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Fabrication of superhydrophobic surfaces on FRP composites: from rose petal effect to lotus effect.

Introduction

In recent times, superhydrophobic surfaces have attracted much attention because of potential industrial applications, such as the anti-ice and anti-snow adhesive, (1,2) surface self-cleaning, (3,4) oil/water separation, (5-7) antibiofouling and drag reduction properties, (8,9) and transportation of microdroplet. (10,11) It is well known that the surface wetting behavior and adhesive property are mainly dominated by surface morphology and chemical composition. Therefore, increasing surface roughness and lowering the surface tension are the main strategies for the preparation of superhydrophobic surfaces. In addition, the adhesive property plays an important role on the dynamic action of the liquid on the surfaces. (12) In order to satisfy the various requirements, the surface wetting ability as well as adhesion property needs to be modified by adjusting surface morphology and chemical composition. According to the difference of water droplet's mobility on the surfaces, two types of superhydrophobic surfaces covering high adhesion surface (13) and low adhesion surface (14,15) are distinguished and defined. For the former, a rose petal is the most representative plant in nature. For the latter, a lotus leaf has gained special attention. In that sense, non-wettability is frequently referred as the "lotus effect."

In the past few years, some research focusing on the surface wetting and transition between the high and low adhesive states, namely from rose petal effect to lotus effect, has been carried out. Balu et al. (16) reported on the fabrication of superhydrophobic paper surfaces with variable stickiness for water drops. The obtained superhydrophobic paper surfaces provided many opportunities in the control of aqueous drops mobility and the transfer of drops on substrates. The variations in stickiness were realized by the transformation from Wenzel state to Cassie state on the nano-scale. Bhushan et al. (17) systematically characterized the superhydrophobic rose petal with high and low adhesion surfaces. Results indicated that the variable adhesion surfaces were obtained by adjusting the density of microstructures and nanostructures. By combining simple hydrothermal process and surface modification using stearic acid, Li et al. (18) fabricated superhydrophobic ZnO surfaces with controllable water adhesion. The as-prepared ZnO surfaces showed an extreme non-wettability but the water adhesion ranged from high to low. It was an important strategy for the adhesion adjustment on superhydrophobic surfaces. Yong et al. (19) realized superhydrophobic patterned polydimethylsiloxane (PDMS) surfaces with tunable adhesion using femtosecond laser. The tunable adhesive superhydrophobic surfaces were potentially used in microfluidic systems to modulate the mobility of liquid droplets. Based on the reaction between an alkyl thiol and the hierarchical structured Cu[(OH).sub.2] substrates, Cheng et al. (20) obtained superhydrophobic surfaces with controlled adhesion. The results would be helpful for the further understanding of the effect of wetting states on the surface adhesion and the fabrication principle for superhydrophobic surface. Peng et al. (21) prepared three kinds of superhydrophobic polymethylmethacrylate (PMMA) macroporous membranes with controlled adhesion using a simple hierarchical alumina template wetting method. The shallow bowl-shaped structure presents a slippery property with a roll-off angle of 3°, the deep bowl-shaped morphology possesses a roll-off angle of 30°, and the deep honey-comb structure possesses a strong sticky water adhesion. This work was a convenient and an effective method to prepare superhydrophobic surfaces on thermoplastic polymers. Li et al. (22) prepared the superhydrophobic paper surfaces with heterogeneous contact angle hysteresis by printing high hysteresis wax islands onto low hysteresis superhydrophobic paper. This work realized a simple method to obtain well-defined microliter sample volumes and to extract several samples simultaneously from the same source. At the same time, it enables the development of two-dimensional paper-based microfluidic devices for biomedical testing.

To date, most of the work superhydrophobic surfaces with controllable adhesion mainly focuses on metal or inorganic substrates, but little work pays attention to superhydrophobicity applied on the polymers' surfaces. (16-18,20,21) Besides, the polymer matrices prepared with superhydrophobic surfaces are chiefly thermoplastics polymers, such as polydimethylsiloxane (PDMS), polymethylmethacrylate (PMMA), polypropylene (PP), polyethylene (PE), polystyrene (PS), polycarbonate (PC), etc. (23-27) Because of many excellent properties covering high specific strength, good dimensional stability, and heat resistance, ease of fabrication, relatively low cost, thermosetting polymer, and fiber reinforced plastic (FRP) composites are widely used for vessel, fluid transportation, aeronautic/space industry, and so on. (28) However, it is difficult to use a simple way to obtain superhydrophobicity on thermosetting polymers or FRP surfaces owing to the infusible and insoluble property after curing. (26) Therefore, it is very important to get the superhydrophobic FRP surfaces with controlled adhesion by a simple and an inexpensive method.

