Natural Superhydrophobic Surfaces and Wetting Regimes: Comparison
Please note this is a comparison between Version 2 by Conner Chen and Version 1 by Xiaonan Kan.

In nature, many kinds of plants and animals have superhydrophobic surfaces, attracting extensive attention due to their unique properties, such as self-cleaning, water-proofing, uniaxial water transport, etc. Although the apparent contact angles (CAs) of these surfaces are similar, the CAHs may be different considering the chemical compositions of these surfaces. Additionally, the important “lotus leaf effect” and “rose petal effect” are proposed according to the water repellency/adhesion.

  • biomimetic
  • superhydrophobic
  • 3D printing

1. Introduction

Being a unique phenomenon, superhydrophobicity has assisted organisms to survive under harsh natural conditions for billions of years [1,2,3,4,5][1][2][3][4][5]. Superhydrophobicity refers to objects with high water contact angles (CAs) (>150°). For instance, a lotus leaf can repel water and keep clean in sludge, cactus spines and desert beetles can collect water in dry environments, water striders can walk freely on water surfaces, and the wings of many butterflies also have anisotropic superhydrophobic properties [6,7,8,9][6][7][8][9]. In nature, two main superhydrophobic effect can be found. One is the “lotus leaf effect”, referring to surfaces with water CAs larger than 150° and contact angle hysteresis (CAH) of less than 10° [10]. These surfaces show superhydrophobic property with low adhesion, which can be attributed to a combination of hierarchical micro/nanostructures and wax with low surface energy. The other is the “rose petal effect”, referring to surfaces with water CAs larger than 150° and contact angle hysteresis (CAH) of more than 10° [11]. These kinds of surfaces show superhydrophobic property with high adhesion and, although possessing unique fractal micro/nanostructures, no low surface energy coatings can be found, leading to high adhesion of water droplets.
Learning from nature, extensive studies have been carried out on fabricating biometric superhydrophobic materials during recent decades, covering from theoretical wetting models and different fabrication strategies to diverse kinds of applications [12,13,14][12][13][14]. To further reveal the wetting regimes, several famous models were proposed, including Wenzel models for complete wetting [15], Cassie–Baxter models considering surface roughness and heterogeneity [16], and intermediate models between these two states [17,18][17][18]. In addition, to mimic natural analogues, numerous kinds of strategies have been developed for fabrication of superhydrophobic materials, such as chemical etching, spray coating, electrochemical deposition, lithography pattering, sol–gel processing, etc. [19,20,21][19][20][21]. The main construction guideline is a combination of surface roughness with low-energy materials. The fabricated multifunctional materials have shown great potential in various applications fields, such as anti-icing, water/oil separation, directional liquid transportation, drag reduction, etc. [22].
Among all these fabrication techniques, 3D printing, also known as additive manufacturing (AM) or free-form fabrication, has attracted extensive attention due to its special role in construction of artificial superhydrophobic materials [23,24][23][24]. The main advantages of 3D printing over others are the fast construction of complex materials without the use of a template with low cost and high precision. During the recent years, several kinds of 3D printing techniques, such as stereolithography (SLA), digital light processing (DLP), fused deposition modeling (FDM), and direct ink writing (DIW), have been reported for the successful preparation of superhydrophobic materials [25,26,27,28][25][26][27][28]. Based on these strategies, different biomimetic micro/nanostructures such as re-entrant structures [29,30,31][29][30][31] and eggbeater analogues [32,33][32][33] can be printed, and the printed objects can be further modified to improve water repellency [34,35,36][34][35][36]. More recently, bulk superhydrophobic objects have been fabricated directly through 3D printing [37,38][37][38]. Although great progress has been made in these areas, urgent challenges still remain. One of the biggest is the lack of structure–function relationships as clear guidelines. In consequence, challenges and opportunities coexist in 3D printed superhydrophobic materials, calling for further advancements in this field.

2. Natural Superhydrophobic Surfaces and Wetting Regimes

In nature, many kinds of plants and animals have superhydrophobic surfaces, attracting extensive attention due to their unique properties, such as self-cleaning, water-proofing, uniaxial water transport, etc. Although the apparent CAs of these surfaces are similar, the CAHs may be different considering the chemical compositions of these surfaces. Additionally, the important “lotus leaf effect” and “rose petal effect” are proposed according to the water repellency/adhesion. The exploring of wetting regimes can provide important guidance for the fabrication of artificial superhydrophobic materials and much effort has been made in this field. Moreover, to further reveal the wettability mechanism, several models have been developed, originating from Young’s equation, including the Wenzel model, Cassie–Baxter model, and some new models. Still, debates exist on whether these models have explained wettability explicitly. As is well known, a lotus leaf can withstand muddy environments due to its self-cleaning property. The apparent CAs of a lotus leaf can be more than 150° and CAHs can be less than 5° [39]. As a result, water can roll down the leaf easily. During the rolling process, the dirt on the leaf can be absorbed into the water drop, so-called liquid marbles are thus formed, and the surface energy can be minimized after adsorption. Numerous efforts have been made in revealing this mechanism [40]. At first, this phenomenon was ascribed to the wax coating and microstructures by Barthlott and Neinhuis [41]. In 2002, Feng et al. revealed that the surface of a lotus leaf is composed of large amounts of micropapillae and nanostructures on top of them [10]. These hierarchical micro/nanostructures play a vital role in superhydrophobic and self-cleaning functions. In addition to the lotus leaf, some other plant leaves were reported to have similar properties, such as Colocasi esculenta leaves [42], Salvinia leaves [43], and so on. Furthermore, these water-repellent structures can motivate design of surfaces which can be applied in anti-icing and anti-fogging [44]. Similar to the lotus leaf, the rose petal also exhibits superhydrophobic property. However, different from the former, water droplets can adhere on top of the petal firmly [45,46][45][46]. The CAH is large in these structures where the pinned water will not roll away until complete evaporation. This can help animals or plants to survive in dry environments. As can be seen from closer observation through SEM, periodic microarrays with an average diameter of about 20 μm and height of 10 μm can be found on the surface, covered with nanosized wrinkles [47]. Jiang et al. revealed that the superhydrophobicity can be ascribed to the microstructures while the existence of the wrinkled nanostructures makes the contact area larger, leading to high adhesion of water [11]. Apart from rose petals, sunflower petals and Chinese Kaffir lilies also exhibit similar properties, showing superhydrophobicity with high adhesion [48]. In addition, the “rose petal effect” could prompt development of biometric superhydrophobic surfaces with high adhesion to retain liquids.


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