**2. Methods**

*Novel Nanomaterials*

surface modification. Besides fundamental understanding of the superhydrophobic phenomenon, researchers currently focuses on extending the application of superhydrophobic surfaces like self-cleaning coatings, friction reduction coatings on ship hulls, corrosion prevention oil–water separation, and antibacterial textiles/bandages [6–9]. Generally, roughness plays a vital role in the hydrophobicity of a hydrophobic solid. Flat solids display almost 100° to 120° contact angles for water and reach up to 160° to 175° if they are rough or micro/nanotextured. Such improved superhydrophobic behavior is not only due to the solids' surface chemistry alone [10]. There are two distinct processes involved in the enhancement of superhydrophobic properties that can be explained based on the (i) Wenzel model and (ii) Cassie-Baxter model [11, 12]. The Wenzel model relies on the surface roughness induced high surface area of the solid resulting in superhydrophobic nature. The Cassie model revealed that the air trapped below the drop could also result in stable superhydrophobic properties. In both models, an apparent contact angle θ\* of a drop on the surface of a rough substrate/solid will be formed via reducing the surface energy of a drop to Young's

Naturally, the water repellent properties (superhydrophobic surfaces) can be seen in many plant surfaces (lotus leaves), and animal furs, so on [13]. For instance, lotus leaves are considered as one of the finest examples of superhydrophobic surfaces. The WCA of the lotus leaf is about ~162° with a hysteresis of 2°. Barthlott and Neihuis study the micro-structure of lotus leaf using a scanning electron microscope (SEM) [14]. They demonstrated the presence of two different ranges of roughness present in lotus leaf viz. (i) one with 10 μm (rough structure) and (ii) other with 100 nm (fine structure). These studies confirmed micro- and nano-textured surfaces on the lotus leaf that lead to the origin of their self-cleaning properties [15]. To date, there are different methods used for the creation of superhydrophobic surfaces that can be broadly classified as (i) Top-down and (ii) bottom-up methods. Top-down approaches include the costly lithographic process, template-assisted fabrication methods, and delicate surfaces' plasma treatment [16–18]. Bottom-up approaches include self-assembled layers/films, chemical deposition, and layer-by-layer (LBL) deposition, etc., [19–21]. The available methods from the bottom-up approaches are considered cost-effective and scalable compared to the top-down methods. More importantly, synergistic methods involving both top-down and bottom-up methods are also in practice. Most of them rely on solution casting, phase separation, electrospinning/spraying, and nanostructure impregnated polymer composite films [22, 23]. The superhydrophobic surfaces/coatings possess numerous applications in automobile windows, optical windows for electronic devices, eyeglasses, fluidic drag reduction, enhanced water supporting force, water corrosion prevention, and humidity proof coatings, antibiofouling, self-cleaning textiles, and oil–water separation, etc. [24–27]. Additionally, the superhydrophobic materials as electrodes for batteries and fuel cells result in their extended shelf-life time as experimentally demonstrated by Lifton et al. [28]. Recently, studies demonstrated that the superhydrophobic coatings could also be used

contact angle θ (determined on a flat surface of the same) [10].

for applications such as anti-icing, anti-fogging, and anti-frosting sectors.

The role of nanostructured materials in superhydrophobic coatings is rapidly rising mainly due to their exceptional physical/chemical properties [29]. The choice of nanostructures over their bulk counterparts for superhydrophobic coatings is due to the increased surface area and high roughness on the surfaces. The nanostructured materials are mostly used as fillers in polymers to modify their surface roughness and porosity of the polymer surfaces, leading to superhydrophobic properties. Additionally, the choice of nanomaterials in superhydrophobic coatings also enhanced their durability and focused on biomedical applications such as creating antibacterial textiles, medical implants, etc. [29–31]. This chapter discusses the recent trends in the development of nanostructured materials based on superhydrophobic coatings and their applications.

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