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Superhydrophobic coating

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A superhydrophobic coating is a thin surface layer that repels water. It is made from superhydrophobic (also known as ultrahydrophobic) materials, and typically cause an almost imperceptibly thin layer of air to form on top of a surface. Droplets hitting this kind of coating can fully rebound.[1][2] Generally speaking, superhydrophobic coatings are made from composite materials where one component provides the roughness and the other provides low surface energy.[3]

Superhydrophobic coatings are also found in nature; they appear on plant leaves, such as the lotus leaf, and some insect wings.[4]

This image shows highly absorbent filter paper coated with a super-hydrophobic paint developed at University College London. This repels water (which has been dyed orange for greater contrast)

Materials used

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Superhydrophobic coatings can be made from many different materials. The following are known possible bases for the coating:

The silica-based coatings are perhaps the most cost effective to use.[12] They are gel-based and can be easily applied either by dipping the object into the gel or via aerosol spray. In contrast, the oxide polystyrene composites are more durable than the gel-based coatings, however the process fof applying the coating is much more involved and costly. Carbon nano-tubes are also expensive and difficult to produce with current technology. Thus, the silica-based gels remain the most economically viable option at present.

As well, surfaces can be made hydrophobic without the use of coating through the altering of their surface microscopic contours. The basis of hydrophobicity is the creation of recessed areas on a surface whose wetting expends more energy than bridging the recesses expends. This relies on delicate micro- and nanoscale structures for their water repellence, and is accomplished using microstructures (or hairs) similar to that of a lily pad coated with some hydrophobic material, which greatly increases contact angle and makes water roll off. This so-called Wenzel-effect surface or lotus effect surface has less contact area by an amount proportional to the recessed area, giving it a high contact angle. The recessed surface has a proportionately diminished attraction foreign liquids or solids and permanently stays cleaner.

These microstructures however, are easily damaged by abrasion or cleaning: with some friction, a lotus leaf will no longer be superhydrophobic. Unlike a lotus leaf which can heal and grow new hairs, an inert coating will not regenerate.[13]

Applications

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Consumer uses

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Durable water repellent is a type of fabric coating to protect them from water.

In addition, superhydrophobic coatings have potential uses in vehicle windshields to prevent rain droplets from clinging to the glass, to improve driving visibility. Rain repellent sprays are commercially available for car windshields.[14][15]

Due to their fragility, superhydrophobic coatings can find usage in sealed environments which are not exposed to wear or cleaning, such as electronic components (like the inside of smartphones) and air conditioning heat transfer fins, to protect from moisture and prevent corrosion.[16]

Industry uses

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In industry, super-hydrophobic coatings are used in ultra-dry surface applications. The coating can be sprayed onto objects to make them waterproof. The spray is anti-corrosive and anti-icing; has cleaning capabilities; and can be used to protect circuits and grids.

Superhydrophobic coatings have important applications in maritime industry. They can yield skin friction drag reduction for ship hulls, thus increasing fuel efficiency. Such a coating would allow ships to increase their speed or range while reducing fuel costs. They can also reduce corrosion and prevent marine organisms from growing on a ship's hull.[17]

Furthermore, superhydrophobic coatings can make removal of salt deposits possible without using fresh water. This has the ability to aid harvesting minerals from seawater brine.[18]

Newer engineered surface textures on stainless steel are extremely durable and permanently hydrophobic. Optically these surfaces appear as a uniform matte surface but microscopically they consist of rounded depressions one to two microns deep over 25% to 50% of the surface. These surfaces are produced for buildings which will never need cleaning.[19] These have been effectively used for roofs and curtain walls of structures that benefit from low or no maintenance.[19]

Medical

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Due to the extreme repellence and in some cases bacterial resistance of hydrophobic coatings, there is much enthusiasm[from whom?] for their wide potential uses with surgical tools, medical equipment, textiles, and all sorts of surfaces and substrates. However, the current state of the art for this technology is hindered in terms of the weak durability of the coating making it unsuitable for most applications.

Criticism

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Instead of using fluorine atoms for repellence like many successful hydrophobic penetrating sealers (not super hydrophobic), superhydrophobic products are coated with a micro- and nano-sized surface structures which has super-repellent properties. These tiny structures are by their nature very delicate and easily damaged by wear, cleaning or any sort of friction; if the structure is damaged even slightly it loses its superhydrophobic properties.[citation needed]

Due to the fragility of certain coatings, objects subject to constant friction like boats hulls would require constant re-application of such a coating to maintain a high degree of performance.

Despite the many applications of superhydrophobic coatings, safety for the environment and for workers can be potential issues.[citation needed] The International Maritime Organization has many regulations and policies about keeping water safe from potentially dangerous additives.[citation needed]

Unless advancements can resolve these identified weaknesses above, the applications are potentially limited.

