The tips of the whisks are chemically distinct, being hydrophilic, and this firmly pins a water layer to the surface with air trapped underneath. The pinning effect keeps the air layer — as large as 3. Here compressed air forms microbubbles.
Air-trapping surfaces are also common in aquatic insects. A series of tiny hairs or bumps, known as setae or microtrichia, trap a thin layer of air that allows the insect to breathe underwater — essentially acting like an external gill. This feature is known as a plastron. He has replicated this principle to create superomniphobic surfaces. Fire ants can cluster together to increase their buoyancy — a property that scientists hope to copy to clean up oil spills.
Repelling oil is always harder than water, explains Banerjee. So to design a superomniphobic surface with plastron-like properties, Banerjee turned to zinc oxide nanotetrapods spray-coated onto stainless steel. These four-legged nanocrystals form when zinc foil is rapidly oxidised in air.
The nanotetrapods are bonded to the steel surface using tetraethylorthosilicate, which creates a silicon dioxide link. But to replicate a superolephobic surface, Banerjee also functionalised the surface with perfluorooctane phosphonic acid C 8 H 6 F 13 O 3 P. Banerjee has created an entirely new filtration process based on a stainless steel mesh coated with zinc oxide nanotetrapods. The mesh membrane forms an interconnected porous plastron network which lets oil through.
But it keeps water droplets suspended above the air pockets formed between the protruding nanotetrapods. The water droplets are in the Cassie—Baxter state, in contrast to oil drops that are in Wenzel mode and permeate the mesh. The filter can reduce the water content of viscous oil to as little as 0.
Another innovative approach to omniphobic surfaces that does not rely on an air pocket came from the lab of Joanna Aizenberg at Harvard University in Massachusetts, US, and also takes it inspiration from nature. The insect-eating pitcher plant captures its prey using a slippery water-lubricated surface that repels oils found on insect feet. The lubricant itself is held in place by an irregular micro-textured surface.
As a postdoc in the Aizenberg group, Tak-Sing Wong, now an assistant professor at Pennsylvania State University in the US, developed surfaces based on the pitcher plant principle, called slippery liquid-infused porous surfaces Slips. Based on these design criteria you can explore all kinds of liquids as a lubricant. The lubricant, typically nm to a few micrometres in depth, is held in place by capillary forces but it must have a high chemical affinity to the underlying material, otherwise the fluid you want to repel will displace it.
Wong has been investigating how lubricated surfaces could also be useful for water collection, in parts of the world where there are water shortages. He found that a hydrophobic Slips did not provide the most efficient way to harvest water vapour or fog as it limited water drop nucleation.
Moreover, coatings fabricated in this study were also found to be stable after ultrasonication test and water-drop impact. Ramezani et al. Initially glass substrate was washed and ultrasonically cleaned. Substrate was vertically dipped in this transparent silica sol. Heating and drying resulted in the deposition of silica film over glass substrate. The size of silica nanoparticles increased from 26 to 42 nm after modification.
Moreover, SH films were highly transparent and had uniform size. Chen et al. Al foil was used as a substrate followed by zinc sheet in second phase. Al foil was properly washed, cleaned and dried before applying fabrication methodology. A solution of stearic acid was made with water followed by addition of HCl in that solution.
The etching process was started by dipping Al foil into the solution for 90 min, and 75 min for zinc sheet. After drying the sample, ladder-like microstructures and wave-like structures were formed on Al-etched and zinc-etched samples, respectively. Anti-corrosion property of SHS was also improved as compared to bare samples. The results of steam-freezing experiment showed that SHS possessed icing delay. Figure 3 shows the process of solution immersion. Solution-immersion process [ 20 ].
Wu et al. In this method, emulsion was created in different steps beginning by dispersion of silica nanoparticles in ethanol followed by modification with PTES. Epoxy resin and curing agent were later mixed in that solution by stirring. Three different types of substrates such as wood, paper and glass were used on which SHS was fabricated.
Emulsion on substrate was also coated by three methods such as brushing, spraying and dipping. This acted as a proof that fabrication method is versatile and not affected by the method of coating. Epoxy resin appeared to be as a necessary ingredient for fabrication of SHS as the one formed without epoxy resin was not mechanically stable. Durability of SHS was tested by sand abrasion, knife scratching and adhesive tape tests resulting in almost constant values of CA.
