Development and Optimization of Micro/Nano Structures for Various Superwettable Surfaces

Abstract

The preparation of superwettable surfaces has attracted considerable attention because of their potential industrial applications, such as self-cleaning surfaces, anti-corrosion surfaces, anti-ice coatings for heat exchangers, and drag-reduction surfaces. However the practical application of previously reported methods is still difficult owing to the required complicated wafer process and lack of adaptability to any target objects. This dissertation presents various approaches to fabricate superwettable surfaces by using inexpensive and easy fabrication processes. Chapter 1 describes superhydrophobic surfaces by aluminum hydroxide nanostructure and surface modification with fluoroalkylsilane coating. And fabrication methods to create dual-scale hierarchical structures are introduced for robust superhydrophobic surfaces. These methods are suitable for diverse applications of aluminum in various industrial areas requiring self-cleaning and anti-frosting property. Chapter 2 proposes selective separation and collection of oils from an oil and water mixture by using a superhydrophobic aluminum mesh. The results may be utilized for filtration of wastewater and oil spill cleanup in various environments. Chapter 3 discusses a new strategy for robust superhydrophilic or superhydrophobic green patina surfaces on copper substrates with superior corrosion resistance and adhesion strength, which have great potential for treating marine pollution. The as-prepared surfaces exhibited superhydrophilicity with underwater superoleophobicity or superoleo-philicity with under-oil superhydrophobicity, which allowed them to selectively separate oil and water with high efficiency. More importantly, the surface displayed not only good mechanical stability but also chemical stability in corrosive liquids owing to the intrinsic properties of the patina and hydrophobic coating. Furthermore, the surface can be utilized as coating material for the decoration of building exteriors and prevention from surface fouling. The proposed method would make it possible to develop engineering materials that require robust anti-fouling, anti-frost, and anti-corrosion properties in marine environments. Chapter 4 proposes an integrative method that improves the stability of surface wettability and extends adaptability to various materials and shape structures through the direct growth of a silica layer on polyaniline nanofibers. The wetting behavior of the fabricate surface was greatly enhanced, and the modified surface maintained superhydrophilicity for more than 2 months. Taking advantage of the adhesive property of polyaniline, a conformable superhydrophobic–superhydrophilic patterned surface is further realized by photopatterning a hydrophobic alkylthiol coating on the polyaniline surface. The proposed method is expected to open up various applications, particularly in water harvesting. Chapter 5 presents a highly conformable superoleophobic surface on a flexible polypropylene film using multi-scale structures and surface functionalization. The multi-scale structures are fabricated via the formation of nanowire arrays and attachment of nanoflakes, realized by inexpensive and easy fabrication processes. Then, after deposition of a fluoroalkylsilane coating, the structured surface shows superoleophobicity with high contact angles and low sliding angles for various liquids with surface tensions as low as 23.8 mN m-1. A patterned surface with extremely different wettabilities by tailoring the surface geometry and surface energy is also realized. Further, the wetting properties of the fabricated surface were maintained during repeated bending and twisting tests (10,000 cycles) and under shrinkage deformation (εshrink = 10 %). Thus, the proposed strategy represents a facile method to develop easily adaptable surfaces with special wettabilities.