**Abstract**

 With the help of biomimetics, superficial characteristics were transposed, through various methods, onto artificially obtained materials. Many industrial fields applied surface architecture modifications as improvements of classic materials/methods. The medico-pharmaceutical, biochemical, transportation, and textile fields are few examples of industrial areas welcoming a "structural change." Antibioadhesion was widely exploited by means of antibacterial or self-cleaning fabrics and cell culturing/screening/isolation. Anti-icing, antireflective, and anticorrosion materials/coatings gained attention in the transportation and optical device fields. Interdisciplinary approaches on extreme wettability include "solid-fluid" formations called liquid marbles, which will be further discussed as a superhydrophobic behavior exponent.

**Keywords:** superficial phenomena, extreme wettability, special surface architecture, liquid marble

## **1. Introduction**

 Since ancient times, humans observed special features which helped plants and animals survive in harsh environments. These properties were unraveled with the help of microscopical investigative techniques, which led to a more thorough understanding of superficial phenomena. Natural unique superficial architectures, like the lotus and rose petal effect, became iconic. Empirical models of wettability were developed (Young, Cassie, and Wenzel) to fully explain the behavior of liquids in contact with special surfaces.

Along with the fulminant expansion of technology during the last decades, a huge progress was also registered in surface sciences. Microscopical analysis techniques revealed surfaces' special architectures and solved many mysteries regarding plants and animals' adaptation to harsh environments (e.g., Namib beetles' survival in the desert). Extreme wettability (superhydrophobicity/superhydrophilicity) was assigned to many natural phenomena, such as raindrops not collapsing while dropping onto ash-covered soil and moss storing the exact amount of water needed to survive. In particular, superhydrophobic surfaces which display a contact angle

higher than 150°, a sliding angle smaller than 10°, and no hysteresis attracted researchers' attention. Apart from theoretical aspects on wettability, which are a part of the paper, natural extreme wettability models will be discussed (lotus leaf, rose petal, and insect wings).

Principles of biomimetics are included in this chapter, as special superficial properties were adapted to human necessities and used as a model in many industrial areas, including nanotechnologies. Biomimetics is, in this case, "the thread that makes the dress complete," or, in other words, "the scene that completes the movie." Applications related to superhydrophobicity will be presented: development of selfcleaning and low friction surfaces, satellite antennas, solar and photovoltaic panels, exterior glass, etc. Studies concerning superhydrophobic surfaces' applications in various domains will also be submitted: prevention of bacterial adhesion, of metal corrosion, of surface icing in humid atmosphere and low-temperature conditions, blood type determination techniques, etc. Efficient, cost-effective, ecological, and reproductible methods are still developing so that mass production of quality materials becomes a fact.

 The chapter will also bring into attention an important exponent of superhydrophobicity: special structures called liquid marbles. The unique "solid-fluid" formations are regarded as soft objects, due to the microliter droplet encapsulated in hydrophobic particles. Practical uses include micro-reactors, miniature cell culturing, or screening devices, successfully replacing classical methods with costand reactive-efficient, low-toxicity analysis techniques. Other properties will be submitted along with applications in various fields.

### **2. Biomimetics: biology vs. technology**

 As human kind evolved, passing through the test of time, many necessities turned out as a result of convenience in everyday life activities. Thus, classical materials like wood, metal, and ceramic became no longer suitable and efficient, lacking performance in many domains (e.g., pharmaceutical, medical devices, weaponry, etc.). Aiming a more complex approach on artificial materials, the concept "materials by design" came to life (Bernadette Bensaude-Vincent, 1997) [1]. The concept refers to developing "composite" materials, which reunite properties of already known ones: heat resistance and time durability of ceramics, lightness of plastic, hardness, and breaking resistance of metals. Depending on quality requirements of the final product, many design possibilities came out, exhibiting improved sustainability, cost-effectiveness, durability, and an environmentally friendly character.

Undoubtedly, a much older concept, "biomimetics," also led to obtaining performant structural materials and is intricated to the "materials by design" concept. The term itself ("biomimetics") was firstly introduced by Otto Schmitt [1], but its principles are considered to be used since Leonardo da Vinci (1452–1519) while designing flying machines after analyzing bird's ability to fly [2].

According to some beliefs, biomimetics is a transfer of ideas between biology and technology, aiming to obtain superior device. A more complex approach refers to it as being "a study of the formation, structure or function of biologically produced substances, materials, biological mechanisms and processes especially for the purpose of synthetizing similar products by artificial mechanisms which mimic natural ones" [2].

