Corrosion behavior of superhydrophobic surfaces: A review

3.1 Contact angle and the Young equation

Atoms or molecules at the surface of solids or liquids possess fewer bonds with neighboring atoms than those in the interior, and thus, have higher energy. This surface energy or surface tension, γ, is equal to the work required to create a unit area of the surface at constant pressure and temperature, and is measured in N/m. The values of surface tension of some materials at 20 °C, found in the literature, are listed in Table 1.

As illustrated in Fig. 4a, when a liquid drop is placed in contact with a solid, the equilibrium of the solid and liquid surfaces will be established at a certain angle called the static contact angle CA, θ₀, given by the Young equation:

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Figure 4

Schematics of a liquid drop on a (a) smooth solid surface (Bhushan and Jung, 2011) and (b) tilted surface showing advancing and receding CAs (Nosonovsky and Bhushan, 2009).

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This equation is applicable only to flat and smooth surfaces and not to rough ones. It agrees with the fact that the WCA increases with decreasing difference between solid–vapor and solid–liquid surface energies, taking into consideration the sign of this difference.

Hydrophobicity is usually determined by measuring the CA of water droplets on a surface. The contact angle hysteresis, CAH, the difference between the advancing CA, θ_adv, and the receding CA, θ_rec, as shown in Fig. 4b, are an important parameter to characterize the wetting phenomena on any surface. A superhydrophobic surface should have a low CAH (<10°) in addition to high CA (>150°). As mentioned in the previous sections, a low CAH allows water to roll-off easily along the surface. Occurrence of contact angle hysteresis is associated with the surface defects, which could be from the surface roughness, chemical heterogeneity, or wetting state. According to the difference of contact angle hysteresis, surfaces can perform as slippery or sticky surfaces. The pinning phenomenon during the evaporation of a droplet is an example of the contact angle hysteresis.

The Young equation was based on the concept of an idealized, atomically smooth solid surface. However, all surfaces have defects and imperfections that contribute to surface roughness, and this roughness will contribute to the surface wetting behavior (Patankar, 2004). In view of this roughness-induced wettability modification, two well-established models have been developed to account for these effects: the Wenzel model (1936) and the Cassie-Baxter model (1944).

3.2 Rough surfaces and the Wenzel and Cassie-Baxter models

On rough surfaces, the Young equation no longer applies. Wenzel (1936, 1949) proposed a model to describe the contact angle, θ, with a rough surface, by relating it to that with a flat surface θ₀. He modified the Young equation as follows:

This is called the Wenzel equation, where r is the non-dimensional surface roughness factor, defined by the ratio of the actual area of a rough surface, A_SL, to its flat projected area, A_F:

Since r > 1, roughness on a hydrophobic surface (θ₀ > 90°) increases the contact angle and renders the surface more hydrophobic, whereas on a hydrophilic surface (θ₀ < 90°), roughness has the opposite effect, decreasing θ toward 0° and yielding a more hydrophilic surface.

The Wenzel equation is only valid for the homogeneous solid–liquid interface shown in Fig. 5a. It no longer applies for heterogeneous surfaces.

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Figure 5

Configurations described by (a) Wenzel equation for homogeneous interface, (b) Cassie and Baxter for the composite interface with air pockets and (c) Cassie equation for the homogeneous interface (Nosonovsky and Bhushan, 2009).

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Therefore, Cassie and Baxter (1944) proposed another model for heterogeneous surfaces composed of two fractions, one with a fractional area f₁ and contact angle θ₁ and the other with f₂ and θ₂, with f₁ + f₂ = 1. The contact angle is thus given by:

Combining Eqs. (2) and (4) gives rise to the Cassie & Baxter equation:

The opposite limiting case with θ₂ = 0°, corresponding to the water-on-water contact when the rough surface is covered by holes filled with water instead of air (Fig. 5c), yields the Cassie equation:

According to Eq. (6), for a hydrophobic surface (θ₀ > 90°), the contact angle increases with an increase of f_LA leading thus, to a more hydrophobic surface. For a hydrophilic surface (θ₀ < 90°), the contact angle can also increase with f_LA, in certain conditions at which the surface shifts from hydrophilic to hydrophobic (Jung and Bhushan, 2006). Boinovich and Emelyanenko (2008) reviewed the theory and fundamentals of hydrophobic materials in more detail.

