Chapter 1
Introduction for Biomimetic Superhydrophobic Materials

Wetting phenomena are found abundantly in our body, our surroundings, and our daily lives. Our eyes repel condensed water for a clear vision, and the articular cartilage is superwetting to achieve ultralow wear. When it is raining, we can glance the raindrop rolling down from some kinds of plant leaves while still leaving a trail of water trace on other kinds of plant leaves. Wetting behavior is also closely related to laundry, froth flotation, printing and dyeing, oil extraction, welding materials, lubricating systems, and so on.

Wettability is determined by the interaction of a surface and interface between a solid and a liquid. The interface is a transition region forming a boundary between two different phases of matter. Exactly speaking, a solid (liquid) surface means the interface between a solid (liquid) and vacuum or a gas. If the liquid is water, the wetting behavior falls into several categories, namely superhydrophilicity, hydrophilicity, hydrophobicity, and superhydrophobicity. As our book will show in detail in the following part, superhydrophobicity is usually defined as the contact angle (CA) of a water drop on a solid surface >150° and a sliding angle (SA) <10°. It is also possible to change a solid surface from hydrophilic (or hydrophobic) to superhydrophobic.

Tuning the wettability of a liquid drop on solid surfaces is an important issue attracting more and more interest from both fundamental and practical perspectives. The answers to such question do help us find the miracle and power of the biomimetic ideology.

Biomimetic materials mean mimicking organisms with the aim of designing artificial materials with desirable functions [1]. In Nature, there are many perfect functional materials with various structures, and the needs of adaptation and survival are indeed the result of esthetic functional systems. Geckos are known for their excellent ability to run on ceilings, and studying their feet would inspire technologists to design of a variety of dry adhesives [1, 2]. The water strider has a remarkable ability to stand, walk on, and even leap from a water surface effortlessly [3]. This skill derives from the superior water repellency of its legs, which possess thousands of tiny setae with fine nano-grooved structures. Investigating the water strider's leg would be helpful in developing some water-standing/walking microdevices employed in monitoring the activities on a water surface [4]. The lotus leaf with nanoscale details superimposed on a microscale structure is a typical example and has an impressive self-cleaning feature. This leaf surface is endowed with a large contact angle >160° and a sliding angle <3°, and this results in the well-known "lotus effect" [5a]. Barthlott suggested that this property primarily originates from the cooperative effect of micrometer-scale papillae on the rough surfaces and hydrophobic epicuticular wax [5b]. This was a very interesting and important discovery. Learning from Nature, tremendous progress in biomimetic superhydrophobic architecture engineering has been achieved using a range of methods. The advantages, disadvantages, and typical examples of each main method will be introduced in this book.

In this chapter, the fabrication methods of certain superhydrophobic materials will be mentioned succinctly, and the amazing applications of biomimetic superhydrophobic materials are explored and elaborately introduced. It is a fact that a full-scale introduction of the applications of biomimetic superhydrophobic materials is beyond us. So the adhesive behavior and oil/water separation properties of superhydrophobic materials, discussed in other chapters, are not intended to be presented repeatedly. The main relative aspects we focus on in this part will cover novel fields such as water harvesting, self-cleaning, corrosion resistance, photochromic superhydrophobicity, robust superhydrophobicity, conductive and transparent superhydrophobicity, anti-fingerprint superhydrophobicity, and the anti-icing ability of superhydrophobicity. We hope this arrangement can help arouse the readers' interest and encourage the reading of other chapters.

1.1 Water Harvesting

An alternating pattern of superhydrophobic and hydrophilic hybrid materials has an important applied background. The available freshwater resources for humans in the poor and rural areas are extremely scarce. In recent years, learning from Nature has motivated extensive interest in the field of bioinspired superwettable materials [6], which show a huge potential to effectively address this very serious issue. Namib Desert beetles feature an interesting fog-harvesting property resulting from the alternating waxy hydrophobic and nonwaxy hydrophilic regions on their dorsal body [7]. Spider silk can collect fog from the atmosphere due to its periodic spindle-knots with a combined structure and chemical gradient [8]. These typical examples from Nature offer the inspiration to synthesize corresponding biomimetic water-collecting materials. Some representative works from our group and other laboratories are described in the following.