As expected, a lot of controversial interpretations arise from different approaches of biology and engineering, considered "baselines" of biomimetics. On the one hand, biology relates to living organisms (cells, plants, and animals) which evolve following a natural DNA-embedded cycle. On the other hand, engineering

*Natural and Artificial Superwettable Surfaces-Superficial Phenomena: An Extreme Wettability… DOI: http://dx.doi.org/10.5772/intechopen.84137* 

relies on human intelligence which develops successive steps in order to obtain a final product [1].

 Conflicts between biology and technology interpretations are classified through a Russian problem-solving system TRIZ (*Teorija Reshenija Izobretatel'skih Zadach*—"Theory of Inventive Problem Solving"), 40-standard features which offer solutions from both perspectives. For example, a few characteristics such as "keep poison out," "self-cleaning," "surface properties," and "waterproof" became conflict nr. 30-"external harm affects the object" [1].

Therefore, it became appropriate to say that "superficial properties" is a concept which can be regarded in the light of both "biology" and "technology." Structural investigation of natural (biological) surfaces is performed using microscopical techniques. Technical interpretations of these surfaces and empirical models arise. Examples include special wettable surfaces like the lotus leaf, rose petal, Salvinia leaf, insect eyes, wings, fish scales, etc. They provide templates in designing new engineered materials exhibiting improved properties compared to classical materials. Such artificially obtained materials and coatings can be considered results of "materials by design" and "biomimetics" concepts, as a reunion of biological inspiration and human engineering. Even though many contradictory assessments take place, it is important to state that biology and technology functioned perfectly together when inventing the Velcro closure system according to the way burdock (*Arctium* sp.) spreads its seeds, the helicopter inspired by the body of the dragonfly, the submarine resembling a whale, etc.

### **3. Extreme wettability: special patterns**

#### **3.1 Understanding wettability**

As is well known, surface wettability characterizes interfacial phenomena between a liquid and a solid support. The liquid's behavior on the studied surface is in fact an indicator of wettability, a superficial property which helps evaluate hydrophilicity/hydrophobicity of a solid. The quantitative indicator of wettability is represented by the contact angle, given by Young's equation (Eq. (1)):

$$\mathbf{Cost}\Theta = \frac{\mathbf{y}\_{\text{v}} - \mathbf{y}\_{\text{v}}}{\mathbf{y}\_{\text{v}}} \tag{1}$$

where θ is the contact angle, γSV is the solid-vapor superficial energy, γSL is the solid-liquid superficial energy, and γLV is the liquid-vapor superficial energy [3].

The equation establishes an equilibrium between superficial energies at the solid-liquid-air interface. However, adaptations of Young's equation were proposed by Wenzel [4] and Cassie-Baxter [5], after it was proven that the original equation only applies to homogenous, smooth surfaces and that the contact angle is influenced by the support's rugosities, as a surface roughness indicator.

Wenzel's equation (Eq. (2)) applies to non-smooth surfaces. Surface rugosity is interpreted through the roughness factor r, defined as ratio of the actual rough surface area to the geometric area projected on a relatively smooth surface. This adapted equation refers to an apparent contact angle θ′, as follows (Eq. (2)):

$$\mathbf{r}\cos\Theta' = \frac{r(\mathbf{\gamma}\_{\rm SV} - \mathbf{\gamma}\_{\rm SL})}{\mathbf{\gamma}\_{\rm LV}} = \mathbf{r}\cos\Theta \tag{2}$$

Another relationship defining an apparent contact angle θ′ is similar to Wenzel's equation, with the difference that the surface's rugosities are separated

by impenetrable air pockets (Cassie-Baxter wetting model). The surface f in direct contact with the liquid is considered, as follows (Eq. (3)):

$$\mathbf{f} = \frac{\sum \mathbf{a}}{\sum (\mathbf{a} + \mathbf{b})} \tag{3}$$

where a and b are the contact areas with the drop (a) and, respectively, air (b). Considering (1 − f) the drop-air contact area and a contact angle of 180°, the calculation formula corresponding to the Cassie-Baxter wetting regime is shown in Eq. (4):

$$
\cos \theta \text{'= f} \cos \theta \text{+ (1 - f)} \cos 180^\circ \text{= f} \cos \theta \text{+ f - 1} \tag{4}
$$

Empirical models of the Young, Wenzel, and Cassie-Baxter wetting states are presented in **Figure 1**.