Unlike the Cassie state, homogenous wetting, or the Wenzel state, leads to a larger solid contact area and subsequently higher CAH that, in turn, results in greater adhesion to the surface. Theoretical calculations and molecular dynamic simulations have also confirmed the fact that CAHs in the Cassie state are weaker than those in the Wenzel state (Koishi et al., 2011; Lafuma and Quere, 2003; Dupont and Legendre, 2010; Kong and Yang, 2006). For example: Lafuma and Quere (2003) compared the CAH of water droplets in the cases of both Cassie and Wenzel states on the same microtextured surface. For the Cassie state, the water droplet was gently dispensed on the surface while, in order to maintain the Wenzel state, the water droplet was condensed onto the surface. In addition to a higher advancing CA, it was shown that the CAH was significantly lower in the case of the Cassie state. Koishi et al. (2011) used molecular dynamic simulations to elucidate the relationship between the values of the CAH and the corresponding wetting regime. They found that the CAHs, in the case of the composite Cassie state, are weaker in comparison with those in the homogenous wetting Wenzel state on the same structure.

It may be concluded that, the Cassie model is a more general model that can be used to predict entire wetting regimes from low extreme to high extreme, whereas the Wenzel model can predict only moderate homogenous wetting regimes between the two extremes. From the Cassie model, it can be noticed that a reduction in the solid fraction and an increase in the air fraction would enhance the water repellency of a surface regardless of whether the surface is hydrophobic or hydrophilic.

4.1 Sol–gel process

The sol–gel process is an efficient, low-cost and low-temperature/low pressure procedure and can produce rough surfaces on a variety of oxides. This approach is a very versatile process for the preparation of superhydrophobic films or bulk materials. The roughness of the surface obtained with this process can be easily modified simply by changing the composition of the reaction mixture and the protocol followed. Silica coatings are the most used in the sol–gel method. Sol–gel processes can produce rough surfaces on a variety of oxides such as silica, alumina, and titania (Roig et al., 2004; Tadanaga et al., 1997). This approach is a very versatile process for the preparation of superhydrophobic thin films or bulk materials. The sol–gel process can form a flat surface coating, xerogel coating or aerogel coating depending on the process conditions; both xerogel and aerogel show rough or fractal surfaces.

Barkhudarov et al. (2008) used organo-trimethoxysilanes as starting materials to make optically transparent superhydrophobic films with contact angles exceeding 155° and reaching 170°. Xiu et al. (2008) generated superhydrophobic isobutyl surface groups by incorporating isobutyltrimethoxysilane (IBTMOS) into silica layers. With a TMOS/IBTMOS ratio of 1:1, a contact angle of 165–170° was observed. As shown in Fig. 6, the obtained surface consists of globules (20–50 nm in diameter) surrounding a network of pores (submicron size), which indicates a hierarchical roughness in two scales: nanoscale and submicron scale.

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Figure 6

High resolution SEM image of a silica film with a TMOS/IBTMOS ratio of 1:1 (Xiu et al., 2008).

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Latthe et al. (2009) managed to prepare superhydrophobic silica films on glass substrates using trimethylethoxysilane as a co-precursor. The silica films obtained were transparent and stable to high temperatures (up to 275 °C) and humidity, having a water contact angle of 151° and a sliding angle of 8°.

