**4. Desert beetle inspired heterogeneous surface**

#### **4.1 Biological model and theoretical analysis of wettability paradox**

Namib desert is one of the oldest and driest existing desert in the world. Plenty of species died out while others which have been adapted to the arid conditions (such as Namib desert beetle, spider, snack, grass, cactus) enjoy a good life. Desert beetle, with superhydrophilic wax-free texture (peaks) and hydrophobic wax-coated array (groove) on its back, has long been imitated for fog collecting [48, 49].

Heterogeneous wettability architecture has also been found in the plant kingdom, i.e., the Salvinia. Salvinia effect [50, 51] represents the stabilisation of an air layer upon a submerged superhydrophobic surface with hydrophilic pins array (which might be a new model for fog-harvesting in the future studies). Salvinia wettability paradox further indicates the phenomenon of a functional entirety with discrete wettability, similar to the heterogeneous construction of Namib desert beetle back.

The principle of its fog-harvesting ability is the fact that superhydrophilicity makes it easier to seize water and superhydrophilic texture combined with hydrophobicity groove gives an appointed route for water drop to transport. The discrete driving force of wettability imbalance can be calculated by Eq. (2), further describing as

$$F\_w = \chi \left(\cos\theta\_{\text{hydrophilic}} - \cos\theta\_{\text{hydrophilic}}\right) \tag{5}$$

where *θ*hydrophilic and *θ*hydrophilic are contact angle of water on two opposite sides with two distinct wettability. If the *θ*ð Þ super hydrophobic> 150° and *θ*ð Þ super hydrophilic<5°, a large driving force caused by wettability imbalance will impel water droplet into more hydrophilic region, which can be further designed as fog-harvesting model.

#### **4.2 Various wettability paradox obtained by diverse advanced technology**

In order to obtain an entirety with discrete wettability, scientists with distinct research backgrounds may utilise diverse advanced approaches. A facile avenue is to composite (in a macroscopic scale) another material with distinct wettability onto the surface of original material. For example, a superhydrophobic copper oxide gauze can be incorporating onto the surface of a hydrophilic PS flat sheet through thermal pressing [52].

Another strategy is to fabricate by various coating or corrosion technology on a basic material (e.g., glass slide) to selectively change the wettability (e.g., via a mask). For instance, primarily, a superhydrophilic surface composed of TiO2 nanoparticles was obtained on a bare glass substrate via a spin-coating method. Then, it was superhydrophobicly treated using heptadecafluorodecyl-trimethoxysilane (FAS). Last but not least, the functional pattern was realised by illuminating the FAS-modified film under UV light with a photomask. The FAS-treated superhydrophobic TiO2 surface becomes superhydrophilic again owing to the photocatalytic decomposition of the FAS monolayer after being exposed to UV light [53]. Via this approach, circle-pattern, 4-, 5-, 6-, 8-pointed star-patterns, and other graphic patterns was manufactured on surface (**Figure 7(a)**). The graphic patterns depend on the geometry of photomask.

Laser treatment can be used in manufacturing wettability gradient. For example, a superhydrophilic-superhydrophobic surface on Pyrex wafer was prepared using femtosecond lasers, through the processes of Teflon-like polymer (CF2)n depositing and Femtosecond laser ablation to selectively remove the superhydrophobic coating [55]. Similarly, selective plasma corrosion [56], electrochemical etching [57], and inkjet printing [54], with similar designed procedures, have been developed to obtain micropatterns with wettability paradox.

#### **4.3 Fog-harvesting efficiency of bioinspired heterogeneous surface**

The strategy of fog-harvesting inspired by desert beetle turns out to be very effective. As is displayed in **Figure 8(a)**, a high-contrast (superhydrophobic array) wetting glass collected more (60%) fog within 30 min than a blank glass [55]. In another research, by changing the size and arrangement density of superhydrophobic array, � 400% improvement in fog-harvesting test (**Figures 7(b) and 8(b)**) was realised (increasing from 149 g/m<sup>2</sup> /h to 618 g/m<sup>2</sup> /h, at about 295 K environmental temperature and � 0.1 m/s fog rate using humidifier, the surface temperature of sample: 277 K and air around sample: 90–95% humidity, vertically placed) [54].

#### **Figure 7.**

*Water collection processes on various kinds of surfaces inspired by desert beetle. (a) Observation of water collecting behaviour on wettability-paradox surface with distinct graphic pattern (fabricated via FAS-modifying and UV selective photocatalytic decomposition): from top to bottom: superhydrophilic surface, superhydrophobic surface, circle-shaped pattern surface, circle-shaped edge surface, star-shaped edge surface, star-shaped circle surface [53], scale bar 100 μm. (b) Observation of water collecting behaviour on wettability-paradox surface with distinct arrangement density and size (fabricated via dopamine ink-jet printing), from top to bottom: superhydrophobic surface with 500/200/200 mm polydopamine patterns and respectively corresponding 1000/400/1000 mm separation [54], scale bar 500 mm.*

*An Application of Bio-Inspired Superwetting Surfaces: Water Collection DOI: http://dx.doi.org/10.5772/intechopen.105887*

#### **Figure 8.**

*Distinct factors which may exert influences on fog-collecting efficiency of wettability-paradox surfaces inspired by desert beetle. (a) fog harvesting comparation of a piece of blank glass and a piece of glass with wettability-paradox surface [55]. (b) Influence of arrangement density and size on water collecting efficiency [54]. (c) Impacts of materials and density of composite on water collecting efficiency [52]. (d) Effects of pattern shapes and placed incline-angle upon fog-harvesting efficiency [53].*

As composite can be made of different materials and density, how these factors affect water-collecting property had been investigated. As demonstrated in **Figure 8(c)**, component and gauze size exert considerable influences on fog harvesting (due to the distinct contact degrees of the hydrophobic gauzes), where the maximum fog collecting rate was 1590 g/m<sup>2</sup> /h at ambient conditions (a simulated flow of fog about 0.12 m/s, relative humidity around the samples: 90–95% and temperature: 295 K, vertically placed) [52].

Fog harvesting performance is also linked to the graphic pattern on wettability paradox surface. Different fog-collecting processes was found on surfaces with various wettability features (**Figure 7a**). On uniformly superhydrophilic surface, the water droplet spread over surface while on total superhydrophobic surface, individual water droplets coalesced in disorder. However, water droplets gathered directionally toward the superhydrophilic place on bioinspired surfaces with wettability pattern. Diverse wettability pattern offers different route for water to combine and move away, resulting in distinct fog harvesting performance. Generally speaking, the fog collecting processes are relative continuous because immediately after a fog-water droplet move away, another one can be captured. Consequently, directional motion of tiny water droplets enhances the fog-collecting efficiency. In terms of fog-harvesting efficiency, the highest value occurred in 5-pointed star wettability pattern, which peaked at amazing 27,800 g/m<sup>2</sup> /h (fog flow 0.75 m/s in velocity under room temperature, inclined by 45° from the horizonal plane). In addition, the placed incline-angle also affects the collecting rate and simply by placing it vertically, the efficiency climbed to 32,000 g/m2 /h (**Figure 8(d)**) [53].