Other interpretations by Quéré et al. [6, 7] consider the Wenzel wetting regime as an equilibrium state of the Cassie model: a critical value of the fraction f determines a critical contact angle θc, determined by the following equation (Eq. (5)):

$$\cos \Theta\_{\mathbf{c}} = \frac{\mathbf{1} - \mathbf{f}}{\mathbf{r} - \mathbf{f}} \tag{5}$$

Since wettability studies continue to unfold, researchers recently proved that Wenzel and Cassie wetting regimes actually co-occur on the same support surface. Hydrophobic surfaces with linear or pillar patterns exhibit both a Cassie levitating state corresponding to drops placed on the support and also a Wenzel pinned state for drops which come into contact with the surface after the impact. Transitions between these states were also reported as a result of external stimuli influence [8, 9]. Wenzel to Cassie and Cassie to Wenzel transitions were analyzed through sequential squeezing and releasing between texture surfaces of nonadhesive plate. Results indicate that both regimes exist at the same time on a double-scaled textured surface, resembling natural micro- and nano-surface architecture: the Wenzel state is characteristic for the larger texture and Cassie to the smaller one [9]. Further investigations consisted in exploiting these characteristics and developing super-repellent materials, also based on natural models, following the principles of biomimetics.

 The "superwettability system," briefly presented in **Figure 2**, includes a much extensive approach on wetting states, depending on the liquid type, the solid support's architecture, and the environment in which the phenomenon is described. Thus, the terms discussed above (hydrophilicity/hydrophobicity) refer to water's behavior in air and upon flat surfaces. Regarding low-surface liquids, such as oils, the "oleophilic/oleophobic" concepts are defining. Moreover, if the support

**Figure 1.**  *Comparison between wetting regimes: (a) Young, (b) Wenzel, and (c) Cassie.* 

*Natural and Artificial Superwettable Surfaces-Superficial Phenomena: An Extreme Wettability… DOI: http://dx.doi.org/10.5772/intechopen.84137* 

**Figure 2.**  *The "superwettability" system.* 

 exhibits a nano-/micro-rough architecture, then the behavior of liquids when contacting such a surface is known as "superhydrophobic/superhydrophilic" and "superoleophobic/superoleophilic." Corresponding wetting behaviors under water for structured rough supports are known as "superoleophobic/superoleophilic" and "superaerophobic/superaerophilic." If placed under oil, then the appropriate approach refers to "superhydrophobicity/superhydrophilicity" and also "superaerophobicity/superaerophilicity" [10].

## **3.2 Natural designs**

 Natural special surfaces transposed as survival skills in animals and plants captivated attention of researchers. They investigated and applied in practice what nature provided. Apart from scientists, novelists like Jules Verne were fascinated by certain elements from the environment and used them as inspirational sources to imagine innovative devices, mostly designed as transformational means and considered eccentrical in that era: the Nautilus submarine whose shape resembled a whale, the eponymous Steam House—a mechanical elephant, the helicopter imagined starting from insects' flight mechanisms and shapes, etc.

 Apart from mechanical devices, natures' kingdom offered humans the possibility to improve artificial materials, based on the evolution of SEM analysis techniques in the 1960s. Detailed investigations of surface structure and properties were performed. As a result, surface architecture was held responsible for many phenomena which were not explained at that time: how plants maintain clean in marshy environments and how their water needs are satisfied during high-temperature exposure. From this category, two types of surface structures, designed as micro- and nano-scaled patterns, confer superhydrophobicity to the leaves of certain plant species: lotus, rice, and taro. Another model which confers water repellency was attributed to a unitary structure of 1–2 μm fibers (Chinese watermelon, Ramee leaves). Also, vertical/horizontal hairs were attributed in the property of water repellency in case of *Alchemilla vulgaris* and, respectively, *Populus* sp. [11–13].

The iconic plant superhydrophobic behavior belongs to the lotus leaf (*Nelumbo nucifera*)*.* Surface wettability of the leaves is considered to be derived from the Cassie-Baxter wetting model: convex micrometric papillae along with nanometric wax needles determine water contact angles higher than 150°. This special architecture, also known as "The Lotus Effect," allows dust particles and other impurities to be collected by raindrops while rolling off (**Figure 3**) [14].

The other side of the lotus leaf presents no waxy crystals, but has tabular nanogroove convex lumps which confer inverse wettability [15].

