Document Type : Original Research Paper


1 Non-metallic Materials Research Department, Niroo Research Institute, Tehran, Iran

2 Islamic Azad University of Tehran, Tehran, Iran


Nowadays, the water shortage has become a severe issue all over the world, especially in some arid and undeveloped areas. Water is a renewable natural resource that can produce freshwater globally. Interestingly, some animals in nature can collect water from fog, which can be the inspiration to develop novel and functional water-collecting materials. A superhydrophobic surface that utilizes no energy has been synthesized from a water-collecting mechanism of the Stenocara beetles back structures. Firstly, this paper reviews the preparation methods of superhydrophobic surface coatings and then it examines patterns inspired by nature. In this paper, hydrophobic nanocoating was manufactured based on polyurethane (PUR) modified by nano-silica. Silica nanoparticles were synthesized via the sol-gel method. Then, silica nanoparticles were dispersed in polyurethane (PU) coatings. In this article, many factors of collecting moisture are evaluated, for example, temperature, pattern placement angle, pattern shape, types of surface patterns, and ambient humidity. Finally, are introduced the optimum situation for absorbing water from the fog.



Freshwater scarcity is increasingly perceived as a global systemic risk. Two-thirds of the world’s lands are experiencing water scarcity, especially in semi-arid and desert regions. Atmospheric water and fog constitute approximately 10% of all fresh water on Earth [1]. Wastewater reuse can therefore be a valuable option for water supply in areas where water is limited. Air humidity could be a potential renewable source. The classic wastewater treatment system includes conventional physical, chemical, and biological methods; but each of these approaches has some limitations. 2-Nitrophenol (2NP) is a poisonous and bio refractory contaminant in petrochemical wastewater which can cause significant injury to the environment and human health based on its high solubility and stability in water [2]. The existence of the nitro group on phenolic rings makes it more resistant to chemical and biological treatment methods [3]. The remediation of wastewater polluted with nitrophenols by traditional methods is difficult. 2-Nitrophenol is considered to be a hazardous waste and toxic contaminant by U.S. Environmental Protection Agency (EPA) [4]. Abstract—Removal of 2-nitrophenol (2NP) from an aqueous solution was performed using the Sono-Fenton process. The Box–Behnken design (BBD) of experiments was used to investigate the influence of operational factors such as pH, initial concentration of hydrogen peroxide, and ferrous ions on the removal of 2NP [5].

Thus, collecting tiny water droplets or fog could help overcome global water shortages [6]. In recent years, controlling dropwise condensation for water collection found extensive application prospects in various industries, including water treatment or recovery in power stations [7], seawater desalination [8], heat transfer [9], microfluidic devices [10] and others. In nature, the Darkling beetle (Stenocara gracilipes of Tenebrionidae family), also known as fog stand beetle, is a species of beetle that is native to the Namib Desert of southern Africa, which absorbs water in the atmosphere through the hydrophilic protrusions of its shell surfaces and then transports it to the mouth through the hydrophobic groove to supply water resources. Inspired by the shell structure of this desert beetle species, a large number of scholars have prepared various biomimetic structure surfaces that combined super hydrophilic/hydrophilic and superhydrophobic/hydrophobic regions. These so-called hybrids super wettable (HSW) surfaces have broad application prospects in water/fog harvesting [11-14].

