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Besides the above techniques, a variety of other methods also exist for facilely enabling states of superhydrophobicity. Today, the known literature is too voluminous to list, but the most notable mechanical and wet-chemical methods are highlighted below (Figure 2.42).

Figure 2.42. Other mechanical methods for achieving superhydrophobic films. a-b) 3D-printed porous membrane (0.37 mm pore size) with c) superhydrophobic properties.[403] d-e) 168 nm and 19 nm thick wrinkled Teflon films with f) superhydrophobic properties.[404] _{g-i) Fractal surface topology} of hot-pulled nano-fur surfaces with j) superhydrophobic properties.[405] k) Petal-like etched structures (Fe2O3).[406] l) Granular structures of FeF3 and CrF3.[406] m-n) Flat platelet-like sheets from etching with o) superhydrophobic properties.[407]

Inkjet and 3D Printing

With the advent of inkjet and 3D printing technologies, their use in micro-patterning superhydrophobic surfaces is slowly gaining momentum. They have been tradiationally used for creating printable microfluidic channels on phase-separation derived superhydrophobic paper,[408] lipid hydrophilicization of superhydrophobic glass,[409] and dopamine hydrophilicization of superhydrophobic textured SiO2.[410] _{Water-soluble ink-jet printed patterns are sometimes also used} as sacrificial scaffolds.[411,412] While hydrophilicization of superhydrophobic surfaces tend to take precedence owing to its simplicity, superhydrophobic patterning has also been achieved by Ngo et al. through the laser-printing of superhydrophobic fluoro-functionalized SiO2 blended toners.[413]

3D printing is a technology that is in its early stages of infancy and the as-printed millimeter-sized printed features still possess pitch distances that are too large for inducing a functional Cassie-Baxter state. However, the resolution of 3D printing has been gradually improving over the years. Lv et al. recently demonstrated the synthesis of membrane mesh lattices with superhydrophobic properties. The ink comprised of PDMS precursors that were impregnated with hydrophobic SiO2 powders for enhanced roughness. The 3D-printed mesh lattices achieved achieving functional states of lotus-like superhydrophobicity, with a CA of ca. 160°.[403]

Thin Film Wrinkling

The dependence of superhydrophobicity on rough hierarchical structures can be exploited by employing the natural surface wrinkling of thin films. Examples in nature range from the wrinkling of bio-cellular epidermal layers of skin[414] to geological wrinkles[415]. In the field of thin film coatings, spontaneous wrinkling[416] is traditionally treated as an unwanted defect, and significant research efforts were once directed towards developing perfectly flat surfaces.[417] _{Wrinkling-induced} superhydrophobicity can be achieved by the use of wrinkled PDMS, formed by sequential pre-stretch and UV-oxidation. The oxidation results in a thin inflexible layer of silica, which forms wrinkled patterns after relaxing the PDMS sub-layer. Functionalization of the wrinkled silica layer results in

lotus-like superhydrophobicity.[418] _{PDMS has also been used as a template for growing thin, rigid} polymeric top layers, such as polyaniline.[419] Other methods also include electrodeposition-induced wrinkling in poly(ethylenedioxythiophene) (PEDOT) polymer films,[420] shrink-wrap wrinkled Teflon[404] _{and hierarchical wrinkling that is induced by nano-imprint lithography of poly(2-} hydroxyethyl methacrylate) (PHEMA)[421], coupled to standard compression methods.

Film wrinkling can be exploited as a dynamic behaviour. Li et al. exploited the concept of wrinkling for an on-line tunable system which responds to different levels of humidity. These stimuli-responsive films were developed based on the sequential cross-linking and wrinkling of LbL-assembled poly(acrylic acid)-poly(allylamine hydrochloride) (PAA/PAH) films that were impregnated with fluoroalkylsilane (FAS) functionalized SiO2 nanoparticles. Resulting wettability ranged from hydrophobicity to superhydrophobicity, achieving a maximum CA and minimum SA of 157° and 2° respectively.[349] Wong et al. demonstrated a mechanically-dynamic wetting system by wrinkling 1D nano-structures, thus culminating in superhydrophobic wave-like nanofibers. This was achieved by combining aligned electrospinning and substrate pre-stretching.[90] _{Superhydrophobic CA, SA and} CAH of 167°, 5° and 7° were respectively attained.

Nanofur and Embossed-Hot Pulling

Nanofibers have always been a foundational cornerstone in superhydrophobicity.[133,230] Since its original inception, minimal changes have been made to the most popular and primary mode of synthesizing nanofibers: electrospinning.[90,133,223,230,273,422] _{In recent years, a technique birthed from} the industrial method of embossing and hot pulling gave rise to a uniquely functional nano-structured fur-like morphology.[405] _{The dense nanohair(fur)-like structures are highly scalable, and can be} simply “hot-pulled” from a slab of PC by a sandblasted mold.[405,423] The fur has a non-uniform diameter that ranges from microns to just 200 nm, naturally enabling the hierarchical profile needed for superhydrophobicity. It has demonstrated its universal functionality across different platforms, and has been shown for applications that extends to SLIPS,[405] long-lived underwater

superhydrophobicity,[405] _{oil-water separation,}[424,425] _{solar cells}[426] _{and even drag reduction.}[177] Superhydrophobicity was showcased at CA, SA and CAH of 170°, 10° and 30° respectively for these series of surface coatings.[405]

Acid-etched Metals

Etching of metals to produce rough corrugated profiles is a well-known procedure. This process is particularly suitable for rapidly developing superhydrophobic metal surfaces, owing to its ability for directly modifying the top-most layers of metallic materials. Metals such as 304 and 316 stainless steel,[406] copper[407] and aluminium[314,427-429] are all suitable candidates. A variety of etching solutions can be utilized, including hydrofluoric acid,[406] _{stearic acid,}[407,428] _{potassium hydroxide} and lauric acid[314,427,429]. Morphologies that are synthesized can be vastly different, ranging from petal / platelet-like,[406] _{granular-like,}[406] _nano-plates,[407] _{micro-structured pits,}[427] _micro- cuboids,[314,428] or nano-flakes[429]. The superhydrophobization of such textured surfaces can simply be conducted by fluoropolymer deposition,[406] or the condensation of other metal-organics such as Zn(AC)2.[314] _{Moreover, they also show enormous potential for one-step etch-functionalization} procedures. For instance, Bahrami et al. and Varshney et al. demonstrated the use of stearic acid and lauric acid for the respective modification of copper and aluminium, achieving inherent superhydrophobicity upon synthesis, with CAs and SAs of 155°, 153° and 7°, 5° respectively.[407,427]