 An example of unique structural characteristics is the carnivorous plant *Nepenthes alata*. Its prey is caught due to oleophobic features which allow insects to slide down to the digestive cavity, capturing them [16]. Contributing to the survival of the *Cladonia chlorophaea* lichen are hydrophobic strains ending in cup-shaped structures which limit water storage, preventing excessive accumulation and further damage to the plant [17].

An exponent of plant adaptation to harsh environment conditions is represented by *Salvinia molesta*, the water fern, who gave rise to "The Salvinia Paradox": hydrophobic hairs ending in hydrophilic peaks retain an air film while submerged, allowing respiration [18]. The thin air film retained at the air-water interface also enables *Oryza sativa* (rice) to carry on photosynthesis, enhancing gas exchange and diminishing Na+ and Cl<sup>−</sup> intrusion through submerged leaves through salt-water floods [19, 20].

"The Rose Petal Effect" reveals how nano-folds covered with micro-papillae of rose petals confer contact angle values of 152°, resembling the Cassie impregnating model: water droplets maintain their spherical shape, adhere to the surface, and do not slip when turned upside down. Compared to the lotus leaf architecture, this wetting regime is characterized by a liquid film which impregnates the papillae, leaving only some dry areas. A dependence was observed between the drops' volume and surface tension: the equilibrium is ruined and the drop falls if it exceeds 10 μL in volume. Thus, smaller drops stay stable, while raindrops slide off, since they are bigger [21]. **Figure 4** illustrates a comparison between the rose petal (a) and the lotus leaf (b) surface structure [22].

Transitioning from the plant to the animal kingdom, it is important to state that apart from "slippery" surfaces discussed above, "adherent" superhydrophobic surfaces were also noted: the gecko lizard's finger structures confer them the ability to climb even perfectly vertical walls, due to micrometric lamellae divided into nanometric setae. A drop placed on this surface retains its shape even in an antigravity

**Figure 3.**  *Lotus leaf structure. Dirt particle removed by rain.* 

*Natural and Artificial Superwettable Surfaces-Superficial Phenomena: An Extreme Wettability… DOI: http://dx.doi.org/10.5772/intechopen.84137* 

**Figure 4.** 

*(a) Rose petal surface structure (Cassie impregnating wetting state) and (b) lotus leaf surface structure (Cassie state) [22].* 

position [23, 24]. The gecko feet model inspired climbing a glass building using Kevlar and polyurethane special gloves [25]. The group of adhesive superhydrophobic natural surfaces includes also the rose petals, as previously discussed.

Regarded at first from a different angle, the insects' ability to fly, to maintain impurity and water-free wings, was later attributed to superhydrophobicity. Microscales hierarchically disposed on insect wings are responsible for maintaining them dry (**Figure 5**) and also exhibit, in some cases, antibacterial activity (cicada wings are bactericidal against Gram-negative bacteria) [26, 27].

 For some insects, patterns joining superhydrophobicity in alternation with superhydrophilicity represent an adaptation to harsh environmental conditions: *Stenocara gracilipes* (the Namib desert beetle) shows on its back a real storage system which captures water from atmospheric humidity, due to superhydrophobic waxy edges and superhydrophilic peaks [17]. In relation with survival skills, *Argyroneta aquatica* (the diving spider) creates around itself a hydrophobic artificial lung, which is permeable to gases and allows underwater living [10].

#### **3.3 Engineered superwettability—materials and coatings: practical applications**

Moving on from the theoretical field, extreme wettability is regarded an open gate for numerous everyday life and also industrial applications.

 Following biomimetic principles and varying surface templates, innovative materials are fabricated, depending on qualitative requirements. The first artificial superhydrophobic materials appeared in the early 1990s: the submicrometerroughed glass plates hydrophobized with fluoroalkyl trichlorosilane (CA = 155°) [28], fractal surfaces covered in n-alkyl ketene (CA = 174°) [29, 30], and ion-plated polytetrafluorethylene (PTFE) coatings with nanometric rugosities [31]. In the 2000s, surface topography studies were correlated with surface chemistry, leading to patterned silicone surfaces with low wettability [32].

**Figure 5.**  *Insect wing-microscaled superhydrophobicity.* 

Techniques used to confer surface roughness are still improving, along with transparency, permeability, resistance, and color change imparting methods [33–35]. Nature proved that hierarchical surface structures are responsible for surface special wettability, and not fluorocarbon derivatives, as it was considered at that time [36].