See also

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References

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  1. ^ Richard, Denis, Christophe Clanet, and David Quéré. "Surface phenomena: Contact time of a bouncing drop." Nature 417.6891 (2002): 811-811
  2. ^ Yahua Liu, Lisa Moevius, Xinpeng Xu,Tiezheng Qian, Julia M Yeomans, Zuankai Wang. "Pancake bouncing on superhydrophobic surfaces." Nature Physics, 10, 515-519 (2014)
  3. ^ Simpson, John T.; Hunter, Scott R.; Aytug, Tolga (2015). "Superhydrophobic materials and coatings: a review". Reports on Progress in Physics. 78 (8): 086501. Bibcode:2015RPPh...78h6501S. doi:10.1088/0034-4885/78/8/086501. PMID 26181655. S2CID 206022154.
  4. ^ Dai, S.; Ding, W.; Wang, Y.; Zhang, D.; Du, Z. (2011). "Fabrication of hydrophobic inorganic coatings on natural lotus leaves for nanoimprint stamps". Thin Solid Films. 519 (16): 5523. arXiv:1106.2228. Bibcode:2011TSF...519.5523D. doi:10.1016/j.tsf.2011.03.118. S2CID 98801618.
  5. ^ Meng, Haifeng; Wang, Shutao; Xi, Jinming; Tang, Zhiyong; Jiang, Lei (2008). "Facile Means of Preparing Superamphiphobic Surfaces on Common Engineering Metals". The Journal of Physical Chemistry C. 112 (30): 11454–11458. doi:10.1021/jp803027w.
  6. ^ Hu, Z.; Zen, X.; Gong, J.; Deng, Y. (2009). "Water resistance improvement of paper by superhydrophobic modification with microsized CaCO3 and fatty acid coating". Colloids and Surfaces A: Physicochemical and Engineering Aspects. 351 (1–3): 65–70. doi:10.1016/j.colsurfa.2009.09.036.
  7. ^ Lin, J.; Chen, H.; Fei, T.; Zhang, J. (2013). "Highly transparent superhydrophobic organic–inorganic nanocoating from the aggregation of silica nanoparticles". Colloids and Surfaces A: Physicochemical and Engineering Aspects. 421: 51–62. doi:10.1016/j.colsurfa.2012.12.049.
  8. ^ Das, I.; Mishra, M. K; Medda, S.K; De, G. (2014). "Durable superhydrophobic ZnO–SiO2 films: a new approach to enhance the abrasion resistant property of trimethylsilyl functionalized SiO2 nanoparticles on glass" (PDF). RSC Advances. 4 (98): 54989–54997. Bibcode:2014RSCAd...454989D. doi:10.1039/C4RA10171E.
  9. ^ Torun, Ilker; Celik, Nusret; Hencer, Mehmet; Es, Firat; Emir, Cansu; Turan, Rasit; Onses, M.Serdar (2018). "Water Impact Resistant and Antireflective Superhydrophobic Surfaces Fabricated by Spray Coating of Nanoparticles: Interface Engineering via End-Grafted Polymers". Macromolecules. 51 (23): 10011–10020. Bibcode:2018MaMol..5110011T. doi:10.1021/acs.macromol.8b01808. S2CID 104394952.
  10. ^ Warsinger, David E.M.; Swaminathan, Jaichander; Maswadeh, Laith A.; Lienhard V, John H. (2015). "Superhydrophobic condenser surfaces for air gap membrane distillation". Journal of Membrane Science. 492. Elsevier BV: 578–587. doi:10.1016/j.memsci.2015.05.067. hdl:1721.1/102500.
  11. ^ Servi, Amelia T.; Guillen-Burrieza, Elena; Warsinger, David E.M.; Livernois, William; Notarangelo, Katie; Kharraz, Jehad; Lienhard V, John H.; Arafat, Hassan A.; Gleason, Karen K. (2017). "The effects of iCVD film thickness and conformality on the permeability and wetting of MD membranes" (PDF). Journal of Membrane Science. 523. Elsevier BV: 470–479. doi:10.1016/j.memsci.2016.10.008. hdl:1721.1/108260. S2CID 4225384.
  12. ^ Shang HM, Wang Y, Limmer SJ, Chou TP, Takahashi K, Cao GZ (2005). "Optically transparent superhydrophobic silica-based films". Thin Solid Films. 472 (1–2): 37–43. Bibcode:2005TSF...472...37S. doi:10.1016/j.tsf.2004.06.087.
  13. ^ Ensikat, Hans J (10 March 2011). "Superhydrophobicity in perfection: the outstanding properties of the lotus leaf". Beilstein J Nanotechnol. 2: 152–161. doi:10.3762/bjnano.2.19. PMC 3148040. PMID 21977427.
  14. ^ "NeverWet Superhydrophobic Coatings – It Does Exactly What Its Name Implies" (PDF). Truworth Homes. Archived from the original (PDF) on 21 December 2016. Retrieved 27 December 2019.
  15. ^ "How to Apply NeverWet Rain Repellent". Rust-Oleum. 2 February 2016. Retrieved 27 December 2019 – via YouTube.
  16. ^ Milionis, Athanasios; Loth, Eric; Bayer, Ilker S. (2016). "Recent advances in the mechanical durability of superhydrophobic materials". Advances in Colloid and Interface Science. 229: 57–79. doi:10.1016/j.cis.2015.12.007. PMID 26792021.
  17. ^ Li, Min; Xiao, Wenbo; Yin, Zuozhu; Chen, Yuhua; Luo, Yidan; Hong, Zhen; Xue, Mingshan (2024). "Construction of a robust MOF-based superhydrophobic composite coating with the excellent performance in antifouling, drag reduction, and organic photodegradation". Progress in Organic Coatings. 186: 108086. doi:10.1016/j.porgcoat.2023.108086.
  18. ^ Kahn, Mariam; Al-Ghouti, Mohammad A. (15 October 2021). "DPSIR framework and sustainable approaches of brine management from seawater desalination plants in Qatar". Journal of Cleaner Production. 319: 128485. doi:10.1016/j.jclepro.2021.128485. hdl:10576/24785.
  19. ^ a b McGuire, Michael F., "Stainless Steel for Design Engineers", ASM International, 2008.
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