UBC Theses and Dissertations

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UBC Theses and Dissertations

Electrochemical evaluation and modulation of surface wettability Kung, Chun Haow


Smart surfaces with dynamic and reversible wettability offer much versatility to meet various application needs. However, application of the smart materials remains challenging, mostly due to complex preparation procedure, a small change in the contact angle, and slow wettability switching. A thorough understanding of wetting phenomena is needed to achieve a breakthrough in the design of superwettability systems. This thesis describes an electrochemical technique to evaluate the surface wettability of rough surfaces based on the wetted area under the droplet. The proportionality between the double layer capacitance and the ion accessible solid-liquid interfacial area is exploited to determine the actual wetted area. While the contact angle fails to describe the wetting mode, the electrochemical approach is capable of discerning the effective wetted area. The electrochemical wettability metric is also used to understand an anomalous wetting behaviour at which the contact angle and interfacial area of an intrinsically hydrophilic substrate show a concurrent increase with roughness. These observations contradict predictions from the Wenzel and Cassie- Baxter models. Based on a coupled optical and electrochemical analysis, the limitations of contact angle are highlighted. An in-situ control over the wettability of electrodeposited copper structure through electrochemical modulation of oxidation state is demonstrated for the first time. Precise control over the rate and extent of the wetting transition is achieved by tuning the magnitude and duration of the applied voltage. Moreover, air drying at room temperature for 1 hour or mild heat drying at 100°C for 30 min restores the initial superhydrophobicity. Microstructural and electrochemical analysis show that the active wetting control is based on the Faradaic phase transformation of the surface-bound CuxO phase shielding the Cu core. Based on the wetting switching functionality of the Cu-CuxO core-shell structures, a smart oil-water membrane is designed. The as-deposited or air-dried Cu mesh exhibits superhydrophobicity and superoleophilicity, thus is effective for heavy oil-water separation. On the other hand, the application of a small reduction voltage (< 1.5 V) remediated light-oil contaminated water. Separation efficiencies greater than 98% are achieved with less than 5% variation over 30 cycles.

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