Superhydrophobic surfaces and emerging applications: Non-adhesion, energy, green engineering
Introduction
The interaction of surface roughness and capillary phenomena during wetting of a solid surface leads to a number of complex effects. The best known is the so-called Lotus effect that involves superhydrophobicity and self-cleaning. Superhydrophobicity is the enhancement of hydrophobic properties due to roughness. An initially slightly hydrophobic solid surface with a water contact angle (CA) θ > 90° becomes very hydrophobic after roughening, and it may have a CA approaching 180°. A roughness-induced superhydrophobic surface, according to the accepted definition, has θ > 150° [1], [2], [3]. The effect of roughness-induced superhydrophobicity was theoretically predicted and experimentally observed in the 1930s [4], although the term “superhydrophobicity” was coined later. Self-cleaning is the ability of many superhydrophobic surfaces to wash out contamination particles with water drops running upon the surface, as opposed to conventional surfaces that have stronger adhesion to contaminants. An increasing number of publications on superhydrophobicity have appeared since the 1990s, when micropatterning technology matured, and it became possible to build superhydrophobic surfaces with desired properties [1], [2], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14].
In general, in order to create a superhydrophobic surface, two factors are needed. First, the surface should have low surface energy (in other words, it should be initially hydrophobic). Second, the surface should be roughened. Surface roughness has a complicated effect on wetting. It increases the surface area so that the same solid–liquid contact area can be achieved for a liquid drop sitting on a rough solid surface with a higher CA than on a smooth surface. More importantly, roughness combined with hydrophobicity often results in air pockets being trapped between the solid and liquid (the composite solid–liquid–air interface), thus leading to a significant decrease in the solid–liquid adhesion and an increase of the CA. In addition, roughness can pin the solid–liquid–air line (the triple line) at the edge of an asperity and thus increase the liquid adhesion to solid. The composite interface is usually needed for superhydrophobicity [15].
Apart from wetting of a solid surface by water, wetting by other liquids is of interest. This includes water solutions and organic liquids, such as oil. Water solutions tend to behave differently than pure water; for example, during evaporation of a protein solution the triple line can be pinned much stronger to surface roughness details. Superoleophobicity, or the ability to have a high CA with solids, has many potential applications. However, the surface tension of organic liquids is much lower than that of water, which makes it extremely difficult to create a superoleophobic surface [16]. Various design criteria have been suggested to overcome this problem and to create surfaces that are both superhydrophobic and superoleophobic [17]. Such surfaces are called “amphiphobic” (i.e., able to repel any liquid), “omniphobic,” or just “superphobic” or “ultraphobic.” Another important area of application is underwater superhydrophobicity, which can be used for various purposes such as antifouling or increasing the slip length for water flow in channels or decreasing the turbulence.
There are many review articles covering various aspects of superhydrophobicity [2], [3], [13], [18], [19], [20], [21], [22], [23]. In this paper we will discuss the recent theoretical advances in superhydrophobicity, the relation of superhydrophobicity to the more general type of “superphobic” surfaces, and new potential applications of superphobicity such as new energy technology, green engineering, underwater applications including antifouling, and optical applications.
Section snippets
Fundamentals of wetting of rough surfaces
In this section, we present the theory of wetting of a rough surface and discuss equations that govern the CA as well as the composite solid–liquid–air interface.
Beyond the Young, Wenzel and Cassie–Baxter models
In this section, we discuss such effects related to the CA with a rough surface as the triple line curvature, size of roughness details, hierarchical roughness, 1D and 2D interactions, and the relationship between the CA and CA hysteresis. These topics are beyond the standard Young, Wenzel, and Cassie–Baxter theories, and they have been discussed actively in the literature in recent years.
New applications for energy and environment
Traditional applications of the Lotus effect include self-cleaning paints, glass coatings, and textiles. A number of emerging applications have been discussed in the literature, ranging from anti-bouncing additives for pesticides, to non-adhesive surfaces for microdevices, to microfluidics [1], [81]. In this section we will discuss new potential applications for self-cleaning optical surfaces, energy conversion and conservation, and environment-friendly self-cleaning underwater surfaces.
Conclusion
Roughness-induced superhydrophobicity has been known since the pioneering studies by Wenzel and Cassie–Baxter in the 1930–40s. The advances in the past decade of the technology enabling manufacturing of microstructured surfaces increased the attention to superhydrophobicity and its application. It was found that the phenomenon in its totality is quite complicated, and it is not comprehensively described by the classical Wenzel and Cassie–Baxter models. The issues that remain beyond the Wenzel
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