Acknowledgements
6.2 Desert Beetle-Inspired Surface with Patterned Wettability for Fog Collection
6.2.3 Direct Methods for Creating Patterned Wettability for Fog Harvesting
heptadecafluorodecyl-trimethoxysilane (FaS) to change the wettability from superhydrophilic (Figure 6.5a) to superhydrophobic (Figure 6.5b), where fog droplets hardly wet the surface and remain in a spherical shape (Figure 6.5b). in the next step, photomasks with a circle-shaped pattern or 4-, 5-, 6-, and 8-pointed star-shaped patterns were used to obtain features of wetta- bility patterns via selective illumination of UV light, which decomposed the FaS monolayer (Figure 6.5c) by the underneath photocatalytic tiO2. On the surfaces with star-shaped wettability patterns, fog droplets could be direc- tionally collected toward more wettable regions (Figure 6.5c). they found that such surfaces with star-shaped patterns realize more efficient water collection compared to other surfaces that are uniformly superhydrophilic, uniformly superhydrophobic, or even circle patterned. the improved water collection by the star-shaped pattern is because the tips of the star generate a Laplace pressure gradient from the shape gradient, which further enhances this directional movement of water droplets toward the center of the stars.
6.2.3 Direct Methods for Creating Patterned Wettability
by a subsequent pattern transfer, which is multistep, indirect, and expensive.
to produce stable hydrophilic micro-sized patterns on superhydrophobic substrates for practical applications, a direct method (i.e., one-step and thus mask-free) of depositing hydrophilic species in well-defined micropatterns is still highly desired. the difficulty of direct deposition of hydrophilic spe- cies onto the superhydrophobic surface to obtain the micropatterned surface lies in the following: (1) the ultralow surface energy of the superhydropho- bic substrate surface greatly decreases the adhesion between the hydrophilic species and the surface, resulting in unstable deposition of the hydrophilic species;42–44 (2) in addition, a superhydrophobic surface entails rough sur- face structure and trapped in the interstices of the rough surface is air. due to the presence of those discrete air pockets, a water droplet sitting on such a solid-air composite surface interacts only with a small fraction of the solid surface, which is widely known as Cassie’s wetting state.45,46 therefore, in this case, both limited interaction area and adhesion between the hydrophilic species in the aqueous droplets and the superhydrophobic substrate make it impossible to directly pattern superhydrophobic surfaces. thus, a direct and general method to this end has to enhance the interaction strength (i.e., adhesion) between the hydrophilic species in the aqueous droplets and the superhydrophobic surface and to increase the contacting area of solid surface and liquid droplets.
Zhai and coworkers first prepared hydrophilic patterns on superhydro- phobic surfaces using a layer-by-layer assembly of polyelectrolytes in a mixed water/2-propanol solvent.47 this approach can be used to create patterned surfaces that mimic the water harvesting structure of the Stenocara beetle’s back. however, this method is not suited for the large-scale fabrication of the patterned surface, and is difficult to control the pattern size.
recently, inspired by high adhesive mussel protein,48 Zhang et al. have reported a facile and, most importantly, direct method for the preparation of a micropatterned surface for fog harvesting by an inkjet printing strategy (Figure 6.6).49 by directly inkjet printing a mussel-inspired ink of dopamine solution with delicately optimized solution composition, stable Wenzel’s microdroplets of dopamine solution with well-defined micropatterns are obtained onto the superhydrophobic surfaces. Upon the formation of poly- dopamine via in situ polymerization, superhydrophilic micropatterns with well-controlled pattern dimension can be readily achieved on the superhy- drophobic surfaces (Figure 6.6a and b). this method is very convenient for the design and fabrication of patterns with different sizes and configura- tions, and is thus suitable for the optimization and selection of the pattern for fog collection purposes. their results have revealed that the micropat- terned superhydrophobic surface with a similar pattern dimension to that of the desert beetle’s back (i.e., ∼500 µm pattern size and ∼1000 µm separation distance), exhibited the highest water collection efficiency among the uni- formly superhydrophobic surface, superhydrophilic surface, and other pat- terned surfaces with different pattern sizes. With this convenient strategy, the design and large-scale fabrication of patterned superhydrophobic surfaces
would be greatly facilitated, which opens up a new avenue for the patterned superhydrophobic surface in practical applications, such as fog harvesting and water condensation/collection in thermal desalination processes.
to further improve the practical applicability, thickett et al. prepared thin polymer coatings with hybrid hydrophobic and hydrophilic regions by a dewetting method.50 in their study, polymer bilayers were prepared on clean, smooth silicon substrates consisting of a polystyrene (pS) under layer and a poly(4-vinylpyridine) (p4Vp) top layer by sequential spin coating. anneal- ing was performed above 160 °C (above the glass transition temperature for both polymers), and a resultant surface morphology consisting of a series of isolated droplets and interconnected cylinders of p4Vp on a pS background Figure 6.6 (a) Scheme of the inkjet printing method for the micropatterning of superhydrophilicity on superhydrophobic surfaces. Optical micro- scopic photograph of the as-printed dopamine droplet (b1) and (b2) on the superhydrophobic surface. (b3) and (b4), SeM images of the poly- dopamine patterns. (c) experimental setup of water collection system from fog. (d) Water collection efficiency by five different surfaces. L.
Zhang, J. Wu, M. n. hedhili, X. Yang and p. Wang, J. Mater. Chem. A, 2015, 3, 2844–2852. reproduced by permission of the royal Society of Chemistry.
was obtained directly (Figure 6.7). they demonstrated that such hybrid films could achieve an enhanced rate of surface water condensation. Such coatings can be prepared on substrates of any shape using conventional techniques such as spin coating, dip coating, or spraying and the procedure is readily scalable, allowing for fabrication on the meter scale.