Superhydrophobic Materials: Versatility and Translational Applications

Abstract

the emergence of bionics. Although it has only been around for ≈60 years, bionics has undeniably spread to and revolutionized almost every aspect of human life. Special wettability, one of the most common phenomena in nature, was described as early as in 1805 with Young's equation. [1] However, it was not until 1976 that the term "superhydrophobicity" was first introduced to describe the particle coating of hydrophobic fumed silicon dioxide, on which water droplets remained spherical and the adhesion force was negligible. Later, in 1997, Barthlott [2] and Neinhuis [3] reported the "lotus effect" and revealed the origin of the self-cleaning property of lotus leaves: the presence of papillae on the microstructure and epicuticular wax. Subsequently, Jiang et al. further deciphered that the nanostructures on the top of the micropapillae impart superhydrophobicity to the surface of lotus leaves. [4] At the same time, it was discovered that many other naturally occurring phenomena are due to the superhydrophobicity of materials, [5] such as the high and directional adhesion of gecko foot, the structural color of butterfly wings, the anisotropic wetting of rice leaves, the low fluid friction of water strider legs, the antifogging and anti-reflection of mosquito compound eyes, the high adhesion of red rose petals, the low adhesion of cicada wings, the high reflection of poplar leaves, and the water capture of Stenocara beetles, etc. To date, great efforts have been made to establish theoretical models to understand superhydrophobic phenomena, develop advanced strategies and techniques to fabricate superhydrophobic surfaces, and exploit the versatile properties and functions of superhydrophobic materials. [6] However, it is still a great challenge to translate superhydrophobic phenomena in nature into practical applications by mimicry. Fortunately, after thorough investigations, researchers have found that sufficient roughness and suitable surface energy are the two indispensable determinants to impart superhydrophobicity to synthetic materials. [7] Applications of superhydrophobic materials in transportation, architecture/building protection, oil/water separation, and seawater desalination to biomedical device fabrication, biosensing, energy conversion and utilization, and textile manufacturing, have been studied extensively over the past decade (Figure 1). [8] An analytical report published by Mordor Inspired by the lotus leaf in nature, superhydrophobic materials have attracted considerable attention in both science and industry over the past three decades. Apart from the most characteristic yet widely used properties such as waterproofing, anti-fouling, and self-cleaning, superhydrophobic materials have also developed exciting new functions such as drag reduction, corrosion resistance, anti-icing, anti-bacteria, and anti-reflection. In this review article, the theoretical models describing superhydrophobic surfaces are first briefly introduced. Then, the most common substrates and strategies for fabricating super...

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“…In the category of super‐wettability, superhydrophobicity is a phenomenon and a notable progressive adaption established by several biological and natural surfaces like lotus leaves, pond skater legs, butterfly wings and more [5] . Simultaneously, some of the natural phenomena are associated to superhydrophobicity of biological materials including structural colour, anisotropic wetting, low fluid friction, antifogging, anti‐reflection and directional adhesion properties [6] . The origin of extreme water repellence, i. e., hydrophobicity in the naturally occurring materials stems from micro/nanoscale hierarchical textures with finely encapsulated low surface energy moieties [7,8] .…”

“…There are several synthetic routes involving chemical modification via covalent and non‐covalent protocols, along with physical techniques which lead to the creation of self‐cleaning materials. There are several theoretical, experimental and industrial groups focusing on mimicking bio‐inspired artificial superhydrophobic materials offering exciting potentials in self‐cleaning, anti‐icing, anti‐fouling, fluid transport, water collection, biological fluid‐repellent surfaces and oil‐water separation applications [5,6,8,10–19] …”

“…In order to achieve the regularization and sustenance of surface roughness (texture), uniform low surface energy coating is essential. The texture and chemical modification are two tales, where optimal trade‐off occurs via various pre‐synthetic and post‐modification techniques [6,26] . It is important to highlight at the molecular level, low surface energy coating over the substrates can be accomplished using fluoro‐alkyl‐silane, long chain alkyl groups, thiols or creation of graphitic/aromatic islands using chemical vapour deposition (CVD) and homo‐coupling polymerization routes.…”

“…[5] Simultaneously, some of the natural phenomena are associated to superhydrophobicity of biological materials including structural colour, anisotropic wetting, low fluid friction, antifogging, anti-reflection and directional adhesion properties. [6] The origin of extreme water repellence, i. e., hydrophobicity in the naturally occurring materials stems from micro/nanoscale hierarchical textures with finely encapsulated low surface energy moieties. [7,8] The main drawback of these natural selfcleaning systems include weak mechanical durability, processability, low thermal and chemical stability.…”

“…There are several theoretical, experimental and industrial groups focusing on mimicking bio-inspired artificial superhydrophobic materials offering exciting potentials in self-cleaning, anti-icing, anti-fouling, fluid transport, water collection, biological fluid-repellent surfaces and oil-water separation applications. [5,6,8,[10][11][12][13][14][15][16][17][18][19] In order to construct a mechanically and chemically robust superhydrophobic substrates, it is important to protect hierarchical roughness and low surface energy coating with respect to external stimuli. The impact of water droplet has profound effect on the aspect ratio, roughness and porosity of the superhydrophobic materials.…”