Dr. S. S. Verma, Department of Physics, S.L.I.E.T., Longowal, Distt.-Sangrur (Punjab)-148106
Nature is full of various engineering marbles as per requirements well known to nature itself and man has always tried to learn and mimic nature’s such mesmerizing innovations. Natural existence of substances exhibiting hydrophobicity (property of repelling water) e.g., Lotus Leaves has always attracted human imaginations to develop such materials which can repel water and can be put to practical applications. Lotus plants are primarily aquatic plants and are exposed to both water and air. This exposure leads to contamination from pathogens like germs or algae, but its leaves have developed a mechanism to fight it—hydrophobicity. When pathogens stick to the leaves of lotus, this “dirt” can be easily washed off by water. So, when water falls on the surface, the droplets roll off, taking the pathogens with them. This property of repelling water is not just seen in lotus leaves but across nature, from rose petals to even grass.
Making of a super-hydrophobic coatingsCreating artificial superhydrophobic surfaces and exploring their versatile applications, such as waterproofing, selfcleaning, drag reduction, and selective absorption, have excited the research frontiers. However, good stability, flexibility, practicability, and universality of superhydrophobic coatings are needed for practical use, and multifunctionality has been a new focus of superhydrophobic coatings. Researchers have been studying this property for a long time as it could help solve many problems that we humans face in everyday life. Scientists all over the world have not only managed to recreate super-hydrophobic coatings but also ensured that the coatings are easy to develop and water resistant. The general strategy is to build the essential topography using hydrophilic building blocks, and at the end, low surface energy molecules are deposited on the top of these featured topographs. While, this approach is widely accepted for synthesizing artificial superhydrophobic interface, this (type of) thin conventional biomimicked coating is incapable of sustaining severe physical and chemical challenges and eventually became inappropriate for several prospective applications in real-world scenarios. This superhydrophobic coating was prepared by mixing a polymer (branched polyethyleneimine) and a small reactive molecule (dipentaerythritol penta-acrylate) in different alcoholic solvents—including ethanol and pentanol. The polymer and the small molecule reacted rapidly in the presence of pentanol, and with the rate of evaporation of pentanol being way less compared with other alcoholic solvents, pentanol was finally incorporated. Interestingly, the super-hydrophobic coating can also be modified to be adhesive. Further, introduction of a facile synthetic approach has provided highly durable and scalable superhydrophobic coating, which is capable of sustain all possible physical and chemical challenges that are relevant at practical outdoor settings. Current approach is capable of providing both adhesive and non-adhesive superhydrophobic coating, and both of these properties are important for several different specialized and smart applications.
Material used: Superhydrophobic coatings can be made from many different materials. The known possible bases for the coating are: Manganese oxide polystyrene (MnO2/PS) nano-composite, Zinc oxide polystyrene (ZnO/PS) nano-composite, Precipitated calcium carbonate, Carbon nano-tube structures, Silica nano-coating, Fluorinated silanes and Fluoropolymer coatings. The silica-based coatings are perhaps the most cost effective to use. 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 of 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.
Growing demand of superhydrophobic coatings
Superhydrophobic coatings find application in sectors such as electronics and telecommunication, textile and leather, building and construction, automotive, healthcare and medical, optical, and power generation. In terms of demand, the electronics and telecommunication sector accounted for approximately 32.0% of the superhydrophobic coatings market in 2015. The segment is expected to witness significant growth during the forecast period owing to the increasing applications of superhydrophobic coatings in consumer electronics, semiconductors, and other electronic components. The building and construction and automotive sectors are also prominent end users of superhydrophobic coatings. Superhydrophobic coatings are environment-friendly and highly compatible with concrete, masonry, ceramics, and composite substrates. These coatings can also be used for waterproofing applications on ceramic floor and wall tiles, cement walls, and roofs in exterior and interior constructions. Textile and leather is anticipated to be the fastest-growing end user during the forecast period, presenting lucrative options for players. The growing need for protective, dirt-resistant clothing in the military sector and self-cleaning apparels for daily use is likely to drive the demand for superhydrophobic coatings in the textile and leather industry.