Because of their high surface energy, metal nanoparticles (NPs) are known to be extremely hydrophilic substances. Hydrophobic properties of the metal NPs can be obtained if these particles are pretreated with low-surface-energy chemicals. n-Octadecyl thiol, which possesses low surface energy, was selected as the modifier to obtain a superhydrophobic-superhydrophilic hybrid fabric depending on the thiol's selective modification of NPs on the fabric. Based on this, two adjacent transition metals, Fe and Co, were selected and coated on a commercially available fabric via a facile in situ growth method [9]. The surface of Co NPs could be modified by n-octadecyl thiol and transformed into hydrophobic NPs. However, the Fe NPs could not be modified by n-octadecyl thiol under our experimental conditions. Thus, they retained the hydrophilic property. The as-prepared fabric with these two kinds of NPs evenly distributed on the surface is termed a superhydrophobic-superhydrophilic hybrid fabric because it shows superhydrophobicity and possesses many superhydrophilic spots. These two kinds of NPs were evenly distributed on the fabrics, and their coating amounts were easily controlled by altering the concentrations and the proportion of the two kinds of ions in the precursor solution. Because of the selective modification, the wettability of the fabrics is linearly dependent of their concentration ratio and varies over a wide range from superhydrophilic to superhydrophobic. To obtain a superior water-harvesting fabric, the concentration ratio must be appropriate because values that are either too small (Fe NPs are dominant) or too large (Co NPs are dominant) are not beneficial for water harvesting. This kind of material might help in solving the global shortage of fresh water via an energy-saving strategy. This material has been found successful in realizing water harvesting similar to the desert beetle that collects micro droplets of water from the morning fog.

Inspired by Nature and beyond it, Peng et al. fabricated a new kind of material that could achieve fog harvesting even under windless conditions [10]. They dispersed 200 mg Co MPs in the microscale with an average diameter of 2 mm into each mechanical hole of the array. Poly(dimethylsiloxane) (PDMS) pre-polymer containing 0.1 equiv. of curing agents was cast on the template filled with cobalt magnetic particle (Co MPs) with the help of a vacuum pump. Here, mechanical perforation and template replica technology were also applied, whereby Co MPs were added in the latter step. Meanwhile, a magnet of 60 mm diameter and 50 mm thickness was prepared. The sample was placed on the top of a permanent magnet with a superficial magnetic field intensity of about 0.9 T during the process of degassing and curing so as to make the Co MPs vertically aligned in the holes. As shown in Figure 1.1a, the conical array responded to the magnetic field. Using a high-speed (charge-coupled device) CCD camera, the movements of cone arrays responding to the magnetic field were recorded (Figure 1.1b,c). When the magnet was placed directly over the sample, the cactus-spine-like cones were totally upright. The cone arrays could collect fog in a spontaneous and continuous way if a moving magnetic field was provided (Figure 1.1d). However, no water drop deposited on the arrays without the magnetic field (Figure 1.1e). By integrating cactus-inspired spine structures with magnetically responsive flexible conical arrays, magnetically induced fog harvesting under windless conditions was achieved. To meet the urgent demand for the basic water requirement, a much larger scale of the as-prepared, magnetically induced fog collector should be fabricated. In windless and foggy regions, the magnetically induced fog harvesting system has great advantages and promising applications. Furthermore, it may serve in our daily lives as an effective method for further optimization of oil mist purification and fog harvesting strategies.

Figure 1.1 (a) Schematic of the fabrication of cactus-inspired conical arrays and (b) the magnetically induced conical array responses. (c) Charge-coupled device (CCD) camera observations in response to a magnet. Scale bar: 1 mm. (d) Magnetically driven cone and (e) a static cone placed in the same fog chamber at different time periods.

(Peng et al. 2015 [10]. Reproduced with permission of John Wiley and...