Feng et al. (2004) managed to grow superhydrophobic aligned ZnO nanorods starting from ZnO sol. Water contact angles exceeding 160° were observed. These nanorod films exhibited a reversible behavior from superhydrophobicity to superhydrophilicity by alternation of ultraviolet irradiation and dark storage: after UV illumination for 2 h, the water contact angle drops from ∼160° to 0° and returns to the initial value after storage in the dark for 7 days. The reason for the reversible behavior is due to the generation of electron–hole pairs in the ZnO surface resulting from the UV irradiation, some of these holes can react with the lattice oxygen to form surface oxygen vacancies. At the same time, both water and oxygen may compete to dissociatively adsorb on them. The defective sites are kinetically more favorable for hydroxyl adsorption than oxygen adsorption. In the case of rough surface, water will enter and fill the grooves of the films, leaving only the upper part of the nanorods not in contact with liquid. This effect results in a water CA of about 0^o (Sun et al., 2001; Bico et al., 2001).

It should be stated that these superhydrophobic coatings synthesized using the sol–gel processing are usually stable due to the formation of covalent bonds between the coating and the substrate (Xue et al., 2010). The resulted coatings are robust and rough (Xiu et al., 2008).

4.2 Layer-by-layer self-assembly

The layer-by-layer (LBL) assembly is a simple and cheap method to construct thin-film coatings by depositing alternating layers, oppositely charged, with intermediate washing steps. It uses electrostatic interaction and covalent bonds to form multilayer grafts. The advantage of this technique is that it possesses a high degree of molecular control over the film thickness, which arises from the linear growth of films with respect to the number of bilayers. Recently, LBL method has been employed by many groups in order to fabricate superhydrophobic coatings with rough surfaces.

Isimjan et al. (2012) prepared titanium thin films on steel substrates by means of LBL deposition process. At first, the steel substrate was deposited with a precursor layer of Poly(diallyldimethylammonium) (PDDA)/Poly(sodium 4-styrenesulfonate) (PSS) by three cycles of alternate immersion of the substrate in PDDA and PSS aqueous solutions. Then, the substrate was alternatively dipped in TiO₂ P25 aqueous solution and PSS. This cycle was repeated many times in order to obtain multilayer films of (TiO₂/PSS)∗n, where n corresponds to the number of deposition cycles. The resulting superhydrophobic coatings on steel substrate exhibited a strong repulsive force to water droplets, with contact angles greater than 165°. Fig. 7 illustrates water droplets on the three layers of TiO₂ nanoparticle coated surface and shows SEM images of one to three layers of P25 on the steel surfaces. Zhai et al. (2004) also prepared a polyelectrolyte multilayer surface by LBL assembly and then overcoated the surface with silica nanoparticles to mimic the hierarchical scale present on lotus leaf surfaces. Superhydrophobicity was achieved after surface fluorination with a fluoroalkyl silane.

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Figure 7

SEM images of different layers of TiO₂ nanoparticles in steel surface: (a) Water droplets on the treated steel surface. (b) TiO₂ one layer (TiO₂)∗1. (c) TiO₂ two layers (TiO₂)∗2 and (d) TiO₂ three layers (TiO₂)∗3 (Isimjan et al., 2012).

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Amigoni et al. (2009) constructed hybrid organic/inorganic surfaces by alternating different layers of amino-functionalized silica nanoparticles and epoxy-functionalized smaller silica nanoparticles. They found out that the hydrophobicity increases with the number of layers and obtained a water contact angle around 150° with the alternation of nine layers.

LBL assembly could be combined as well with electrodeposition to make superhydrophobic coatings as described by Zhao et al. (2005).

4.3 Etching

Etching is another simple and efficient method to produce superhydrophobic coatings with rough surfaces. Different etching techniques, such as chemical, plasma and laser etching have been recently used (Dong et al., 2011).

Li et al. (2012) used hydrochloric acid as chemical etching solution to prepare superhydrophobic surface on aluminum alloy AA2024. It has been shown that the roughness depended on the chemical etching time. A contact angle larger than 150° was achieved at an etching time of more than 3 min.

Pan et al. (2010) established a simple cetyltrimethylammonium bromide (CTAB) and ultrasonication assisted nitric acid HNO₃ etching technique to fabricate a rough surface on the copper substrate. A superhydrophobic coating was obtained with a contact angle around 155°.

Gao et al. (2012) fabricated a rough surface on the zinc substrate using a glow discharge electrolysis plasma (GDEP) reactor to etching. This apparatus consists of a high voltage supply unit and two electrodes submerged in an electrochemical cell separated by a dielectric wall with a diaphragm.

