**3. Cactus inspired conical structure**

#### **3.1 Biological model and theoretical analysis of cone**

Plants survived in extraordinarily droughty conditions tend to have advanced fogharvesting ability inevitably. Inspired by a cactus [37], conical structures were developed as a perfect model for water collecting. Agaves [38] and wheat awns [39], which have similar needle-like construction, also can harvest fog. The geometry of cone can be deemed as half of a spindle knot to some degree when analysing driving force. The Laplace force is an attraction to gather fog-water, which can be calculated by Eq. (1). Also, roughness gradient (Eq. (2)) can be introduced.

By integrated the cones into oblique conical micro- and/or nano- array (or barbed array, bioinspired by Eremopyrum orientale or ryegrass leaves) [40, 41] on 2D planar materials, anisotropy surface with enhanced water-collecting property from foggy atmosphere were developed. As for the mechanism, oblique array with anisotropy gives rise to a difference in concomitant retention force (*Fr*) on hindering water transport to two directions. When the droplet moves, the retention force can be estimated by [42]

$$F\_r = w\gamma(\cos\theta\_{RO} - \cos\theta\_{AO})\tag{4}$$

where w is the width of water droplet, *θRO* and *θAO* indicate the intrinsic contact angle. Moving along the oblique direction, decreasing solid–liquid contact width is beneficial to postpone the water meniscus resulting in water release, while moving against, increasing solid–liquid contact width causing pinning of water droplet. Experiment also illustrates that the retention forces of the droplet against the oblique direction are larger than those along the oblique direction [40, 43]. As a result, the distinction of retention force along two opposite direction can ultimately lead to water directional removal.

#### **3.2 1D and 2D patterns via different methods using distinct materials**

With regards to 1D pattern, research was frequently carried on metals with hydrophilicity. Thanks to the electroconductivity of metals, electrochemical method (e.g., controlled electrochemical corrosion or gradient anodic oxidation) can be employed [44, 45]. For instance, a copper wire (usually �800 μm in diameter) was placed vertically and attached to the anode of a 10 V DC power with a curled copper sheet connected with the cathode. For one thing, a container filled with CuSO4 solution (electrolyte) pumps from a syringe pump which increases the liquid level of electrolyte at a constant velocity, causing a gradient of electrochemical corrosion of the copper wire and thus producing a conical shape. For another, controlled by a periodic current (from 0.05 to 0.8 A, pondered for 5 s, and then declined to 0.05 A) using the DC power, the roughness gradient was obtained.

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

In terms of 2D planar model, an interesting method called soft lithography [40, 41, 44, 45] was developed to directly copy biological surfaces. Primarily, fresh biological sample was fixed flat on a petri dish. Non-fizzy PDMS mixture (10:1 polydimethylsiloxane and its curing agent after uniform stir and vacuum treatment) was decanted into the dish and heated at 325 353 K for 3 5 hours (heating temperature and time contributing to hardness and flexibility of as-prepared PDMS model). After free cooling to room temperature, PDMS model was apart from biological sample with caution. Whereupon, the mirror symmetric PDMS mould of target architecture had been prepared. Eventually, an analogous step was repeated to get the target structured model. If the material of target model must be PDMS, original PDMS mirror was fluoridized (fluorination in vacuum at 363 K for 5 h) beforehand. As for PVDF material, PDMS mirror was pressed on the PVDF powders (which has spread on a clean glass slide and heated to 543 K until the organic material were fully melting in advance) at 2 106 Pa for 10 seconds. Microstructure of biological sample can be easily coped by such patent. In addition, for conical array surface pattern without using biological sample, the mirror mould mentioned above can also be manufacture via machine spotting, drilling or punching on PE (polyethylene) or PVC (polyvinyl chloride) board [46]. By adding magnetic powders (such as Co) into PDMS before curing, flexible magnetic PDMS pattern was made whose cone array could tilt at desirable angles under controllable external magnetic field [43, 46, 47].

#### **3.3 Fog-harvesting efficiency of cactus inspired structure**

Multi-gradient copper wire shows a satisfying efficiency in fog-harvesting tests. As illustrated in **Figure 6(a)**, the first three water drops transported and combined into one droplet within 0.18 s directly the first water drop was formed on the cone copper wire [44]. In another research [45], oxidised copper wire with multi-gradient was compared with the original copper wire, showing a unidirectional movement and enhanced (increasing 70%, in 60 s) fog harvesting efficiency (**Figure 6(b)**). Fog harvesting performance of such 1D cone sample is associated with the tilt angles of placing sample. As demonstrated in **Figure 6(c)**, the collecting efficiency peaked at 0.36 mg/s (experiment done in temperature 288 K, and fog flow velocity 1.8 m/s) when gradient cone copper wire was placed horizontally [44].

2D biomimetic models also show high efficiency or show larger-scale watercollecting capacity. As demonstrated in **Figure 6(d)**, PDMS surface with vertical needle-like array showed a fog harvesting function at 80% humidity air while magnetic cone with flexibility under alternating magnetic field exhibited 50 times higher fog-harvesting efficiency (at about 0.2 g/h under the quasistatic foggy environment) in 1 hour than the static cone [46]. Biomimetic copied artificial leaf with oblique barbed array on surface involves a fog-harvesting capacity of 95.4 g/m<sup>2</sup> /h (air flow rate of 10 L/min and 3.5 bar pressure delivering a water nebulization rate of 0.4 mL/min at 296 K) and it was improved to 136.8 g/m<sup>2</sup> /h after an initiated chemical vapour deposition of hydrophobic surface nanocoating. It is worth noting that by integrated 18 copper wire 1D patterns (fixed on a 6.5 6.5 0.5 cm Teflon frame, side-to-side horizontally setup) into a tiny water-collecting device, the highest value of fog-harvesting rate can reach up to surprisingly 6180 g/m2 /h under 2.4 m/s fog velocity in temperature 288 K [44].

From what is cited above, it is safe to draw a summary that different gradient in surface morphology or shape of cone structure, arrangement of cone or cone array, and wettability of materials will affect fog-harvesting performance.

#### **Figure 6.**

*Water-collecting of diverse cone structured patterns. (a) Transport process of first three drops on a multi-gradient cone copper wire (by controlled electrochemical corrosion) [47], scale bar 1 mm. (b) Fog-water collection behaviour between an original copper wire and a multi-gradient copper wire (by gradient anodic oxidation) [45], scale bar 2 mm. (c) Fog-water unidirectional transport properties of a multi-gradient cone copper wire (by controlled electrochemical corrosion) at different tilt angles [44]. (d) Water harvesting of a magnetic cone with and without the external magnetic field in the same chamber [41].*
