Translated Abstract
After billions of years of evolution, the surfaces of many animals and plants exhibit various special wettable phenomena, such as superhydrophobicity and self-cleaning of lotus leaf, ultrahigh adhesion of red rose petal, anisotropic wetting and sliding of rice leaf, directional adhesion of butterfly, underwater superoleophobicity and oil repellence of fish scale, and so on. The materials with special wettabilities play a very important role in many practical applications, including water repellence, oil repellence, anti-icing, anti-rust, drag reduction, lab-on-a-chip, cell engineering, microfluidic, microdroplet manipulation, oil/water separation. It is found that the wettability of a solid surface is mainly governed by both the chemical composition and geometrical topography. By mimicing the specially wettable surfaces in nature, many micro-fabrication technologies have recently been used to construct different artificial surfaces showing special wettabilities. However, most reported methods to control surface wettability generally require high-cost, complex fabrication process and have tight restriction on materials. In addition, those methods are not good at complex and fine adjustment of the surface wettabilities of various materials. As a new microfabrication technology, femtosecond laser microfabrication has been developing rapidly in recent years, and been applied in interface science to control the wettability of solid surfaces. This technology can directly create micro/nanoscale hierarchical structures by a simple one-step scanning method. Here, different microstructures, pre-designed patterns and arrays are prepared by this technology on a wide variety of materials, such as semiconductor, polymer, metal, dielectric. On those as-prepared surfaces, many special wettabilities are achieved, including superhydrophobicity, underwater superoleophobicity, controllable adhesion, anisotropic wettability, and smart wettability.
The main works in this dissertation are detailed listed as follows:
1. A micro/nanoscale hierarchical rough structure can be formed by the femtosecond laser line-by-line scanning process. The superhydrophobicity and ultralow adhesion are obtained on intrinsic hydrophilic Si surface and intrinsic hydrophobic PDMS surface, respectively. The water contact angles on those two ablated surfaces reach uo to 158° and 157°, while the sliding angles are only 4° and 1°. In addition, experimental result and theoretical analysis demonstrate that the superhydrophobicity on the edge of the upper surface is responsible for the superhydrophobic microboat’s large loading capacity.
2. Underwater superoleophobicity is fistly achieved by femtosecond laser. When the femtosecond laser-induced rough Si surface with micro-mountain array is immersed in water, the oil droplet on the sample shows a oil contact angle of 159.4° and sliding angle of only 0.5°. A kind of nanoscale rough structures is induced on silica glass surface. The as-prepared surface exhibits not only underwater superoleophobicity but also high transparency in water. The transmittance of the superoleophobic glass surface is more than 90% for visible light. A real liquid microlens array without evaporation problem is fabricated based on an underwater oleophobic/superoleophobic heterogeneous pattern.
3. Controllable adhesion is realized on various materials by femtosecond micrifabrication. Several new mechanism and methods are found, including micro-airbag effect, hydrophobic/superhydrophobic patterned structures, adjusting the contact states. The adhesion of the as-prepared surfacescan is controlled from ultralow to ultrahigh. The controllable adhesive superhydrophobic/superoleophobic surfaces can be used to modulate the droblet and fluid, such as droplet rebounce, droplet quick location, no-loss droplet transfer.
4. A microgroove array structure is prepared by the wide-spaced femtosecond laser scanning process, and the resultant surface achieves anisotropic wetting and anisotropic sliding, being similar to the natural rice leaf. The maximum difference of contact angles perpendicular to and parallel to the microgrooves, respectively, reach up to 18.3°, while the maximum difference of sliding angles is 45°. In a similar vein, anisotropic underwater oil-wetting is also realized. Inspired by butterfly wings, a kind of superhydrophobic triangle array patterns showing directional adhesion is demonstrated.
5. Smart and tunable wettability is obtained on femtosecond laser ablated Ti and Zn surfaces. For the materials like Ti, oxidation also happens with rough microstructure formation during femtosecond laser ablation. The generated rough TiO2 layer exhibits smart wettability; that is, superhydrophobicity-superhydrophilicity, as well as underwater superoleophilicity- superoleophobicity, can be reversible switched by alternate UV irradiation and dark storage. In addition, no-loss oil droplet transportation is firstly realized in water through switching the density of water solution with the aid of underwater superoleophobic surfaces.
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