The most popular known procedures used to artificially obtain superhydrophobic surfaces include chemical reactions in a humid atmosphere [37], thermic reactions [38], electrochemical deposition [39], individual/layer-by-layer assembling [40], etching [41], chemical vapor deposition [42], and polymerization reactions [43]. Substrates include glass, metals (Cu, Ti, Zn), and cotton, and resulted structures exhibit CA > 150°, mimicking natural patterns [44]. For example, the rose petal was used as template in order to obtain polymeric coatings, resulting in "adhesive" superhydrophobicity [21]. Patterns resembling surface design of the lotus leaf were also fabricated through eco-friendly methods, without toxic solvents [45].

 Fluorocarbon and silicone derivatives were preferred as substrates in fabricating superhydrophobic surfaces, assuming that the larger the number of flor atoms, the higher is the hydrophobicity [46]. Nowadays, these materials are replaced with biodegradable ones, such as agricultural residues [47]. Recent studies indicate that lignocellulose can be successfully used in obtaining fire-proof coatings [48]. Another example of eco-friendly superhydrophobic coatings includes waterborne resins from aqueous silanes and siloxane solutions with silica nanoparticles applied as protective coatings to cultural heritage (marble, sandstone, cotton, ceramic artifacts) [49].

 The *Slippery Liquid-Infused Porous Surface* (SLIPS) technology includes smooth coatings applied onto military uniforms and medical gowns in order to avoid biological fluid contamination, due to surface fluids incorporated into a micro-/ nano-porous substrate [50, 51]. The field of *anti-bioadhesion* also involves protein adsorption, bacterial adhesion, and cell culturing media, all of them wettabilitydependent phenomena [52]. Thus, in vitro studies regarding platelet adhesion on implants reveal that no adhesion happens on TiO2 nanotube-covered supports. Moreover, polydimethylsiloxane (PDMS) surfaces with various sized-rugosities, superposed scale plates, submicron structures, and nanostructured and smooth surfaces, proved the highest effectiveness against blood platelet adhesion in superposed scale plate surface [53]. Antibacterial cellulose fibers modified with siloxanes and silver nanoparticles show durable activity against *Escherichia coli* and *Staphylococcus aureus* [54], while bactericidal action against *Pseudomonas aeruginosa*  was discovered for fluoroalkyl silane-hydrophobized glass [55].

The result of joining extreme wettability surfaces are patterns which promote development of cells planted as hydrogels/solutions in the hydrophilic zone. Advantages of the method include lack of lateral contamination risks due to hydrophobic separative borders, efficiency, economic analysis method, the possibility of real-time screening, and noninvasive diagnosis [56–58].

Other high-impact applications of superhydrophobic surfaces include *anti-icing*, *antireflective*, and *low friction* properties, mostly popular in the marine and aviation transportation fields, mirrors, and lens industry [59–62]. These properties along with a low adhesion degree, a high contact angle, and a low sliding angle allow impurity collection, while rolling off represent desired characteristics for windshields, exterior windows, and solar panels [63].

 Since *metal corrosion* is a contemporary problem, superhydrophobic anticorrosion treatments were developed: coating techniques (microwave chemical vapor deposition, followed by immersion) with fluorochloride silanes of magnesium alloys and substrate modifications (Al with hydroxides, Zn immersed in superhydrophobic solutions), proving resistance against acids, alkaline, or saline solutions [64].

*Natural and Artificial Superwettable Surfaces-Superficial Phenomena: An Extreme Wettability… DOI: http://dx.doi.org/10.5772/intechopen.84137* 

Closely following the corrosion issue is *friction reduction*, which is of interest in aeronautics and ships. The shark skin and the lotus leaf are models in designing continuous surface films with self-contained air bubbles, able to reduce laminar and turbulent liquid flow, lowering friction forces. Moreover, recent progress includes high-pressure-resistant special surfaces, with a high impact in the submarine industry [65]. Apart from enhancing classical transportation devices, inspirational novel ones were developed flowing the water strider's model. Prototypes of miniature robots which walk-in straight-line and function as water-pollutant monitors, displaying high transport capacities [66, 67]. Water collecting/storing systems are still developing as a solution for dry areas, starting from the Namib beetle's back special architecture [68].

 Interdisciplinary researches on surface extreme wettability will be continued by discussing an intrinsically superhydrophobic behavior, characteristic for versatile structures entitled liquid marbles.
