**1. Introduction**

Superhydrophobic surfaces received many research interest from academic to industrial sectors due to their intriguing self-cleaning properties [1–4]. The study on superhydrophobic surfaces was initiated during early 1907 when Ollivier observed a contact angle of ~180° for surfaces modified with soot, lycopodium powder, and arsenic trioxide [1]. Later, Coghill and Anderson (in 1923) studied the surface modification of galena via deposition of stearic acids leading to achieve a water contact angle (WCA) of 160° [1]. Until the 1990s, minimal research work has been carried out on superhydrophobic surfaces. In contrast, it was reactivated in 1997 when Neinhuis and Barthlott discussed the origin of superhydrophobicity via the principle of "lotus effect" [5]. Many researchers have focused on superhydrophobic surfaces by mimicking nature and fabricating similar structures via artificial methods through

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

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 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.

**319**

*Nanostructured Materials for the Development of Superhydrophobic Coatings*

A facile precipitation cum calcination route was used for the preparation of porous ZnO nanoparticles [32]. Briefly, appropriate amount of zinc nitrate (4 g) was dissolved in distilled water, followed by the addition of polyethylene glycol (PEG) (3 g) and hexamine (3 g). The entire precursor solution is subjected to a vigorous stirring process using a magnetic stirrer for 30 minutes at a temperature of 60 °C. Following this, the ammonia solution was added to the precursor solution until their pH reached 9. After that, the solution is allowed to gelation process by placing them in a hot plate at the temperature of 90 °C, which results in the formation of a dark brown gel. After 4 hours, the dried brownish gel was placed in a silica crucible and calcined at a temperature of 500 °C for 2 h that finally lead to the formation of white-colored porous ZnO

The metal (copper/magnesium stearate powders) were prepared via a precipitation method using metal salts, stearic acid, and ammonia, as reported in our recent

A spray coating process is used to fabricate the superhydrophobic films comprising ZnO/metal stearate with the various weight ratio of ZnO in our recent study [32]. The coating thickness was varied by spraying the solution from different time

**Figure 1(A)** shows the X-ray diffraction (XRD) pattern of ZnO nanostructures prepared via the calcination route explained in section 2.1. High intense sharp diffraction peaks are seen in **Figure 1(A)** matched with the wurtzite structure of ZnO ((JCPDS No. 89–7102) [34]. The Raman spectrum of the prepared ZnO nanostructures (given in **Figure 1(B)**) indicated a sharp band located at 437 cm−1 as a result of E2 (high) mode vibrations [35]. **Figure 1(C, D)** represents the deconvoluted X-ray photoelectron spectroscopy of Zn and O states present in the ZnO nanostructures. The Zn 2p states (given in **Figure 1(C)**) showed the presence of two peaks corresponding to the Zn 2p3/2 (at 1022 eV) and Zn 2p1/2 (at 1045 eV), respectively. A value of 23 eV is obtained for the difference between the peak positions of Zn 2p3/2, and Zn 2p1/2 states that matched with the reported ones and indicates that Zn possesses an oxidation state of +2 in the synthesized ZnO nanostructures [32]. The O 1 s spectrum evidences the broad peak centered at 532 eV (given in **Figure 1(D)**) arises from the oxygen content present in the wurtzite ZnO [35]. The field emission scanning electron micrographs (FE-SEM) of the as prepared ZnO nanostructures (given in **Figure 1(E)** and **(F)**) showed the presence of nanoparticles with a high amount of pores. The high magnification micrograph (**Figure 1(F)**) evidences the honeycomb-like porous ZnO nanostructures with pore sizes ranging from 200 to

*DOI: http://dx.doi.org/10.5772/intechopen.96320*

**2.1 Preparation of porous ZnO nanostructures**

**2.2 Synthesis of copper stearate and magnesium stearate**

intervals ranging from 30 seconds to 30 minutes.

**3.1 Characterization of porous ZnO nanostructures**

**3. Results and discussion**

**2.3 Fabrication of superhydrophobic films via spray coating process**

**2. Methods**

nanostructures.

works [32, 33].