In this article, FRP specimens were first prepared and coated using resin glue mixed with different content of CaC[O.sub.3] and Si[O.sub.2]. After the subsequent treatment covering sandblasting, etching, and modification, superhydrophobic surfaces with the variable adhesion were successfully obtained. In this experiment, no expensive reagents, toxic chemicals, or expensive instruments were employed.

Experimental

Materials

Plain glass fiber cloth (SW100A-90a) was provided by Sinoma Science and Technology Co., Ltd. The epoxy resins with types of EPOLAM resin 2008 (EP2008) and EPOLAM curing agent 2008-S (EP2008-S) were supplied by AXSON Co., and used as the matrix of FRP. The commercial CaC[O.sub.3] (2000 mesh) and Si[O.sub.2] (about 100 nm) were selected as the surface coating components. Their morphologies are shown in Fig. 1. Stearic acid ([C.sub.18] [H.sub.36] [O.sub.2]), ethanol, and acetic acid were analytical grade. All materials were used as received; no further purification or other treatments were made.

The preparation of FRP and surface coating

The manufacturing of FRP was carried out by hand lay-up method. Five plies of plain glass fiber cloth were cut. The first ply of plain glass fiber cloth was placed on a ceramic which had been covered with demoulding reagent. Next, a mixture of epoxy resin and curing agent (the weight ratio for the EP 2008 and EP 2008-S was 100:25) was applied on the glass fiber mat. It is very important to ensure that the mixture wets the entire surface of glass fiber. The procedure was repeated until all the plies were superimposed. Then, the sample was pressed with a metal roller from one side to the other. Finally, another mixture of EP2008/EP2008-S/CaC[O.sub.3]/ Si[O.sub.2] was continuously coated on the surface of as-prepared FRP composite. A ceramic board covered with demolding reagent was placed on the top of the coating with a certain pressure. After lying at room temperature for 24 h, the laminated plate was completely cured at 60°C for 6 h. The thickness of the laminated plate and surface coating was approximately 1.0 ± 0.1 and 0.1-0.2 mm, respectively.

The preparation of superhydrophobic surfaces of FRP

The as-prepared FRP with surface coating was sandblasted using corundum particles (particle diameter of 50 mesh, at sample-nozzle distance 2 cm and air pressure of 1.0 MPa), and the subsequently etching was performed using acetic acid solution (8.5 mol [L.sup.-1]) for 5 min. After being ultrasonically cleaned using deionized water for 10 min and dried at 60°C for 1 h, the samples were immersed in 1 wt% stearic acid ethanol solution for 60 min and naturally dried for 24 h. The detailed preparation procedures are schematically illustrated in Fig. 2. In terms of the mass ratios (before sandblasting) of EP2008/EP2008-S/CaC[O.sub.3]/Si[O.sub.2], the prepared samples were defined as [S.sub.1](EP2008:EP2008-S:CaC[O.sub.3]:Si[O.sub.2] = 100: 25:100:0), [S.sub.2] (EP2008:EP2008-S:CaC[O.sub.3]:Si[O.sub.2] = 100:25: 40:0), [S.sub.3] (EP2008:EP2008-S:CaC[O.sub.3]:Si[O.sub.2] = 100:25:40:5), [S.sub.4] (EP2008:EP2008-S: CaC[O.sub.3]:Si[O.sub.2] = 100:25:40:10), [S.sub.5] (EP2008:EP2008-S:CaC[O.sub.3]:Si[O.sub.2] = 100:25:40:20), respectively.

Characterization

Field emission scanning electron microscopy (FESEM) with the type of XL FEGSFEG-SIRION (FEI Ltd., Netherlands) was applied to investigate the morphological features of the obtained specimens. The sessile drop method was used for the measurement of water contact angle with a Dataphysics OCA15Pro contact angle system operated at room temperature. The water droplets with volume of 4 and 10 µL were used to measure static contact angle and roll-off angle. The average contact angle value was determined by measuring 5 times at different spots. The friction coefficients were conducted on a MM-200 friction and wear tester (Shanghai University of Technology, China) at room temperature. The rotation speed of the steel ring was 200 rpm, and the load and measurement time were, respectively, 50 g and 30 min.

