Author: Fu, Yexiang
Title: Investigation on superhydrophobic micro/nano-structured surfaces fabricated by ultraprecision machining and hydrothermal synthesis for self-cleaning and corrosion resistance
Advisors: To, Sandy (ISE)
Cheng, Ching-hsiang (ISE)
Degree: Ph.D.
Year: 2022
Subject: Hydrophobic surfaces
Surfaces (Technology)
Nanostructured materials
Hong Kong Polytechnic University -- Dissertations
Department: Department of Industrial and Systems Engineering
Pages: xvi, 183 pages : color illustrations
Language: English
Abstract: Superhydrophobic surfaces have attracted researchers' interest because of the academic aspects and the industrial applications. Many types of superhydrophobic surfaces been developed and investigated in many application fields including self-cleaning, anti-fogging, drag reduction, corrosion resistance, etc. In these studies, the hierarchical surfaces integrating the microstructures and the nanostructures are believed to possess enhanced hydrophobicity and could present some superior properties in certain applications. However, research related to superhydrophobic micro/nano-structured surfaces is still limited and incomplete. The effect of the integration of micro and nano structures has not been fully investigated and is difficult to determine simply through basic contact angle measurements. Therefore, the role of superhydrophobic micro/nano structures in application fields including self-cleaning and corrosion resistance is well worth studying and could provide additional ways to evaluate hydrophobicity and help achieve a better understanding of wetting mechanisms.
In this thesis, the theoretical and experimental study on superhydrophobic micro/nanostructured surfaces can be divided into three parts. In the first part, two types of micro/nano-structured surfaces, i.e., the Cu2O-STA microgroove surface and the NiO-FAS-17 microgroove surface were developed, and the wettability evaluated. The micro/nano-structured surfaces were fabricated by integrating ultraprecision machining (UPM) and hydrothermal synthesis. The UPM fabricated microgrooves were the base structure on which a thin layer of Cu2O or NiO micro/nanostructures with stearic acid (STA) or FAS-17 treatment was prepared by hydrothermal synthesis, thus forming hierarchical structured surfaces. Contact angle measurements were conducted on the prepared surfaces and satisfactory hydrophobicity was achieved. The Cu2O-STA microgrooves presented a contact angle of 143.2° ± 1.2° while the NiO-FAS-17 microgrooves presented a contact angle of 161.3° ± 1.2° and contact angle hysteresis of 3.3° ± 1.2°.
In the second part, the Cu2O-STA microgroove surfaces were investigated for corrosion resistance. Electrochemical characterization was performed through the techniques of electrochemical impedance spectrometry and Tafel polarization to reflect the corrosion resistance properties. The electrochemical measurements showed that the hydrophobic Cu2O-STA film on flat substrates presented good corrosion resistance. The performance of the Cu2O-STA microgroove surfaces was even better with improved impedance and reduced corrosion current density. This reveals an enhancement of hydrophobic corrosion resistance via introduction of UPM fabricated microgrooves to form a hierarchical architecture.
In the third part, water droplet bouncing experiments were performed on superhydrophobic NiO-FAS-17 microgroove surfaces to study the self-cleaning effect. The process of droplet impact was recorded to determine properties which included the first rebound height, restitution coefficient, water-surface contact time and spreading factor against the governing parameter Weber number (We) for bouncing behavior analysis. The results reflected good superhydrophobic self-cleaning effects on both the NiO-FAS-17 coated flat surface and the NiO-FAS-17 microgroove surface, while a larger first rebound height and stronger tendency of droplet break-up were recorded on the latter one. This indicates that the micro/nano-structured surfaces possess an improved hydrophobic self-cleaning effect.
The originality and significance of the present research lie in the following aspects: (i) a novel method integrating UPM and hydrothermal synthesis is developed to fabricate superhydrophobic micro/nano-structured surfaces; (ii) enhancement of micro/nano-structured surfaces in hydrophobic corrosion resistance is achieved; (iii) enhancement of micro/nano-structured surfaces over nanostructures in regard to the self-cleaning effect is revealed through analysis in droplet bouncing behaviors. Both (ii) and (iii) can provide the means to compare the hydrophobicity of micro/nano-structured surfaces and nanostructured surfaces with the same surface chemical composition and can improve the understanding of wetting of micro/nano-structures.
Rights: All rights reserved
Access: open access

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