Applications in electronics
Water is the main cause of lost function in electronic devices. Millions of mobile phones and many other devices are damaged by water annually. In the present civilization of life totally dependent on the use of electronic devices, we rely on waterproof artificial materials to protect electronics from being damaged. As wearable electronics become increasingly popular, multifunctional coatings to that do more than just repel water are being sought after. There is therefore a large opportunity to offer consumers protection for their electronics against water and moisture damage. Liquid repellent, thermal, conductive, magnetic and anti-corrosive nanocoatings have been applied inside and outside. Most major handheld electronics manufacturers have made their devices water resistant in the last few years.
There are several innovative application developers now producing protective hydrophobic, superhydrophobic and oleophobic (HSHO) nanocoatings to treat electronic devices, including: cellular phones, smart phones, personal digital. Hydrophobic repellency treatments can have a significant impact on the performance and reliability in electronics. Repellency treatments can be used to modify a broad range of materials used in electronic applications including metals, metal oxides, polymers, and ceramics. Nanoscale technology for electronics is also an excellent solution for precision electronic components such as printer heads, PCBs and other SMT applications, stainless steel components, microfluidics, or other highly sensitive electronics components requiring a repellent treatment. Hydrophobic treatments for electronics can be used to increase reliability, improve performance characteristics, and add value to consumer electronics by repelling water and oil and also allowing for easy clean finger print resistant surfaces.
Hydrophobic applications include: antiwetting applications, inkjet printer nozzles, microfluidic channels, inkjet repellency, hard disc drives, stainless steel components, needles and syringes and many more. Hydrophobic electronic repellency treatments for electronics are compatible with the following types of materials: Metals, Polymers, Glass/Ceramics, Particles, Semiconductors, Fabrics/Other and many more. Some main applications of super-hydrophobic coatings in electronics are mentioned as:
In flexible and wearable electronic devices: Superhydrophobic surfaces have shown versatile applications in waterproofing, self‐cleaning, drag reduction, selective absorption, etc. Superhydrophobic materials integrating stretchability with conductivity have huge potential in the emerging application horizons such as wearable electronic sensors, flexible power storage apparatus, and corrosion-resistant circuits. The most convenient and universally applicable approach to forming superhydrophobic surfaces is by coating; however, currently, superhydrophobic, smart coatings with flexibility and multiple functions for wearable sensing electronics are not yet reported. Here, a highly flexible multifunctional smart coating is fabricated by spray‐coating multiwalled carbon nanotubes dispersed in a thermoplastic elastomer solution, followed by treatment with ethanol. The coatings not only endow various substrate materials with superhydrophobic surfaces, but can also respond to stretching, bending, and torsion—a property useful for flexible sensor applications.In underwater electronics: Sensitive and expensive electronic equipment used in marine environments are generally subjected to harsh extremes. Component lifetimes are typically extended by physical isolation using bags, hard containers or with conformal impermeable coatings. However, over time, protective measures become compromised, the resulting exposure causing corrosion and short circuit-driven malfunctions. There is a need for advanced technologies that effectively protect electronics and sensitive components used in marine environments. Technologies can further benefit from efficient heat dissipation (high thermal conductivity), electrical insulation (low electrical conductivity) and compatibility with the interconnectivity of electronic assemblies.Transparent superhydrophobic glass coatings for electronic devices: Department of Energy’s Oak Ridge National Laboratory has developed a transparent coating that repels water that carries away dust and dirt, reduces light reflection and resists fingerprints which resulted from superhydrophobic research on glass-based coatings. Samsung Electronics has exclusively licensed this optically clear superhydrophobic film technology to improve the performance of glass displays on smartphones, tablets and other electronic devices. Beyond electronics, the technology holds significant potential for applications in solar panels, lenses, detectors, windows and many other products. The fields of solar panels and architectural windows are still available for licensing.
Other industrial uses
In industry, super-hydrophobic coatings are used in ultra-dry surface applications. The coating causes an almost imperceptibly thin layer of air to form on top of a surface. 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 ships’ 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. In addition to these industrial applications, superhydrophobic coatings have potential uses in vehicle windshields to prevent rain droplets from clinging to the glass. The coatings also make removal of salt deposits possible without using fresh water. Furthermore, superhydrophobic coatings have the ability to harvest other minerals from seawater brine with ease. Superhydrophobic coatings rely on a delicate micro or nano structure for their repellence—this structure is easily damaged by abrasion or cleaning; therefore, the coatings are most used on things such as electronic components, which are not prone to wear. Objects subject to constant friction like boats hulls would require constant re-application of such a coating to maintain a high degree of performance.