Lu, Y; (2017) Robust Self-Cleaning Coatings. Doctoral thesis , UCL (University College London).

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Abstract

Self-cleaning, refers to a surface that has the ability to repel contamination (e.g. mud water, spilt ink etc.) or get itself cleaned under a natural circumstance (e.g. rain etc.). This thesis presents the synthesis, characterization and application of two types of self-cleaning surfaces, which are Lotus leaf-inspired superhydrophobic surfaces that repel water, and Nepenthes pitcher plant-inspired omniphobic surfaces that repel both water and liquid hydrocarbons (e.g. cooking oil). The surface durability of these surfaces is also investigated to engage practical applications. The self-cleaning properties and surface durability are studied in three stages. In the first stage, superhydrophobic mild steel surfaces were fabricated through chemical etching of CuCl2 solution followed by low surface energy modification with fluorosilane or Sylgard. To reduce the pressure on the environment, some of the by-products from the chemical etching and fluorosilane processes were used to treat soft porous materials such as sponge, cotton and paper to make superhydrophobic surfaces. In one pot, this method was used to make superhydrophobic coatings on both hard (steel) and soft (cotton etc.) substrates. Due to the superhydrophocity and oleophilicity, mild steel mesh was made superhydrophobic using this method for oil-water separation. Mechanical robustness of the superhydrophobic mild steel plate was further tested via sandpaper abrasion; most areas on the surface lost superhydrophobicity after 6 cycles of abrasion. To improve the mechanical durability of superhydrophobic surfaces and further reduce the pressure on the environment, the second-stage of the work introduces a paint-like suspension that can be treated on both hard (glass and steel) and soft substrates (cotton and paper) to make superhydrophobic coatings without by-products. The suspension, fabricated through mixing dual scaled TiO2 nanoparticles and fluorosilane-ethanol solution, can be simply sprayed or dip coated onto various substrates. Mechanical robustness of painted superhydrophobic surfaces can be greatly improved through combining the substrates and the paint using commercial adhesives such as double sided tapes and spray adhesive. The superhydrophobic surfaces retained water repellence after knife cut, finger print and even 40 cycles of sandpaper abrasion, showing remarkable robustness. Although the superhydrophobic paint treated surfaces retained water repellence after being contaminated by oil, they would eventually be stained by oil. To resist oil contamination, the third stage of this thesis presents an omniphobic coating, which is known as slippery liquid infused porous surfaces (SLIPS). The SLIPS were fabricated by adding a lubricating layer onto the superhydrophobic painted surface. The prepared SLIPS repelled water, coffee, red wine, cooking oil and even ketchup with a low contact angle hysteresis. Apart from the surface mechanical durability, thermal and chemical stability have to be considered because the lubricating layer can be subject to extreme temperatures or corrosive liquids. The SLIPS samples retained omniphobicity after thermal tests at 200 °C and -196 °C, mechanical tests of knife scratch and Newton meter press at ~850 kPa, and chemical tests using corrosive liquids with pH from 0 to 14. Hopefully in near further, there would be innovative products available in the market based on this technique.

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