Tang et al. Copper channel was used as substrate. First surface was roughed by laser, then electrodeposition was performed to obtain microstructures on surface and then dried. No coating material was used for lowering surface energy. Moreover, the effect of microstructures on pressure drop was also studied.
Pressure drop increased with increase in channel width and was also smaller in super-hydrophobic channel as compared to smooth channel. Friction factor was reduced in super-hydrophobic micro-channels by maximum of Li et al. Zinc sheets were used as a substrate material.
Laser ablation process was used to increase surface roughness of zinc sheet dipped in aqueous solution of H 2 O 2. Two types of laser ablation were used in the study such as nanosecond ns and femtosecond fs laser ablation.
ZnO and Zn OH 2 were generated on the zinc surface as a result of laser ablation. Both types of laser ablation created different microstructures on surface. Clustered flower-like microstructures were formed on ns-laser ablated sample while non-directional flaky nanostructures were formed on fs-laser ablated sample.
Roughness was also found to be higher for ns-laser ablated sample due to which it was super-hydrophobic with WCA and WSA of On the other hand, fs-laser ablated sample was highly hydrophobic with WCA of Chun et al.
First, copper substrate was exposed to the nanosecond laser beam originating from laser beam machining of 3. Earlier before this study, it had already been found that laser beam produced layer of CuO on sample which was hydrophilic and it took a long time about 27 days to be converted into SHS which had Cu 2 O on the surface. As copper is also stable with ethanol, so additional reduction in time to get SHS was achieved when ethanol was used by which SHS was formed in less than 5 h.
Figure 4 shows the schematic illustration for fabrication of SHS by laser beam machining. Table 3 summarizes the work of different researchers who have used electrodeposition technique for SHS fabrication.
Schematic illustration of SHS with laser beam machining and post-process [ 24 ]. Wang et al. The fabrication method consisted of three steps. First step was to prepare template from polystyrene microspheres powders. Second step was to put copper plate in polystyrene colloidal microspheres with CuSO 4 solution as an electrolyte.
During this process, copper atoms filled the voids in the template. In final step, surface was modified using fluorosilane solution resulting in a SHS.
WCA increased with increase in deposition time to 19 s and then decreased after that. Bhagat and Gupta [ 26 ] developed a purely physical process of fabricating super-hydrophobic polycarbonate surfaces PC. Silicon wafers were used as substrate which along-with PC were initially washed and cleaned properly.
In first step of fabrication, micro-textures were produced on substrate by high power laser and in second step, those microstructures were thermally replicated on PC surface by sandwiching them between two hot plates having different temperatures. Upon cooling, both surfaces were then separated from each other. This physical process of fabrication resulted in robust SHS with good mechanical properties. Figure 5 shows schematic diagram for the fabrication of super-hydrophobic PC by electrodeposition.
Schematic diagram for fabrication of super-hydrophobic PC by laser micro-texturing of Si wafer and its subsequent replication [ 26 ]. SiC paper was first sonicated, rinsed with ethanol and then baked. It was peeled from PDMS cast after cooling. SHS formed was also found to possess excellent adhesive properties described by hybrid wetting state. Momen and Farzaneh [ 28 ] fabricated exceptionally stable SHS using a simple and cost-effective approach of spray coating. Glass substrate used was first ultrasonically cleaned followed by preparation of three separate samples.
Then all three samples were separately spray-coated on glass slides by using spray gun. WCAs for all samples a, b and c were measured to be UV and humidity had also little effect on WCA. Ipekci et al. A matrix was required for better dispersing of silica nanoparticles functionalized with fluorinated silanes.
Hydroxyl-terminated polystyrene was used as a matrix which reacted with substrate to form polymer brushes through covalent bonding. Glass slides were used as a substrate. Enhancement of mechanical robustness was different for different substrates and polymers. The method used was salt-spray method which increased surface roughness, followed by modification of surface reducing its surface energy.
Two substrate materials such as steel and Mg alloy were separately used. Both substrates were first properly polished, cleaned and then dried.
These were placed for about 2 h in the neutral spray chamber which contained NaCl solution creating humid and fog condition. This generated needle-like structures on steel and flower-like structures on Mg alloy surface. Finally, super-hydrophobicity was achieved when above samples were put in solution of FAS WCA measurements showed that CA of Mechanical durability of SHS formed was tested by water fall test in which kinetic energy of falling water varied.