Shiu et al. (2005) developed an approach for making tunable superhydrophobic surfaces using oxygen plasma etching. Water contact angles up to 170° were obtained.

4.4 Chemical and electrochemical deposition

Chemical and electrochemical deposition techniques have been intensively employed to prepare superhydrophobic surfaces. Darmanin et al. (2013) reviewed the most published researches showing the details of electrochemical oxidation processes, such as oxidation of metals in solution, anodization of metals or electrodeposition of conducting polymers, and reduction processes, e.g., the electrodeposition of metals and galvanic deposition.

Ou et al. (2012) investigated the corrosion behavior in NaCl solution of superhydrophobic surfaces prepared by 1H, 1H, 2H, 2H-perfluorooctyltrichlorosilane (PFOTS) chemically adsorbed onto the etched titanium substrate. It has been shown that the strong chemical interfacial bonding between PFOTS and Ti leads to a higher stability and corrosion inhibition efficiency. Yin et al. (2008) also prepared a stable superhydrophobic film by mystic acid chemically adsorbed onto anodized aluminum substrate. The contact angle was in the order of 154°, and corrosion in seawater was significantly decreased.

Huang et al. (2013) demonstrated that the copper surface covered with copper stearate, which was formed by an electrochemical reaction with a DC voltage of 30 V in stearic acid solution, showed superhydrophobic properties. Shirtcliffe et al. (2004) electrodeposited copper from acidic copper sulfate solution onto flat copper substrates to obtain superhydrophobic surfaces of varying levels of roughness, depending on the current density during deposition.

4.5 Other techniques used

Other approaches to the fabrication of superhydrophobic surfaces have been developed and used, these include spraying nanoparticle suspensions onto substrates (Ogihara et al., 2013), chemical vapor deposition (Ishizaki et al., 2010), electrospinning (Acatay et al., 2004; Ma et al., 2005), sublimation (Nakajima et al., 1999), hydrothermal synthesis (Shi et al., 2005), and wax solidification (Onda et al., 1996).

Spraying is a one-step, time-saving, low-cost and repairable method for preparation of superhydrophobic surfaces. Xu et al. (2011) fabricated superhydrophobic copper meshes by evenly spraying an emulsion of n-octadecanethiol and silver nitrate in ethanol onto the copper mesh with nitrogen gas by a spray gun. Alkanethiols contain long-chain alkyl groups that have low surface free energy. One advantage of this method is that in the case of mechanical damage of the surface, it can simply be repaired by partial spraying. In a similar way, Ogihara et al. (2013) prepared superhydrophobic paper using mixtures containing nanoparticles and ethanol, manually sprayed over paper from 20 cm away with a vaporizer. In this case, the surface energy and the roughness structure could be controlled by the spray-coating conditions, such as the type of nanoparticles and their size.

Electrospinning is another powerful, simple and practical one-step method to generate continuous ultrathin fibers with micrometer and sub-micrometer diameters from a variety of polymeric materials. Electrospun fibers intrinsically provide at least one length scale of roughness for superhydrophobicity because of the small fiber size. The fiber mats composed solely of uniform fibers could be obtained by electrospinning a hydrophobic material (i.e., poly(styrene-block-dimethylsiloxane) block copolymer) blended with homopolymer polystyrene (PS) (Ma et al., 2005). Table 2 shows the most important parameters controlling the electrospinning process.

Acatay et al. (2004) electrospun a thermoset fluorinated polymer onto an aluminum foil substrate, by applying an electrical bias from the tip of the polymer solution-filled syringe to a grounded collection plate. The formed electrospun film consisted of a continuous web or mat of randomly aligned fibers.