At present, there are numerous methods of HSW surface preparation, such as reactive ion etching [15-16], laser processing [17], photocatalysis [18], and others. Hou et al. fabricated superhydrophobic surfaces with an array of superhydrophobic patterns using lithography and deep reactive ion etching technology. These surfaces were found to enhance the dropwise condensation effect and to have an excellent droplet-stripping effect and heat exchange performance. Mahapatra et al. [19] used carbon dioxide laser-processing techniques to create patterns on superhydrophobic aluminum surfaces, which promoted the condensation of droplets on these surfaces. Bai et al. [20] designed super hydrophilic star-shaped patterns using the mask photocatalysis technology for the titanium dioxide (TiO2) superhydrophobic surface, which patterns enhanced the water collection effect of the surface. Zhu and Guo [21] fabricated a titanium dioxide-copper (TiO2-Cu) composite surface modified with thiol and produced an HSW surface with excellent water collection efficiency and outstanding abrasion resistance. According to recent studies, hybrid HSW surfaces with super hydrophilic and superhydrophobic parts have considerable advantages regarding condensation, water harvesting, and heat exchange over more conventional surface structures. Therefore, it is also a vital issue in the field of bionic materials to develop a low-cost, readily prepared HSW surface. The surface of TiO2 modified by a low-surface-energy substance such as fluoro-alkyl silanes (FAS) can become superhydrophobic [22]. Due to the excellent photocatalytic properties of TiO2, the FAS monolayer deposited on the surface of TiO2 can be catalytically decomposed by UV irradiation [23]. This leads to a wetting transformation of the TiO2 surface. On the contrary, the decomposition of FAS monolayer modified on the surface of silicon dioxide (SiO2) nanoparticles is hard to achieve by UV irradiation. Given this, an HSW surface with mixed super hydrophilic TiO2 and superhydrophobic SiO2 nanoparticles can be produced by maskless UV irradiation. To the best of the authors’ knowledge, this is the first attempt to apply maskless photocatalysis to the fabrication of HSW surfaces for water harvesting.

Today, the only industrial-scale device for fog harvesting is the polymer mesh installation, which is now available in many dry areas of the world [24]. One of the most important parameters in fog harvesting techniques is the hydrophobicity of the collector. Polypropylene, polyethylene, nylon, and stainless steels with different hydrophobicity grades are examples of the materials used for this purpose. Superhydrophobic materials, which have attracted attention in recent years, have the properties such as self-cleaning, stain retention, and antifouling [25].

To produce such superhydrophobic surfaces; the sol-gel process, chemical vapor deposition, lithography, chemical etching, and electrospinning methods were used [26-27]. Electrospinning is a simple method that can be used to obtain the necessary structure. The porous film using hydrophilic polymer or containing hygroscopic nanoparticles can capture water droplets from the moist air due to their high specific surface area. Production involves the random delivery of a hydrophilic material onto a hydrophobic surface. However, this method is not suitable for repeated production [28-29]. Although the lithography method can accurately produce pre-designed patterns, it is not preferred due to its disadvantages such as the difficulty in controlling the process and the use of hazardous chemicals [30-31]. Ink-jet printing is a one-step procedure, but expensive. For all these reasons, low-cost methods are required for practical applications along with high fog harvesting efficiency.

It has been reported that the fog-harvesting ability of polyethylene terephthalate fibers is affected by the cross-section and surface structure [32]. Also, silicone elastomers and fluoropolymers are widely used in the preparation of superhydrophobic surfaces [33-34]. Polyacrylonitrile is a stable polymer that is widely used as a raw material in the process of functionalization with its good mechanical and chemical properties [35]. Recently, various microfabrication and chemical treatment strategies have been developed to produce polydopamine-coated bumps, titanium dioxide nanoparticles, and Trigonella foenum graecum on a hydrophobic surface [36-37]. In addition, it was found that almost all of the rough areas on the surface of various insect species were covered with a wax layer and this increased the efficiency of fog harvesting [38]. This result brought a new perspective to fog harvesting efforts. It was found that knop-like tissues with convex surfaces maximize the fog condensation rate [39]. The type and the content of nano-silica play significant roles in water uptake. Moreover, the results obtained from the gravimetric method showed that the coatings tend to absorb water rapidly in the initial days. After such periods a further slow increase in water uptake is initiated for hydrophilic-silica-filled coatings [40].

