Superhydrophobic Textiles: Review of Theoretical Definitions, Fabrication and Functional Evaluation

Engineering of superhydrophobic textile surfaces has gained significant scientific and industrial interest for its potential applications in outdoor wear and protective textiles, resulting in many publications especially on theoretical models and fabrication methods. In this review, progress in theoretical definitions to explain the wetting behavior and realization techniques for superhydrophobic textile surfaces is discussed. Firstly, theoretical models from Young, Wenzel, and Cassie-Baxter to the more recent re-entrant angle model are overviewed to understand the design strategy for superhydrophobic surfaces. Secondly, major surface manipulation techniques to produce superhydrophobic textiles were reviewed for: modification of surface energy, addition of surface roughness by depositing or growing nanoparticles either in spherical form or in high aspect ratio, etching by plasma or caustic chemicals. Particular attention is paid to evaluation methods to measure the level of hydrophobicity for superhydrophobic textile surfaces, as a limitation of static water contact angle (WCA) on differentiating superhydrophobic surfaces has been reported elsewhere. The challenges in application of superhydrophobic textiles to clothing materials in terms of comfort properties and durability are discussed with the suggestion of further research opportunities to expand the application. INTRODUCTION Recently, there has been active research in biomimetic technology for developing highly functional materials that mimic nature. In superhydrophobic research, superhydrophobic surfaces refer to surfaces with excellent water repellency with a water contact angle (WCA) exceeding 150° and low contact angle hysteresis (CAH) of less than 10°. The most well-known example is the development of superhydrophobic self-cleaning materials that mimic lotus leaves. The lotus-leaf, which is one of the best known natural superhydrophobic surfaces, effectively removes impurities, such as mud, with water. It was found that this is because nano-level hydrophobic wax crystals on top of micro-level bumps on lotus-leaf surfaces come together to have strong superhydrophobic attributes [1]. At such surfaces, dirt and soils are loosely attached, and a rolling water drop can easily attach the loosely bonded substances, removing them from the surface, giving self-cleaning effects. Due to this, the phenomenon of self-cleaning resulting from a superhydrophobic surface that does not become wet is called the lotus effect. This surface characteristic is applicable in industries for oil repellency, anticorrosion, anti-fogging/frosting, anti-bioadhesion, and water-oil separation. Because of this, there has been active research for the past several decades on various methods and materials that propose superhydrophobic and ultra-oil repellency that control wettability for water, oil, and non-polar liquids through the chemical makeup of solid surfaces and designing geometrical surface structures [2-7]. In the past few years, various studies have been focused on the textile applications of such superhydrophobic/superoleophobic characteristics, and textile materials with large WCA and selfcleaning effects have been commercialized [8]. Superhydrophobic textiles can grant not only excellent water repellency and oil resistance, but also active self-cleaning performance, and thus they can be used as high protective clothing textiles and functional outdoor clothing materials [2]. Furthermore, it can reduce the number of launderings thanks to the self-cleaning performance. When the number of launderings is reduced, the performance of the highly functional textiles can be maintained for long times and can lead to the development of environment-friendly materials that can reduce the use of resources and energy needed for laundry.