After the roughening of the surfaces, most substrates still do not show superhydrophobicity. Consequently, the surface energy of such coatings should be lowered using low surface energy materials. The most common ones are fluorocarbons, silicones, and some organic materials (polyethene, polystyrene, …). Although, inorganic materials such as ZnO and TiO₂ have high surface energy, the nanoparticles from these materials are easily covered by airborne organic contaminations. Due to such contaminations the surfaces modified with such nanoparticle layers may demonstrate superhydrophobic behavior. For instance, fluoroalkylsilane (FAS) molecules were intensively used by several groups in order to modify the surface and enhance superhydrophobicity due to their extremely low surface free energy and the simple reaction of the silane groups with the hydroxyl groups on coatings (Hu et al., 2012; Ishizaki et al., 2011; Liang et al., 2013; Song et al., 2012).

5.1 Anti-adhesion and self-cleaning

As previously mentioned, the high water contact angle (above 150°) and the low contact angle hysteresis of superhydrophobic surfaces characterize them with low adhesion and self-cleaning properties that are extremely useful for practical applications such as self-cleaning window glasses.

In a natural environment, surfaces get contaminated very easily and frequently. Cleaning them requires effort, time and money. Thus, the creation of substrates that are self-cleaning and that have low-adhesion to contaminants has been a hot topic of research for several decades. As illustrated in Fig. 8, when a water droplet rolls off a superhydrophobic surface, it entrains the dust and contaminants with it. This is not the case for normal surfaces, where the dust remains.

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Figure 8

Water droplets rolling off substrates with a normal hydrophobic surface (left) and a self-cleaning superhydrophobic surface (right) through dust particles (Xue et al., 2010).

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Nimittrakoolchai and Supothina (2008) studied the anti-adhesion and self-cleaning properties of a superhydrophobic film by applying red powder and dust onto it. The superhydrophobic coating was prepared by the deposition of a polyelectrolyte film on a glass substrate, followed by etching in HCl solution to roughen the surface, and by the deposition of SiO₂ nanoparticles onto the etched film. They showed that much of the red powder was still spread on the uncoated surface after cleaning with water droplets as compared to the superhydrophobic surface, which was much cleaner. Moreover, dust particles were heavily sprinkled on the coated surface and were simply removed by flipping it, showing the anti-adhesion property for contaminants of such films.

Superhydrophobic surfaces with self-cleaning property also have many applications in the textile industry. For example, self-cleaning textile shirts, blouses, skirts and trousers, which are stain-proof, have already been synthesized (Xue et al., 2010). Furthermore, the self-cleaning effect is used in optical applications, such as solar panels, lenses and mirrors that have to stay clean (Nosonovsky and Bhushan, 2009).

5.2 Anti-biofouling applications

Biofouling of underwater structures and ships’ hulls, in particular, increases operational and maintenance costs (Gudipati et al., 2005; Townsin, 2003). It can be reduced through underwater superhydrophobicity, i.e., forming a hydrophobic rough surface that supports an air film between itself and the water (Marmur, 2006). The reduction of the wetted area minimizes the probability that biological organisms encounter a solid surface. The design of such surfaces should involve optimization between mechanical stability and minimal wetted area. The anti-biofouling properties of superhydrophobic coatings have been investigated (Zhang et al., 2005). Compared to normal substrates, which fouled within a day, almost no micro-organisms attached to the superhydrophobic surfaces in the first week after immersion.

5.3 Corrosion inhibition

The concept of preparing surfaces that repel water creates huge opportunities in the area of corrosion inhibition for metals and alloys. Given their water repellency, superhydrophobic coatings form an important and successful method to slow down the breaking of the oxide layer of metals and thus prevent the metal surface underneath from further corrosion.

Several works have been carried out in order to study the corrosion resistance of metals coated with superhydrophobic surfaces. Some of the results obtained will be overlaid and discussed in the next section.

5.4 Drag reduction

Turbulent flows of a liquid along a surface experience frictional drag, a macroscopic phenomenon that affects the speed and efficiency of marine vessels, the cost of pumping oil through a pipeline, and countless other engineering parameters. The drag arises from shear stress, the rate per unit area of momentum transfer from the flow to the surface. Superhydrophobic surfaces can be fabricated in order to reduce the drag based on its highly water repellent property and capable of forming a thin air film over an underwater surface which stops the surface from becoming wet. The air film formed over the surface has the property of being able to take in air supplied from outside because of the surface tension of water. The superhydrophobic surfaces to reduce frictional drag have been used in ships. When air is supplied from the bow section to a ship’s hull with superhydrophobic coatings, it becomes attached to the superhydrophobic surface and forms an air film on it. The frictional drag can thus be reduced by an air lubricant effect.

