Introduction
1.6. Limitations of Conventional Methods
relevant severe settings is rare in the literature, likely due to the poor durability of artificial superhydrophobicity in challenging scenarios. Xue et al.147 utilized underwater superoleophobicity unprecedentedly for oil/water separation. An underwater superoleophobic hydrogel-coated mesh, which consists of rough nanostructured hydrogel coatings and microscale porous metal substrate (Fig.
1.20D-E), was able to separate water from various oil/water mixtures (such as vegetable oil, gasoline, diesel, and even crude oil/water mixtures) selectively and effectively ( > 99%) without any external intervention as shown in Fig. 1.20F-G. A lot of other research articles were published later where underwater superoleophobicity was used for efficient oil/water separation.148-151
These two properties are especially advantageous over the conventional methods because the purity of the separated phase meets the requirements for reuse and such approaches do not have any adverse effect on the environment. I intend to develop materials with durable and extreme anti-wettability properties keeping the practical application of oil/water separation in mind because of its huge impact on the environment.
several prospective applications, however, the use of such materials is drastically restricted at practical scenarios where exposure to different physical (scratching, creasing, etc.) and chemical (high salt, acidic and alkaline conditions) insults are very common. Appropriate hierarchical (micro-/nano-
scaled) topography and essential chemistry are likely to be perturbed even when a small amount of force is applied to the material. The damaged interface under mechanical stresses exposed hydrophilic interior as shown in Fig. 1.22A, and thus, the superhydrophobicity was compromised.154 Often, the essential low surface energy is optimized on top of the hierarchical features by depositing a thin (in nm scale) layer of low surface energy molecules through weak covalent bonds (e.g. metal-sulfur chemistry155-156) or ionic interactions157-158 which is likely to fail under chemically complex and harsh environments like extremes of pH or salt. Silane chemistry159-160 has also been widely employed to achieve this extreme water-repelling property, but this approach has downsides as well, which is mainly its inability to sustain prolonged UV exposure. These kinds of functionalization failed to maintain low surface energy upon exposure to various chemical environments. By and large, it can be inferred that physical and chemical insults lead to compromise of the anti-wetting property of the artificial superhydrophobic materials. To overcome these limitations, self-healing (Fig. 1.22B) and post-repairing approaches were introduced with improved durability. However, the performance of such designs under harsh settings are limited for few cycles and such approaches also demand external intervention to cure the damaged wettability.161-162 Thus, the development of the chemically and physically durable superhydrophobic interface remained an exciting research topic.
1.6.2. Underwater superoleophobic surfaces
The two prerequisites (i.e. hierarchical topography and high surface energy) to achieve underwater superoleophobic surfaces are mostly fulfilled by the following approaches - (i) Hydrogel-
Figure 1.22: (A) Illustrating the loss of superhyphobicity upon physical abrasion which causes removal of top surface and exposes the hydrophilic bulk. (B) Schematic representation of self-healing superhydrophobic coatings that sustained physical abrasions for few healing cycles through migration of 1H,1H,2H,2H-Perfluorooctyltriethoxysilane (POTS) to the abraded top surface. Reprinted with permission from Ref. 154 Copyright 2011, Wiley-VCH. Reprinted with permission from Ref. 161 Copyright 2010, Wiley-VCH.
based approach7,110-111, (ii) metal oxide-based coating112,116-117 and (iii) Electrostatic multilayers118-120. These approaches have been discussed earlier. A large number of reported underwater superoleophobic surfaces, fabricated by the aforementioned methods one or the other, are available in the literature, but
there are many disadvantages associated with these methods restricting their practical application. For the hydrogel-based approach, the issue is the delicate nature (Fig. 1.23C-F) of the hydrogels whenever exposed to aqueous phase163-164, thereby unable to withstand any mechanical stresses (Fig. 1.21A) which are inevitable during transportation, packing or intended use. In comparison to that, metal oxides are much more rigid and tough. However, due to the reactive nature, such hydrophilic metal oxide coatings are highly prone to disintegrate in practically relevant severe chemical environments like extremes of pH, saline water, etc. and as a consequence, the bio-inspired wettability of the coating is eventually compromised. On the other hand, the weak electrostatic interaction between the oppositely charged polymers/nanoparticles in electrostatic multilayer coating that tightly holds the special orientation of each polymer in the multilayer coating, easily gets compromised on various harsh
Figure 1.23: (A-B) Schematic illustration of the underwater oil wetting state on gold nanoparticle (AuNPs)/
poly(diallyldimethyl‐ammonium chloride) (PDDA) film consisting of 7 bilayers in presence (A) and absence (B) of salt. Presence of ions effects roughness through coiling of polyelectrolyte chains and the ratio of water content, and thereby changes the underwater superolephobicity (C-F) Swelling of clay-nanocomposite (NC) gel before (C,E) and after (D,F) their immersion in seawater for one year which affected the topography of the gel material (E-F). Reprinted with permission from Ref. 118 Copyright 2013, Wiley-VCH (A-B). Reprinted with permission from Ref. 164 Copyright 2019, Elsevier (B)
physical as well as chemically complex exposures.118,165 For instance, Xu et al.118 prepared an electrostatic multilayer using gold nanoparticle (anionic in nature) and a cationic polymer which changes its wettability depending on the salt concentration of the medium (Fig. 1.23A-B), and therefore it cannot be used for real-life application. In short, the underwater superoleophobic interfaces that are designed following these approaches are not suitable as far as practical applications are concerned.
Therefore, it is very evident that the practical implementation of both the properties is highly challenging due to the poor durability of the materials against physical and chemical manipulations.
Further, a common synthetic approach for adopting two different biomimicked wettability properties, i.e. superhydrophobicity and underwater superoleophobicity by appropriate optimization of chemistry and topography could be useful for various applied and fundamental contexts.