Results and discussion

By means of the etching reaction occurred between the CaC[O.sub.3] and acetic acid, it is easy to obtain rough surfaces on the FRP matrix. One can deduce that a special micro/nanohybrid structure can be formed on the FRP. Such rough surfaces benefit in the achievement of the hydrophobic property. Due to the difference in the content of CaC[O.sub.3], the etching level and the derived rough morphologies should be different. As shown in Table 1 that the S3 and [S.sub.5] exhibited the superhydrophobic property, reflecting the effect of the concentrations of CaC[O.sub.3] and Si[O.sub.2] on the hydrophobic properties. The corresponding static contact angles are respectively 152 ± 1.5° and 160 ± 1.6°. At the same time, we can see that the hydrophobic property was greatly improved with the increasing concentrations of Si[O.sub.2] from [S.sub.2] to [S.sub.5]. The Si[O.sub.2] aggregates also played an important role in building micro/nanohybrid structure.

However, a static contact angle cannot solely reflect the overall wetting properties of the sample surface. The roll-off angle is also an important indicator for the superhydrophobic materials. After etching and modification, the FRP surface gives rise to different adhesions, because of different content of CaC[O.sub.3] and Si[O.sub.2]. In this work, the surface adhesion is our major interest. The as-prepared [S.sub.1] and [S.sub.5] surfaces have similar superhydrophobicity, but the dynamic properties are rather different. A water droplet can be firmly pinned on the [S.sub.1] surface in a vertical position or even turned upside down (Fig. 3a). In contrast, in Fig. 3b, a water droplet can hardly stand on a sloped [S.sub.5] (tilting angle of about 9°) and would immediately roll-off.

Different dynamic properties mean the surfaces with different adhesions. The adhesion work is a method used to research the physical force, such as adhesive force or adhesion at interfaces between solid substances and liquid. (29) It is usually defined by the following equation: (30)

W = [?.sub.LV] (1 + cos?), (1)

where W is the adhesion work, and [?.sub.LV] and ? refer to the surface tension of the liquid-vapor and apparent contact angle, respectively. In our experiments, according to the corresponding static contact angle, the adhesion work of [S.sub.1] and [S.sub.5] for different water volume is calculated, and the results are shown in Fig. 4. When the water volume is less than 4 µL, no apparent distinction is found whether contact angle or adhesion work both [S.sub.1] and [S.sub.5] . In order for a drop to be axisymmetric, it has to be sufficiently large compared with the scale of roughness or heterogeneity. Thus, only large enough drops should be used for contact angle measurements, and the small water droplet does not provide true superhydrophobic surfaces. (31,32) When the water volume is more than 4 µL, as the droplet volume increases, the contact angle decreases and adhesion work increases. When the droplet volume has the 10µL, the adhesion work of [S.sub.5] surface is about one-fourth of that [S.sub.1].

To have a better understanding of the water adhesion behavior on the sample surface, we carefully analyzed the effect of surface morphology on the wetting behavior at various stages of sample. The surface morphology was examined using SEM. FRP surface morphology with the coating component of EP2008:EP2008-S:CaC[O.sub.3]:Si[O.sub.2] = 100:25:100:0 and EP2008:EP2008-S:CaC[O.sub.3]:Si[O.sub.2] = 100:25:40:20 is shown in Figs. 5a and 5b. They both present the relatively smooth epoxy resin surface with the CAs of 79° and 83°. This is consistent with the previous literature. (33) After sandblasting, the contact angle achieves 117° (Fig. 6a) and 142° (Fig. 6b). A large deviation of contact angle indicates a significant effect on the surface property of the sample. The change of surface property is associated with the formation of the rough surface (Figs. 6a and 6b). After etching, most of CaC[O.sub.3] is reacted with C[H.sub.3]COOH and many inhomogeneous holes (Fig. 6c) or valleys (Fig. 6d) are formed on the surfaces. The results are in line with the morphology of CaC[O.sub.3] and Si[O.sub.2] shown in Fig. 1. Correspondingly, the amount of calcium declines to 3.89% from 14.42% and 1.73% from 5.54% obtained by energy dispersive spectroscopy (EDS). There is an important reason that water contact angle decreases from 117° to 64° and 142° to 84°, respectively. However, interestingly, after modification with stearic acid, the surface morphologies of [S.sub.1] and [S.sub.5] show a huge difference. The stearic acid stacking is randomly distributed on the etched surface, and the surface of stacking is very smooth (Fig. 6e). The adhesion between stearic acid and etched surface may mainly depend on the electrostatic force. Nonetheless, in Fig. 6f, the surface structure is well proportioned and it comprises numerous nanoparticles, which form the micro/nanobinary structure. Obviously, the micro/nanobinary structures play a very important role in the CA and RA.

