Advanced Functional Surfaces With Extreme Wetting Behaviors:Modeling, Fabrication And Application

As a ubiquitous and fundamental interfacial phenomenon in nature, wetting has long been exerting profound influences on our daily life not only by providing vitally important industrial applications but also by serving as helpful inspirations for scientists. Since "self-cleaning effect" of lotus leaf was reported in Planta1997, surfaces with special wetting behaviors have aroused extensive research interests. A surface with extremely liquid-hating or liquid-loving property is considered to be special in wetting. One of the most important physical parameters of surface wettability is liquid contact angle, and a surface is considered superhydrophobic (or superoleophobic) if water (or oil) contact angle is greater than150°, while, a surface is considered superhydrophilic (or superoleophilic) if water (or oil) contact angle is virtually approaching0°. In cases the wettability switches from a dewetting state to a more wetting state and vice versa due to external stimuli, the surface is considered stimuli-responsive with special wettability. Attributing to tremendous potentials in anti-fouling, self-cleaning, anti-fogging, chemical shielding, heat transfer, anti-icing, drag reduction, liquid-liquid separations, miniature reactors, fluid manipulations and so on, innovations and developments inspired from natural counterparts with special wetting behaviors have been biomimetically achieved in the past decade, which also have greatly promoted the development of surface science. Nevertheless, issues including theoretical relations between surface wettability and surface geometrical or chemical structures, techniques and wetting mechanism for surfaces with heterogeneous and counterintuitive wetting behaviors and a perfect resistance to virtually all contacting liquids, although extremely pressing, are all still far from being successfully addressed.This dissertation, devoted to originating the secret between surface wettability and surface structure, is presented by starting with the biomimetic investigations of distinct wetting behaviors of ginkgo leaves’ sunny and shady surfaces contrastively. Thereafter, mathematical models and theoretical derivations have been proposed and validated via systematic design, fabrication, characterization and investigations with practical applications of surfaces with photo-responsive superhydrophobicity, counterintuitive superoleophobic but superhydrophilic wetting behaviors and/or functional superomniphobicity.During the development of biomimetic superhydrophobic surfaces, the shady surface of a fresh ginkgo leaf was found exhibiting extremely stable and robust superhydrophobicity owing to its special spherical micro-humps or protuberances and also the hydrophobic wax nature. Inspired from which artificial surfaces with comparable superhydrophobicity of the natural counterparts were not only chemically synthesized but also further achieved with dynamically tunable and reversibly switchable wettability displaying smart response to UV or visible light, owing to the photoisomerization of azobenzene derivatives modified onto the artificial surfaces.As for conducting theoretical analysis and deducing equations for predicting surface wettability, mathematical models for liquid wetting on spheres or cylindrical fibers was built based on the distinct structures of sunny and shady surfaces of a fresh ginkgo leaf. Equations for contact angle predictions both for fully wetted Wenzel state and composite Cassie-Baxter state, the Gibbs free energy and also the robustness of the wetting interface, might serve as theoretical references for systematically characterizing the wettability of practical surfaces.A simple route to counterintuitive superoleophobic but superhydrophilic surfaces in air was successfully developed by investigating the wettability and heterogeneity of silanized films via vapor chemical deposition. Instant superwetting for polar solvents and ultra-resistance to nonpolar liquids even with ultra-low surface tensions were achieved all together, for the first time, on the commercially available cotton fabrics. A heterogeneous wetting mechanism was proposed and affirmed by systematic characterizations of surface elemental mapping and energy dispersive X-ray spectrum, and further approved with the test results of over100kinds of liquids. Surprisingly, the unprecedented performances, demonstrated in surface self-cleaning and solely gravity-driven oil-water separations in a membrane-based single unit, made the heterogeneous fabric surface a promising candidate in practical applications.In fulfilling the ultimate goal of limitations in surface extreme repellency to liquids, hierarchically structured superomniphobic surfaces with ultra-low surface energy were developed through an electrospinning technique. The obtained hierarchical surfaces were displaying ultra-high contact angles and ultra-low roll-off angles for hundreds of liquids with different physical properties, i.e. polar and nonpolar, Newtonian and non-Newtonian, inorganic and organic, acid and base, et al, which had never been previously achieved in the literatures. The robust superomniphobicity, demonstrated by impact tests of liquid droplets and jets, liquid rolling tests using a U-shaped surface, tests of loading capacity on fluids, metal protection tests by immersion in concentrated hydrochloric acid and/or concentrated sodium hydroxide, made the surface a perfect candidate in fields of stain-proof, self-cleaning, chemical shielding, fluid manipulations, cargo carrier, drag reductions, anti-icing, et al.As for a perspective, the investigation methods, experimental results and theoretical analysis presented in this dissertation served as useful references, may inspire new ideas in inter-disciplines including chemistry, biology, physics, materials, mechanics and engineering, and hopefully will exert profound influences both in scientific and industrial practices.