact angles and pinning that affect the sliding behaviour on inclined surfaces. These effects result in a poor correlation between the measured dynamic contact angles and the observed critical sliding angles. Significant variations in the values of these dynamic wetting parameters are inherent to the heterogeneity of surface roughness, which limits the utility of standard dynamic wetting criteria. These findings have implications for academic and industrial research that focuses on achieving a uniform wettability of coating materials throughout their service life. This study examines the droplet impact dynamics and freezing process on the above-fabricated surfaces through experiments and analytical analysis. The freezing delay of water droplets on the metal surfaces is measured in a cold room below the freezing point of water. The experimental results demonstrate that the superhydrophobic surfaces can significantly delay water droplet freezing, with up to a 57.47 ± 5.22 s freezing delay. Also, the freezing delay time increases with the static contact angles of water on the sample surfaces. Furthermore, droplet impact from a higher distance on the same target surface leads to faster freezing. The heat transfer analysis demonstrates that a poor wetting condition (e.g., on the superhydrophobic surfaces) contributes to a smaller final contact area of a water droplet on the surface. The average freezing rate per unit mass is approximately proportional to the final contact area. This indicates the longer freezing delay on the (super)hydrophobic surfaces. The analytical framework coupled the droplet impact dynamic and the freezing process. This model can estimate the total freezing delay time for a water droplet impact and freezing on a surface by evaluating the droplet impact and freezing processes. The experimental results validate the analytical predictions, which verify the feasibility of the method and assumptions used in this study. The analytical consideration not only provides a straightforward and valid solution but also simplifies the rather complicated mechanism of the droplet dynamic and freezing process on metal surfaces and provides a directly estimated total freezing delay time under different conditions. This approach benefits engineers and reduces extensive and sophisticated computations. This fast and effective method for predicting the droplet impact and the freezing process has also filled the gap in the literature. This comprehensive study of wettability, droplet impact dynamics and freezing processes connects surface wettability and ice repellency by investigating the wetting behaviour of water droplets on irregular rough solid surfaces; at the same time, connecting droplet impact dynamics and freezing delay and providing a better theoretical analysis of droplet impact, subcooling, nucleation, recondensation and solidification processes will benefit the research field.

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