Polyurethane contains block copolymers of a urethane group (—NHCOO—) and a ureido group (—NHCONH—) [41]. This material has the advantages of high strength, wear resistance, and tear resistance. As an adhesive, polyurethane has been widely used in the preparation of superhydrophobic surfaces [42-45]. To create a polyurethane that has a lower surface energy, silicone or organic fluorine is usually used as a modifier. Organic fluorine-modified polyurethane has poor low-temperature resistance and toxicity [46-47], which has great limitations on the promotion and application of fluorine-containing materials. Therefore, silicone has been modified in the present experiment.

In the current study, to achieve a coating with good performance, especially in hydrophobicity, nano SiO2, fluorine components, and polyacrylate-polyurethane are combined via a copolymerization process followed by cross-linking. A set of techniques including Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM). Then, surface hydrophobicity, chemical resistance, and mechanical properties have also been studied. Additionally, amounts of collecting water by nanocoating surfaces based on varieties of natural patterns were checked [48].




Silica particle, alcohol, acetone, tetraethyl orthosilicate (TEOS), ammonia solution, methyltriethoxysilane (MTES), octyltriethoxysilane (OTES), vinyltriethoxysilane (VTES), methoxy peroxy silane, butyl acrylate (BA), methyl methacrylate (MMA), 2-hydroxyethyl methacrylate (HEMA), 1,6-hexamethylene diisocyanate trimer butyl acetate, t-butyl hydroperoxide (TBHP), dibutyltin dilaurate, styrene (St).


Preparation of polyurethane/silica nano-coating

Silica nanoparticles were synthesized by chemical method from tetraethyl orthosilicate (TEOS) and ethanol (C2H5OH) (Fig. 1).

The ammonia molar ratio is 1:4:8:0.2. The original silica sol obtained is colloidal silica in ethanol, which was exposed respectively to MTES, OTES, VTES, and MATMS at 35°C for 8 hours.

Silane coupling agents with 0.25 mol per 100 parts by weight of silica sol were added to the reaction system.

First, TEOS: 60 mg/L, H2O: 20 mg/L of, acetone: 20 mg/L, and ethanol: 30 mg/L are mixed for 20 minutes with ultrasound at a temperature of 25 °C to create a sol of silica, to produce silica nanoparticles [49].

Ammonia:10 mg/L was added and mixed at 40 °C for 20 min. The concentration of ammonia is the more significant factor in dictating the size of the SiO2 nanoparticles. With the increasing volume of ammonia solution, an increase in the size of silica nanoparticles was observed. [50].

Typically, polymer hydrogels were isolated in manufacturing as wet gels that contain manufacturing impurities such as monomers, initiator residues, salts, solvents, and soluble oligomers. Depending on their intended application, the polymers are purified and dried to the required extent. Drying usually is necessary for the centrifugation/washing process and is repeated five times. The obtained gel is finally oven-dried for 20 minutes at 110 °C.

Next, silica polyurethane nanocoating, polyester-grade polyurethane (pu140A80): 15 gr with silica nanoparticles:15 gr was mixed and obtained by the sol-gel method at room temperature. There were two reasons for choosing the amounts of polyurethane added to silica nanoparticles. First, the obtained nanocoating is transparent and without turbidity. Secondly, it has the property of sticking to surfaces.


Experimental Tests

This nanocoating was examined on the glass and aluminum surfaces. First, to remove dust and oil can be used alcohol and acetone. Then, the nanocoating is applied to surfaces on various patterns with different designs (Fig. 3). The nanocoating is covered where on the middle and edge of the slides. The glass and aluminum slides without nanocoating were chosen for checking tests. Increasing surface roughness is beneficial for improving the hydrophobicity of the coating. So, a glass slide was roughed and then coated by nanocoating.

Another was aluminum surfaces, which were cut to the size of a glass slide (7.5X2.5 cm). All glass slide patterns were repeated on aluminum surfaces. Just in case, soft sandpaper was used for making rough surfaces. In addition to the elementary designs, sheets of aluminum were cut to the size of glass slides and then crumpled and flattened for the aluminum coating coverage.

