3.2 How to Construct Nanomaterials with Superwetting Surfaces
3.2.3 Natural and Artificial Examples of Superwetting Nanomaterials
a classic and natural example of a superhydrophobic material is the lotus leaf (Figure 3.2a), which is composed of micro- and nanometre-scale hierar- chical structures and hydrophobic cuticular wax (Figure 3.2b and c).48 people believe that the main reason for its superhydrophobicity is the combination of the hydrophobic surface chemical composition with low surface energy
Figure 3.2 (a)–(c) digital photograph and SeM images of the superhydrophobic lotus leaf.48 reprinted with permission from ref. 48. Copyright 2008, aip publishing LLC. (d) digital photograph of a water strider which stands on the water surface.49 (e) SeM images of the water strider’s legs. reprinted by permission from Macmillan publishers Ltd: Nature (ref. 49), copyright 2004. (f) and (g) SeM images of the porous micro- spheres and nanofibres of the superhydrophobic pS film.50 (h) a water droplet on the superhydrophobic pS film. L. jiang, et. al., a lotus-leaf- like superhydrophobic surface: a porous microsphere/nanofibre com- posite film prepared by electrohydrodynamics, Angew. Chem., Int. Ed., 2004, 43, 33. Copyright © 2004 john Wiley & Sons, inc.
and the micrometre- and nanometre-scale surface roughness. another clas- sic and natural example of a superhydrophobic material is the water strider’s non-wetting legs which can enable the water strider to stand on the water sur- face effortlessly (Figure 3.2d).49 the superhydrophobicity of the water strid- er’s legs has been traced to the micrometre-scale needle-shaped setae with numerous nanometre-scale grooves and the secreted hydrophobic wax on the elaborate structures (Figure 3.2e). this is the result of the combination of the hydrophobic surface chemical composition and the micrometre- and nanometre-scale surface roughness, too. inspired by these natural superhy- drophobic materials, and combining the theories of wettability, people have invented a series of artificial superhydrophobic nanomaterials by generating highly rough surfaces of low surface energy materials or integrating low sur- face energy coatings on as-fabricated rough surfaces. jiang et al. developed a lotus leaf-like superhydrophobic polystyrene (pS) film with a novel compos- ite structure composed of porous microspheres and nanofibres (Figure 3.2f) by a electrohydrodynamics technique.50 Like the hydrophobic cuticular wax on the lotus leaf, the pS is a low surface energy and hydrophobic material as well. Sizes of the microspheres and nanofibres are 3–7 µm and 60–140 nm, respectively. there are also many protuberances and cavities with dimen- sions of tens to hundreds of nanometres on the surfaces of the microspheres (Figure 3.2g). the porous microspheres play the leading role in the super- hydrophobicity of the film by increasing surface roughness, and the nanofi- bres interweave to form a 3d network and act as a skeleton to reinforce the composite film. this pS composite film exhibits an excellent water-repellent property with a water Ca of 160.4 ± 1.28° (Figure 3.2h).
Seeger and co-workers presented a simple method of growing polysiloxane nanofilaments with diameter ranging from 20 to 50 nm (Figure 3.3a) on var- ious substrates to fabricate superhydrophobic surfaces.51 the combination of the hydrophobicity of the silicone and the nanostructured surface rough- ness formed by the nanofilament contributes to the superhydrophobicity.
What is worth mentioning is that the two main requirements for superhy- drophobic property of a material, namely a low surface energy composition and the nanometre-scale roughness are obtained in a single fabrication step.
a frosted glass coated with these silicone nanofilaments shows a water Ca of 166 ± 3° and a water sliding angle (Sa) lower than 3°. other coated sub- strates like cotton fabric, wood, silicone rubber and aluminium, etc. all show a superhydrophobicity (Figure 3.3b). Most importantly, no fluorine-contain- ing reagents are needed in the fabrication of this superhydrophobic coating.
however, the silicone nanofilament coating still behaves superoleophilically.
When the silicone nanofilaments are further modified with a lower surface material 1H,1H,2H,2H-perfluorodecyltrichlorosilane (pFdtS, Figure 3.3c), the nanostructured coating exhibits a superoleophobic property for vari- ous non-polar liquids, such as mineral oil, toluene, hexadecane, decane and cyclohexane (Figure 3.3d and e).52 owing to the introduction of fluoroalkyl groups, the glass coated with these superoleophobic nanofilaments show oil Cas higher than 155° and oil Sas lower than 6° for all of the oils investigated.
oil droplets can easily roll off from the slightly tilted (3°) glass. even jets of toluene and decane can bounce off the nanofilament-coated glass without leaving a trace (Figure 3.3f). this superoleophobic nanofilament coating also possesses excellent transparency, chemical and environmental stability.
a series of typical superlyophobic nanomaterials containing fluorodecyl polyhedral oligomeric silsesquioxane (f-poSS) have been reported by tuteja
Figure 3.3 (a) SeM image of the polysiloxane nanofilaments.51 (b) Water Cas and Sas of a series of substrates coated by the polysiloxane nanofilaments.