The results showed that WCA decreased after 20 minutes but water-repellent properties still remained in the solution. Although durability of SHS formed by this method was not excellent, it is still good. Figure 6 shows the schematic diagram for fabrication of SHS by salt-spray method. Table 5 summarizes the work of different researchers who have used spray coating technique for SHS fabrication. Schematic diagram for fabrication of SHS by salt-spray method [ 30 ].
Feng et al. Fabrication process was completed in three steps. First, copper powder was oxidized with AgNO 3 solution after it was ultrasonically cleaned with nitric acid and distilled water. Second, surface hydrophilicity was changed into hydrophobicity by lauryl mercaptan DM. In last step, mold compression was performed on copper powder under 80 MPa to obtain super-hydrophobic characteristics.
Tests showed maximum WCA of Super-hydrophobicity of surface was also dominant for other fluids. Copper-based SHS was also found to be very long-term durable, storable, regenerable and of excellent cleaning property. Sun et al.
Experimental device was made at first which consisted of tank, linear motor, pump, substrate cathode, steel anode, plating solution, etc. Copper plate was polished mechanically, ultrasonically cleaned and then fixed in experimental device.
Ni plating solution consisted of thoroughly mixed nano-Al 2 O 3 particles was prepared. Electro-brush was moved reciprocately by motor to rub the substrate surface. The resulting sample was immersed in FAS solution for surface modification. The WCA increased with increasing plating voltage to 12 V and then decreased after it. Similarly, WCA increased with plating time to 2 s and then remained almost constant. Final SHS obtained possessed excellent mechanical properties and stability.
Huang and Leu [ 33 ] used spin coating method to fabricate SHS. Substrate used was copper heat sink. In first step, chemical polishing was done on substrate to remove oxides and impurities. The category in oil removing method which by my personal opinion is a more efficient way because the amount of oil is always less than the amount of water; so, it is logical that we try to remove oil from water and not water from oil.
To remove oil from water, the material should be superhydrophobic and superoleophilic. Superhydrophobic oil removing filters are the main part of the oil removing category. Gao et al. As shown in Figure 2b , water droplet cannot pass through the filter but toluene can easily pass through. There are several ways to protect a surface from corrosion.
During the past two decades, scientists have been using superhydrophobic nanocomposite coatings without any toxic materials in order to protect various surfaces from corrosion. The corrosion protection capability of the superhydrophobic coatings mainly is because of the presence of air packets between surface and corrosive solution, and these packets act like a barrier and prevent from corrosive ions diffusion and protect the substrate [ 1 ].
Superhydrophobic metallic surfaces could be able to decrease the corrosion rate of metals by several orders of magnitude through imparting hydrophobization.
Several reports have been published that demonstrated the enormous capability of superhydrophobic surfaces on the corrosion mitigation. The potentiodynamic polarization test revealed a significant decrease in the corrosion current density Figure 3 of metallic surfaces by using a commercial hydrophobic surface modification [ 4 ]. Potentiodynamic polarization curves of bare metallic surfaces hydrophilic and surface modified samples with a commercial hydrophobic material hydrophobic and with developed commercial hydrophobic materials superhydrophobic [ 4 ].
This leaf has a unique surface structure coated with wax and shows superhydrophobic properties, and sliding angle is very low so water can easily slide on the surface of the leaf and remove any contaminants. The aforementioned properties of superhydrophobic surfaces and coatings are called self-cleaning properties. There are many superhydrophobic coatings which were synthesized with different methods and used in industries.
It is worth to mention that the actual self-cleaning surface is the surface exhibiting the combined superhydrophilicity and photocatalytic behaviors to decompose the dirt. The use of the term, self-cleaning surface, is not appropriate for superhydrophobic surfaces, which are extremely dry and repel water drops. As schematically shown in Figure 4 , these surfaces do not actually clean themselves but they wash away the dirt when the water drops roll over the surface.
Schematic illustration of self-cleaning process in a non-hydrophobic and b hydrophobic surfaces. In recent years, superhydrophobic coatings have been suggested as anti-icing coatings. As mentioned before, the presence of air packets on the superhydrophobic surfaces causes the water droplets to slide easily on the surface; therefore, there will not be enough time for the droplet to freeze on the surface; consequently, this reduces the side effects of frosts on the surfaces.
Every year ice storms harm the equipment such as electrical transmission equipment, communication systems, aerospace facilities, highways, etc. In order to reduce this kind of damages, different methods have been developed such as local warming and preventing of ice formation by chemical activities and additives, which have some limitations in practical applications. Suggestions or feedback?
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