Relative humidity is a measure of the percentage saturation of the air. Therefore, to prepare the placement of tests was chosen place, at 57% relative humidity in a cold (18±2 °C) and hot (35±2 °C) temperature. In another test, to investigate the effect hydrophobic of nanocoating, the whole surfaces were studied at angles of 45°, 90° and 120° degrees (Fig. 2). Several times, this test was evaluated, immediately after stopping the humidifier, one, two and three hours after that. To ensure nanocoating and the process of tests was performed five times.

Finally, glass and aluminum slides are coated with nano coating in various designs and evaluated in ideal situations. (Fig. 4).


Pattern.3 is similar to the elytra of the Namib beetle S.Gracilipes longitudinally. Pattern.4 is identical to pattern.3, with the only difference being in lines that are in cross-sections. Pattern.6 is the same as the elytra of the O.Unguicularis beetle. Pattern.7 surface without nanocoating was used as a control for comparison. Pattern. 8 is another control of the test, which has the nanocoating. Pattern.13 is similar to the surface pattern of the P.Cribripes beetles. Pattern.15 includes a surface that was first sanded in preparation for the placement of the nanocoating. Pattern.16 was covered by the nanocoating and then sanded down (sanding on aluminum, do not possible to apply rough sandpaper on the surface. Soft sandpaper has been used). Pattern.17 was designed checkered. Pattern.18 is similar to a spider's web and was designed with rough puffs. Pattern.22 is without coated and has only been sanded.


Examining the degradability and stability of the nanocoating

Glass and aluminum slides are covered to check the stability of hydrophobic the nanocoating in different circumstances.



Contact angle results:

To check the contact angle of the nanocoating, glass, and aluminum slides are about 7.5X2.5 cm coated by the nanocoating, and surfaces without nanocoating were controlled again.

The findings of the tests conducted are according to Table 1 performed that the contact angle of the glass surface without nanocoating is equal to 48.2. However, the contact angle of the aluminum surface without nanocoating is 71.3 º, which is more hydrophobic than the glass surface. After covering the nanocoating, the contact angle of glass in a static state is 99.3 º, and in aluminum is 147.6 º. The aluminum surface is a more hydrophobic character. Generally, a superhydrophobic surface was defined as a surface with a high-water contact angle (WCA) greater than 150°.

Experimental dynamic contact angle results of Table 2 showed that glass and aluminum surfaces covered with the nanocoating were 103° and 160°, respectively. Other studies showed that the PEA coating film is somewhat hydrophilic due to the presence of 2-hydroxyethyl acrylate parts in water. Interestingly, the WCA increased from 97.5° for FPEA to 108.0° for SiO2-FPEA with SiO2 nanoparticles incorporation [51].

In this examine was observed that the PU/SiO2 nanocoating is super hydrophobic. PU/SiO2 nanocoating is a suitable coating for having adhesion force and structural stability. This produced coating is also resistant to wear. Therefore, both materials have the properties of PU/SiO2 composite super-hydrophobic coating [52]. Surface modification is a suitable method for improving the dispersion of SiO2 nanoparticles.


FT-IR analysis

Fig. 5 shows the FT-IR spectrum of SiO2 nanoparticles and polyurethane polymer. The peaks are visible clearly. Stretching vibrations Si-O asymmetric is the strong peak at 1117 cm-1. The peak at 844 cm-1 and 474 cm-1 are attributed to Si-O-Si symmetric stretching vibration absorption and Si-Si-O bending vibration. The characteristics of the absorption peaks of Si and O-H appear respectively at 1602 cm-1 and 3437 cm-1. In the absence of peaks at 2800-3000 cm -1 in nano SiO2, peaks at 2930 and 2965 cm-1 appear, which is attributed to C=H, -CH2- stretching vibration. The new peaks observed at 1731 and 1635 cm-1 for C=O and C=C are shown. Bending vibration (2200-2400 cm-1) for the Carbonyl ester group appeared characteristic of C-O-C near 1270 and 1150 cm-1. The absence of characteristic absorption of C=C at 1637 cm-1 indicates the complete reaction of all monomers in the copolymerization process. The incorporation of silica nanoparticles synthesized is confirmed successfully in the coatings. Comparing the findings with previous articles shows that the results obtained from FT-IR are close to the previous results and are confirmed based on the tables related to the spectroscopy of all structure factors in nanocoating [53].