G. r. j. artus, et. al., Silicone nanofilaments and their application as superhydrophobic coatings, Adv. Mater., 2006, 18, 20. Copyright © 2006 john Wiley & Sons, inc. (c) Schematic showing the growth of silicone nanofilaments using (trichloromethylsilane) tCMS and subsequent modification with pFdtS.52 the SeM images below are the silicone nanofilaments before and after pFdtS modification. (d) Cas and Sas for water and various non-polar liquids on the tCMS and tCMS/pFdtS nanofilaments-coated glasses. digital images of non-polar liquid drop- lets (e) and a jet of toluene (f) on the tCMS/pFdtS nanofilament-coated glass. W. Choi, et. al., Superoleophobic coatings with ultralow sliding angles based on silicone nanofilaments, Angew. Chem., Int. Ed., 2011, 50, 29. Copyright © 2011 john Wiley & Sons, inc.
and co-workers.53–56 the f-poSS possesses an extremely low surface energy of about 10 mn m−1, which plays an important role in the superoleophobicity of the blends. among these materials, a superlyophobic coating of cross-linked poly(dimethylsiloxane) (pdMS) + 50 wt% f-poSS (γSa ≈ 11.5 mn m−1) was fabricated on a stainless steel mesh with hierarchically structured surface (Figure 3.4a–c).54 the fabrication process was achieved by a simple technique based on electrospun coating. the reason for choosing pdMS in this work is its excellent chemical resistance against a range of different chemicals upon cross-linking. this superoleophobic coating exhibits very high apparent Cas, ultralow Sas and ultralow Ca hysteresis for a range of polar and non-polar
Figure 3.4 (a) SeM image of the hierarchically structured surface illustrating the elec- trospun coating of x-pdMS + 50 wt% f-poSS on a stainless steel mesh.54 (b) elemental mapping of fluorine on the hierarchically structured sur- face. (c) SeM image illustrating the re-entrant curvature of the electrospun texture. (d) roll-off angles for various newtonian liquids on the surface shown in (a). droplets (e) and jets (f) of various low surface tension new- tonian liquids on the surface shown in (a). reprinted with permission from ref. 54. Copyright 2013 american Chemical Society. (g) SeM image of a stainless steel mesh coated with electrospun microbeads of 50 wt%
f-poSS + pMMa blend.55 (h) SeM images of the electrospun microbeads obtained using aK225 with 5 vol% dMF. (i) droplets of different low sur- face tension liquids on the surface shown in (g). a. K. Kota, et. al., hierar- chically structured superoleophobic surfaces with ultralow contact angle hysteresis, Adv. Mater., 2011, 24, 43. Copyright © 2011 john Wiley & Sons, inc.
low surface tension newtonian liquids, including various acids, bases and solvents, due to the hierarchical texture along with re-entrant curvature and the ultralow surface energy of the coating (see Figure 3.4d and e). due to the low Ca hysteresis, even jets of different newtonian liquids can easily bounce on the surface of the pdMS/f-poSS coating (Figure 3.4f). they also prepared a superoleophobic surface by employing an electrospun coating of 50 wt% f-poSS + poly(methylmethacrylate) (pMMa) on a textured substrate (Figure 3.4g and h).55 the hierarchical texture and the low surface energy of this material render the surface superoleophobic with very high apparent Cas (Figure 3.4i) and ultralow Ca hysteresis even with extremely low surface tension liquids such as n-heptane (apparent Ca = 155°, Ca hysteresis = 4°).
For most of the superhydrophilic materials, they simultaneously exhibit a superoleophilic property in air as is introduced in Section 3.2.2, but behave superoleophobically underwater, as is widely reported by researchers.57–60 a typical natural example of a hydrophilic and underwater superoleophobic material is fish scales, with an underwater oil Ca of 156.4 ± 3.0°.61 the surface of fish scales is composed of a thin hydrophilic mucus layer with a hierarchi- cal structure of oriented micropapillae and nanostructured roughness. jiang et al. believes that the water surrounding the fish scales leads to wettability reversion. inspired by this, a novel strategy to generate underwater supero- leophobic materials without the assistance of fluoride compounds was exten- sively studied and the oil–water–air–solid coexisting four-phase system was discovered.61–63 jiang and co-workers prepared a series of underwater super- oleophobic polyacrylamide (paM) hydrogel films by a casting technique.61 different paM hydrogel films with fish-scale structures, microstructures or micro/nanostructures all behave underwater superoleophobically with underwater oil Cas as high as 174.8 ± 2.3°. Superhydrophilic–underwater superoleophobic materials, especially separation membranes which possess opposite superwetting behaviours for water and oil in a water-rich environ- ment, are thus widely explored in the field of oil/water separation.
Besides the superwetting nanomaterials introduced above, superwetting absorbing nanomaterials including sponge-, foam- and textile-based nano- materials, and superwetting separation membranes with nanostructure morphologies including meshes and textile-based films, polymer-dominated membranes as well as the one-dimensional (1d) nanomaterial-based ultra- thin films with nanometre-scale thickness are also widely prepared and studied by researchers to achieve the separation of oil/water mixtures and emulsions. these works are discussed in Section 3.3 and Section 3.4 in detail.