7.1 Solvent stability

Many works have been carried out in order to study the stability and durability of superhydrophobic coatings on different metallic substrates, in water and various organic solvents. For instance, Isimjan et al. (2012) investigated the effect of water, chloroform and decane on the contact angles of TiO₂/SiO₂ coated steel substrates measured after every 24 h for a period of eight days. It was shown that the contact angles remained constant and the surfaces were stable even after 8 days. In another work, coated zinc substrates were immersed in toluene for 24 h at room temperature (Ning et al., 2011). The resulting contact angles had no variation, suggesting a good stability of the surface.

The stability of superhydrophobic surfaces on different substrates in seawater was also intensively studied and revealed the conservation of superhydrophobicity even after a long duration exposure (He et al., 2009; Liu et al., 2007, 2008; Yin et al., 2008; Yu et al., 2013). Zhu and Jin (2007) created a superhydrophobic film by using electroless Ni-P composite coating on carbon steels. The superhydrophobic film has good stability in the air at room temperature and good corrosion resistance in 5 wt% NaCl solution, neutral salt spray test and water erosion test. Boinovich et al. (2012) also studied the corrosion behavior of highly hydrophobic and superhydrophobic coatings for low C-steel steel under atmospheric conditions and aggressive media. Their results showed that the most durable protection against corrosion is obtained by the formation of multilayer coatings which contain the nanocomposite superhydrophobic layer in combination with oxide layer. The potentiodynamic polarization curves in 0.5 M NaCl aqueous solution for low-carbon steel samples with different surface layers are shown in Fig. 12.

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Figure 12

Polarization curves in 0.5 M NaCl aqueous solution for low-carbon steel samples with different surface layers: (1) Native oxide; (2) MC-1 + NSC (MC-1: Magnetite coatings formed at 98 °C and at high stirring, NSC: nanocomposite superhydrophobic coatings); (3) MC-2 + NSC (MC-2: Magnetite coatings formed at 92 °C); (4) PEO + NSC (Boinovich et al., 2012).

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Boinovich et al. (2013) also investigated the effect of different corrosive active media on the Mg–Mn–Ce alloy coated by nanocomposite syperhydrophobic materials. They indicated that the preliminary plasma electrolytic oxidation of alloy improves the adhesion and water repelling properties of the coatings and consequently increases the corrosion resistance of Mg alloy. Xu et al. (2011) fabricated the superhydrophobic coating for Mg alloy using a facile electrochemical machining process. Their results showed that the superhydrophobic Mg alloy surface has excellent corrosion resistance in acidic, alkaline and salt solutions. Table 4 presents the corrosion potential (E_corr) and corrosion current density (I_corr) of untreated and superhydrophobic Mg alloy in different corrosive media.

Barkhudarov et al. (2008) investigated the behavior of superhydrophobic coatings as corrosion inhibitors. They concluded that the extreme case of a superhydrophobic coating with a contact angle of >160° decreases the rate of corrosion roughly tenfold compared to the unprotected aluminum. Fig. 13 shows the changes in aluminum thickness after the test with different surface conditions.

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Figure 13

The change in aluminum thickness versus time for samples protected by films of varying contact angle and a sample with only native Al₂0₃ layer. The superhydrophobic films improved corrosion resistance by a factor of six compared to unprotected aluminum surface (Barkhudarov et al., 2008).