SEM Results

Fig. 6 shows the surface morphology of the nanocoating polyurethane/silica. The surface is smooth, uniform, and without defects (Fig 6-B) and was used as a suitable coating. In the magnified image (Fig 6-A), nanoparticles of SiO2 with an average size of about 47 nm and a good dispersion index (PDI) of ~0.197 are scattered on the surface of the template and do not cause aggregation. The closely packed SiO2 nanoparticles formed a dense surface structure with a uniform roughness, probably between 40 and 55 nm, which could act as a protective layer and UV-resistant coating.

Compared to other articles, the nanoparticle of silica is 35 nm. In this Article, due to the production of silica nanoparticles by the sol-gel method and the use of ammonia, the size of nanoparticles is 47 nm has arrived [54].

In other investigations, it was observed that the higher the amount of ammonia in the sol-gel method for stability, the larger the size of the nanoparticles and the higher the porosity in the gel [55].


Experimental tests

The results are shown in Table 3 that the most effect on obtained water including moisture in cold temperatures and patterns placements angle of 120º with designed nanocoating on the edge of the surfaces. The inappropriate situation for collecting the water of fog is that placements f the surface of 45º and warm environments by covered with nano coating in the middle part of surfaces.

Table 3 shows that the volume of water obtained in aluminum without nanocoating is much more than in glass slide in the same qualification.

The results in the Table 4 are similar to those in Table 3. Important parameters for collecting water consist of the placement angle, temperature, and designs of the pattern.

Namib beetle can collect water behind its wings or elytra in the morning fog at 18-20 °C. On the other hand, the Namib beetle has acted similarly to an aluminum plate due to having hydrophilic and hydrophobic ridges and peaks [56].

Some structures made to collect water from air humidity are placed at a zero-degree angle. The remaining consists of plastic patterns for water consumption in agriculture or the Hulk net pattern placement at a 90º angle. This article examines collecting water without energy consumption. So, there are no comparison models with the mentioned.

All surfaces without nanocoating and the nanocoated patterns increased significantly, At the placement angle of 120º. The results observed that the quantity of water obtained in cold temperatures is more than in hot environments. According to data acquired from the Namib beetles, when the elytra placement angle is at 45º they have moved water, and the speed and movement of the droplet are increased at higher angular [57].

In all tests, the amount of water in the wrinkled aluminum patterns for creating macroscopic topography is more. According to the schematic shape of the wing of the Namib beetles, surface topographies can be produced in nano dimensions using H or fs lasers like the surface pattern of the beetle’s wing. For further investigation, variant patterns are designed on glass and aluminum, using schematic shapes in spider webs and points like the Namib beetle wings in surface patterns. Several surface patterns close to nature patterns were prepared and described in Fig. 4. All surfaces have been placement an angle of 120º.

This test was for checking the best-designed pattern. Fig. 7 shows that in the pattern.1, the volume of water obtained is different depending on the kind of placement. The highest and lowest water collected in the glass design is in pattern.2 and 8. Pattern.3 and 4, which are longitudinal and transverse and similar to the wing shape of the S. gracilipes beetle, both obtained water in the same way and there was no difference between transverse and longitudinal shapes.

According to the results of Fig. 8, the lowest water collected is in pattern.13. Pattern.15 and 16 both obtained the same amount of water.

