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Wetting Phenomena on Structured Surfaces: Contact Angle, Pinning, Rolling and Bouncing
Published in Akihiro Miyauchi, Masatsugu Shimomura, Biomimetics, 2023
The surface of raw lotus leaf is fully covered by double-roughness structure consisted of the smaller-sized structures and the larger-sized structures as shown in Fig. 10.6a,b [27]. The smaller-sized structures are made from plant waxes [28]. The waxes are removed by dipping in ethanol for 20 min (Fig. 10.6c,d). On the other hand, the waxes remain during air-drying (Fig. 10.6e,f). The smaller-sized structures made from plant waxes plays a significant role in lotus effect, super water-repellency and rolling behavior of water droplets. For example, it was observed that water droplets on raw and dried lotus leaves (Fig. 10.6a,c) rolls down when lotus leaf is slightly tilted. The angles, rolling angle, are 2.7° and 2.3°, respectively [27]. In contrast, the rolling behavior was not observed on the dipped lotus leaf. The importance of the smaller-sized structures on lotus leaf has been found.
Potential Utilization of Nanofluids for Concentrating Solar Power (CSP)
Published in K.R.V. Subramanian, Tubati Nageswara Rao, Avinash Balakrishnan, Nanofluids and Their Engineering Applications, 2019
Amine Allouhi, Mahmut Sami Buker
The second contributor in the water consumption in CSP plants is cleaning processes of the mirrors. The rarity of water in deserts incurs huge expenses. Some procedures have been suggested to clean receiver mirrors while minimizing the quantity of water used. One promising alternative is the use of super-hydrophobic coatings, which cause lotus effect inducing self-cleaning [36].
Introduction to Nanotechnology
Published in Wesley C. Sanders, Basic Principles of Nanotechnology, 2018
Hydrophobic surfaces cause water to readily bead up and roll off without wetting them (Verbanic et al. 2014). There are several examples of surfaces in nature exhibiting hydrophobicity due to the presence of nanoscale structures. The lotus plant growing at the bottom of ponds emerges above the water surface while remaining untouched by the contaminants in the dirty water. Water droplets roll over the leaf's surface taking away all the dirt and leaving a clean surface behind; this is referred to as the lotus effect (Balani et al. 2009). The lotus effect is generated by an organized arrangement of microscale and nanoscale spires covering the surface of the leaf (Figure 1.5) (Verbanic et al. 2014). A similar hydrophobic effect is observed with cicadas due to the presence of arrays of nanosized pillars 100 nm in diameter and 300 nm in height on the wings of the insects (Hong, Hwang and Lee 2009).
Development of durable superhydrophobic and UV protective cotton fabric via TiO2/trimethoxy(octadecyl)silane nanocomposite coating
Published in The Journal of The Textile Institute, 2021
Muhammad Zaman Khan, Jiri Militky, Vijay Baheti, Jakub Wiener, Michal Vik
Recently, self-cleaning surfaces have received great attention within the scientific community due to increased demand of anti-wetting surfaces and their industrial applications. Superhydrophobic surfaces have been explored with substantial attention over the past decade and phenomenal progress has been made in this field. The physical self-cleaning concept is based on the superhydrophobic approach where the water droplets achieve spherical shape and subsequently roll-off the surface taking away the dirt particles. This self-cleaning concept is based on the superhydrophobic approach which is also known as the lotus effect. To obtain superhydrophobic surfaces, surface roughness in combination with low surface energy of material are considered as the two important factors (Bae et al., 2009; Feng et al., 2002; Yan et al., 2011).
A brief review on bio-inspired superhydrophobic electrodeposited nickel coatings
Published in Transactions of the IMF, 2018
In recent years, there has been a great interest in the synthesis of novel functional coatings for special applications. They include particularly the so-called superhydrophobic surfaces characterised by self-cleaning, anti-icing, anti-fouling, anticorrosion, reduced fluid drag and enhanced heat transfer properties.1–4 Increased hydrophobic behaviour of many biological surfaces has been known from ancient times and investigated since the eighteenth century, but the term ‘superhydrophobicity’ was introduced in 1996 by Japanese scientists to describe unusually high repellence of water on fractal (rough) surface formed by alkylketene dimer solidified on a glass plate.5,6 Based on systematic research of a variety of plant surfaces (over 200) using high-resolution scanning electron microscopy, Barthlott and co-workers7 discovered a self-cleaning effect of leaves of lotus Nelumbo8 and permanent retention of air under water by floating ferns of genus Salvinia.9 They concluded that superhydrophobicity of many plant species results from hierarchical surface structures formed by convex to papillose epidermal cells and a very dense arrangement of three-dimensional epicuticular waxes of different shapes.7 Thus, wetting of such hierarchical surfaces is minimal due to air trapped in the cavities of the convex cell sculptures. This motivated and facilitated development of biomimetic technical materials with micro- and nanostructured surfaces of superhydrophobic properties (e.g. known under trademarks of Lotus-Effect® and Salvinia® Effect10).1–4,11,12
Self-cleaned zirconia coatings prepared using a co-precursor sol–gel method
Published in Surface Engineering, 2021
Uzma K.H. Bangi, Akshay A. Ransing, Kyu-Yeon Lee, Hyung-Ho Park
The self-cleaning coating technologies are found to have high potential in the commercial products owing to their ability to reduce cleaning labour costs. These coatings exhibit a wide variety of applications in window glasses, cements, textiles and paints [1]. There are two categories of the self-cleaning coatings, namely hydrophobic and hydrophilic coatings. Both of these coatings possess the property to clean themselves in the presence of water. The contact angle of water on a surface plays a vital role in determining the ability of that surface to self-clean. The ‘lotus effect’ is a common case of the self-cleaning, where water droplets can be seen on the surface of a lotus leaf owing to its hydrophobicity [2]. Hydrophobic coatings have high water contact angles above 90° and serve as self-cleaning coatings. These coatings are highly water repellent and water tends to form spherical droplets that roll away dirt from their surfaces. Chemically, the coating surfaces are made hydrophobic using the organosilane compounds by replacing the surface polar hydroxyl groups with the non-polar alkyl groups. On these hydrophobic coatings, the non-polar substances aggregate in an aqueous solution and exclude the water molecules from their surface. There are many reports available on the hydrophobic self-cleaning coatings based on silica (SiO2) [3–5]. The hydrophilic surfaces have low contact angles (below 90°), and the hydrophilic self-cleaning coatings are based on photocatalysis. Such coatings are able to break down the impurities when exposed to light taking away the dirt from their surface. Titanium dioxide (TiO2) coatings are well-known for the hydrophilic self-cleaning surfaces [6–9] owing to their favourable physical and chemical properties.