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Liu et al. (2009) successfully created a superhydrophobic surface on aluminum surface by an anodization process and chemical modification using myristic acid. Their results showed that, the superhydrophobic surface significantly improves the corrosion resistance of aluminum in sterile seawater. The superhydrophobic surface affects mainly the aluminum anodic reaction, whose currents (I_corr) are reduced by about three orders of magnitude, the corrosion potential (E_corr) shifts positively by 0.2 V when the anodized aluminum is covered with the myristic acid. The significant point of this research is the possibility of applying this method to a large scale production of superhydrophobic engineering materials for ocean industrial applications. In another research, Liu et al. (2009) studied the use of superhydrophobic surfaces on aluminum as a method for inhibition of microbially influenced corrosion. Their study showed that the superhydrophobic film does not only decrease the corrosion current densities, but also microbially influencing corrosion acceleration inhibition due to preventing colonization of microorganisms.

7.2 pH stability

The stability of superhydrophobic surfaces over a wide pH-range is crucial for their use as engineering materials in several industrial applications. Guo et al. (2005) investigated the stability of coated aluminum alloy, over the pH range from 1 to 14. All the measured contact angles were around 160° to 162°, showing that the pH of the aqueous solution had little or no effect.

Similar results were obtained by Ishizaki et al. (2011) where the chemical stability of the color-tuned superhydrophobic magnesium alloy surface and untreated magnesium alloy surface was examined, in the pH range from 1 to 14. The results showed static water contact angles (>145°) for the coated substrates for all pH solutions. On the other hand, all the untreated substrates showed hydrophilic behavior (contact angles <40°) and thus, evidence of corrosion.

7.3 Thermal and humidity stability

High temperatures usually reduce the durability of superhydrophobic coatings and lead to an irreversible change of the hydrophobic character to a hydrophilic character. However, some surfaces exhibit resistance toward heat up to a certain temperature.

Xiu et al. (2008) generated porous inorganic silica films on glass substrates, by the sol–gel process, using tetramethoxysilane and isobutyltrimethoxysilane as precursors. At elevated temperatures (>200 °C), the hydrophobic isobutyl group in the silica surface decomposes and leads to the loss of superhydrophobicity. The contact angle decreased with increasing temperature and the contact angle hysteresis increased. However, when the as-prepared silica films were modified with fluoroalkyl silanes, the thermal stability was significantly improved: at a temperature of 400 °C, the surface remained stable. Further heating resulted in a decrease of contact angle and when the temperature reached 500 °C, the film was no longer hydrophobic (contact angle almost became 0°).

Latthe et al. (2009) studied the influence of temperature on silica coatings by putting the samples in a furnace for 5 h. It was found that the films retained their superhydrophic property up to a temperature of 275 °C. Above this value, the films became superhydrophilic with contact angles smaller than 5°.

Furthermore, the influence of humidity was also investigated. The silica films were exposed to a relative humidity of 85%, at 30 °C for a duration of 60 days. The results showed a strong stability and durability against humidity.

Similar results were obtained by Rao et al. (2011) who investigated the effect of humidity on the wetting properties of silica films on copper substrate. The experiment was carried out at a relative humidity of 95% at 35 °C for a period of 90 days. No significant change in the superhydrophobicity of the films was observed, showing the durability and resistance of such surfaces against humidity.

7.4 UV stability

The stability of superhydrophobic films against UV irradiation is extremely important, especially for outdoor surfaces that are exposed to UV light. Feng et al. (2004) obtained superhydrophobic ZnO nanorod films that lost their superhydrophobicity under UV illumination for 2 h: the water contact angle dropped from 160° to 0°.

On the other hand, the results obtained by Isimjan et al. (2012) revealed that the TiO₂/SiO₂ coated steel surfaces strongly resist against UV radiation: the contact angles remain constant even after a period of 5 h UV exposure. This was explained by the fact that in the presence of SiO₂ nanoparticles, the high energy electrons generated by TiO₂ under UV cannot diffuse to the surface and thus, no hydroxyl radicals can be formed and no oxidation will occur.

Xiu et al. (2008) managed to fabricate one of the most stable superhydrophobic silica surfaces treated with fluoroalkyl silanes, which showed resistance after 5500 h UV irradiation without degradation of either contact angle or contact angle hysteresis. The fact that the C—F bonds are much stronger than the C—H bonds was the reason of this improved UV stability.