Fig. 9 shows that collecting water in pattern.1 in both ways of placing has been collected water. Patterns.3 and 4 show that the transverse lines collect more water than the longitudinal lines due to the increased sequence between the hydrophobic and hydrophilic surfaces. Pattern.6 is similar to O.unguicularis and obtained more water volume than other designs.

Patterns.15 and 16, the coated nano-coating was removed from the aluminum and obtained less water (Fig. 10).

In this review, compared to other articles, O.unguicularis is only one of the four beetles that consider the standing head fogging mode in a low-temperature environment with artificial fog. The efficiency of fog removal from the elytra water of fogging and non-fogging beetles shows that S.Gracilipes is small and O.Unguicularis foggers are the better harvesters of fog water. P.cribripes is the largest and worst species of collecting moisture from the air [58].

Based on previous investigations, a super hydrophilic-superhydrophobic pattern inspired by the desert beetle for efficient fog harvesting using a simple spray method and selective modification consisting of a hierarchical hydrophobic microparticle/hydrophilic nanoparticle structure fabricated. As for the Mercaptan, When the mass ratio of Cu2O microparticles to ZrO2 nanoparticles in the spray suspension stood at 8:1, the prepared super hydrophilic-superhydrophobic hybrid sample had the highest fog collection efficiency of approximately 1707.25 mg/cm-2/h-1. In other words, at a typical mist velocity of 50 cm/s, the super hydrophilic-superhydrophobic hybrid sample constructed with an area of ​​1 square meter can collect approximately 17 L of fresh water, which can fully meet the water consumption [59]. The ratio of the surface's size in this research is 7%.


Examining the degradability and stability of the nanocoating

First, all patterns remained in a jar with 70% alcohol. It observed that a stability period was six days. Second, the consistency was three hours when samples were placed in a 3% HCL solution. Patterns were put in an oven at 120 °C. The samples were healthy at 105 °C, and after that, they started to destroy. The main point is that the aluminum surface covered by nanocoating was unstable by scratching.



Experimental results showed that the polyurethane/SiO2 nanocoating, with its high hydrophobic properties, can be collected air humidity without using energy on surfaces.

This study showed that the glass surface has a contact angle of 48.2° and the aluminum surface has a contact angle of 71.3°. The static contact angle of the covered glass is 99.3°, and the dynamic contact angle is 103.0°. In this test for coated aluminum, the static contact angle is 147.6°, and the dynamic contact angle is 160.0°.

Therefore, covered aluminum is a superhydrophobic surface.

SEM analysis, nanocoating particles are 47 nm in size. The prepared nanocomposite is rough and can create a hydrophobic surface. So, due to the use of silica nanoparticles in the nanocomposite, it is more transparent than other models.

The results of the experimental tests also showed that the best pattern is the aluminum surface with cold humidity of 57% at a temperature of 18±2 °C. The more alternating patterns between the surface and the nanocoating, the more water is collected, like in the checkered pattern. Cold humidity helps to collect more water because cold temperatures prevent the collected water from evaporating. Experiments revealed that the patterns’ placement angle has a significant effect on air moisture collection, and the higher this angle is higher the amount of water collected will be. The best placement angle is 120°.



The authors hereby declare that there is no conflict of interest.



Purpose: This study aimed to identify the dimensions of the innovative university to provide a model for Islamic Azad University.



HSW           Hybrid Superwettable

FAS             Fluoro-alkyl silanes

SiO2            Silicon dioxide

WCA          Water Contact Angle

SEM            Scanning Electron Microscopy

FT-IR          Fourier transform infrared

PEA             Poly-Ethyl acrylate

PU               Polyurethane

TEOS          Tetraethyl orthosilicate

MTES         Methyl triethoxysilane

OTES          Octyltriethoxysilane

VTES          vinyltrimethoxysilane

BA               butyl acrylate

MMA         methyl methacrylate

HEMA        2- Hydroxyethyl methacrylate

TBHP         tert-Butyl hydroperoxide

ST                Styrene

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