Chen, Zhuyang; (2019) Robust superhydrophobic nanocomposite coatings. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

A solid surface is described as superhydrophobic when it possesses a large water contact angle (θw ≥ 150°) and a small contact angle hysteresis (Δθ ≤ 10°). These surfaces have many applications, such as microfluidic channels, anti-icing, self-cleaning and anti-bacterial surfaces. Superhydrophobic surfaces have been achieved through the control of chemical composition of the surface, to acquire low surface energy and suitable roughness. Applying nanocomposite coatings by wet chemical spraying is one of the most facile, low cost and scalable ways to create superhydrophobic surfaces. The nanocomposite can be simply prepared by dispersing nanosized particles such as SiO2 , carbon black, or/and metallic oxide particles (such as ZnO, TiO2, Al2O3, etc.) in a hydrophobic polymer matrix such as polystyrene (PS), polyethylene (PE) and polyurethane (PU), where the nanoparticles provide the necessary roughness and the polymer reduces the overall surface energy. Durability of superhydrophobic nanocomposite coatings is an important issue. The micro/nanostructure that is formed by particles with some of the non-flexible polymers is easily damaged under mechanical shears/stresses and liquid impact. Reserches have shown that the hydrophobicity of certain nanocomposite coatings would degrade after UV/sunlight or strong chemical exposure. The damage of the particles or polymer would lead to an increase in the liquid/solid contact area and a change of the composition associated with the surface chemistry, resulting in a loss of hydrophobicity. In this thesis, the fundamental/theoretical aspects of the surface wetting property were briefly introduced and explained. This was followed by a thorough literature review which covered more than 50 literature published in the past decade related to the fabrication of mostly nanocomposite based superhydrophobic surfaces, also showing their material selection and durability testing methods/results. It is clear that development/applications of superhydrophobic surfaces are still facing many challenges such as achieving mechanically and chemically stable surfaces. In experimental work, the material selection and fabrication process of two types of superhydrophobic nanocomposite coatings: water-based oxide nanocomposite and PKFE (Polytetrafluoroethylene particle mixed with Kytox oil and Fluoropolymer-grafted epoxy resin) were demonstrated, followed by the descriptions of different durability and multifunctional tests that were relevant to superhydrophobic coatings. The durability test performance of the superhydrophobic nanocomposite coatings were then presented and discussed accordingly. All the nanocomposite coatings are scalable and facile as they are produced via off-the-shelf material and wet chemical spraying. The coated surfaces demonstrated good mechanical (remained superhydrophobic after at multiple cycles of tape peeling, abrasion and sand abrasion tests), chemical (remained superhydrophobic after immersing in strong acid/alkane solution for at least 2 hours) and environmentally stable (remained superhydrophobic under UV exposure up to 2 weeks) properties. Furthermore, an excellent droplet impalement resistance was noted, with PKFE superhydrophobic surfaces withstanding impact by a water jet travelling at maximum speed of 32.5 m/s, without losing its anti-wetting property. The extreme liquid impalement resistance could be attributed to the flexible nature of the epoxy which was used in the coatings, and it also provided the mechanical/chemical stability. Water-based oxide superhydrophobic nanocomposite possesses transparent and others unique optical properties. The 1-2 μm thick coated sample was found to have an optical transmittance of over 90%. However, transparency decreased due to the Mie scattering of light from the increase of surface roughness. Mie scattering on nanocomposite superhydrophobic coatings has rarely been experimentally studied previously. Therefore, in my work, ASTM standard Haze and Clarity measurements were used to evaluate wide (scattering angle larger than 2.5) and narrow angle light scattering. Experimental results indicated that the increase of coating thickness and the use of larger particle size (particle agglomeration) would lead to a poorer Haze and Clarity performance. In practice, low clarity would result in the transmitted image to blur more as the observation distance is increased and a low Haze would result in the transmitted image being unclear.

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