For the geometry of rough surface, its structure-dependent superhydrophobicity has been verified both theoretically and experimentally by a photoresist material. (34) If the structure is too low and too wide, the microlens surface (Fig. 6g) will be wetted by the droplet and the surface will enter the Wenzel regime, but that is based on the hypothesis of a saturated surface. In addition, with the increase of droplet volume, the CAs of [S.sub.1] and [S.sub.5] both decrease at different degrees. This may be attributed to the different surface topography (microlens and microbowls). This is why [S.sub.1] has a superhydrophobic surface (CA > 150°) when the droplet volume is below 4 µL. However, it has a CA of 130° when the droplet volume achieves 10 µL (Fig. 4). At the same time, such as [S.sub.5] surface, the wettability of microbowls will follow the Cassie-Baxter model in which the air trapping mode is dominant (Fig. 6h). (35) Many researchers have contributed to the related experiment work, (33-35) the Cassie-Baxter model is shown in the following equation:

cos [?.sub.r] = [f.sub.1] cos ? - [f.sub.2] (2)

where [?.sub.r] and ? represent the contact angles on rough and smooth surface, respectively. Here, [f.sub.1] and [f.sub.2] are the fractions of the solid surface and air in the composite surface, respectively (i.e., [f.sub.1] + [f.sub.2] = 1) According to the Cassie-Baxter equation, [f.sub.1] and [f.sub.2] are calculated to be about 0.049 and 0.951. This means that air occupies about the 95.1% of contact areas when the surface of [S.sub.5] contacts with a 4 µL water droplet. The large fraction of air trapped in the pores of microstructures will greatly increase the air/liquid interfaces and effectively prevent the penetration of the liquid into the pores, which plays an important role in gaining superhydrophobicity.

The photograph in Fig. 7a shows that water droplets are put on the [S.sub.5] (right) and blank FRP (left) surfaces. The CA of the blank FRP is about 80° due to the hydrophilic nature of the epoxy resin. As anticipated, the introduction of surface treatment on the FRP transforms it from hydrophilic to superhydrophobic. The high CA value and low RA that are 160° and 9° are obtained under optimal conditions. The asymmetric wettability is obviously seen, and the water droplets on the [S.sub.5] present an almost perfect round shape. In Fig. 7b, a mirror-like surface can be observed. The phenomenon was firstly reported by Larmour et al. (36) on a surface which passed McCarthy's test for 180° CA, suggesting a Cassie-Baxter state. A theoretical discussion of the possibility of such surfaces displaying underwater superhydrophobicity has also been published. (37) The mirror-like surface is owing to an air layer between the water and the superhydrophobic surface. Then, Li et al. (38) and Basu et al. (39) reported the similar phenomenon. This is an additional indication of the Cassie-Baxter state when [S.sub.5] is upon submersion of the surface in water. However, the phenomenon is not seen from [S.sub.1] surface in water. It is chiefly because the [S.sub.1] surface enters the Wenzel regime in water and not enough air layer exists between the water and the superhydrophobic surface.

The durability of the [S.sub.1] and [S.sub.5] surfaces at different time and temperature was investigated. When stored in ambient environment for 3 months, their water contact angle and sliding angle remain essential constant, and this indicates that their surface microstructures do not change within the storage time. However, in practical applications, FRP are often used for different temperatures. Thus, it is necessary to study the superhydrophobicity of the [S.sub.1] and [S.sub.5] for different temperatures. Figure 8 shows the contact angle values of the [S.sub.1] and [S.sub.5] after thermal treatment. When the thermal treatment temperature is lower than 60°C, the water contact angle of [S.sub.1] and [S.sub.5] is still higher than 150°, even when the samples are treated for 20 h. When the treatment temperature is at 70°C, the water contact angle falls below 150° for the Si. But the [S.sub.5] still remains in a superhydrophobic state until 80°C. The reason is that the binding force between stearic acid and etched surface mainly depends on the electrostatic force between [Ca.sup.2+] and -CO[O.sup.-] for [S.sub.1] surface. However, for the [S.sub.5] surface, the binding force mainly relies on the chemical bond between the -OH (from Si[O.sub.2] surface) and -COOH (from stearic acid). At the same time, Si[O.sub.2] may act as nuclei, which will facilitate the crystallization of stearic acid occurring at low temperature.

It is noted that FRP surfaces have better antifriction properties after modification by stearic acid, as shown in Fig. 9. Meanwhile, the [S.sub.5] surface has a smaller friction coefficient than that of [S.sub.1]. This is because the stearic acid forms molecular assemblies in which one end of the long chain molecules is attached to the substrate surface by COOH group. The alkyl chains have a significant freedom of swing and will rearrange along the sliding direction, and yield a smaller resistance. The similar

Conclusions

In conclusion, by simply changing the content of CaC[O.sub.3] and Si[O.sub.2], after sandblasting, etching, and modification, the adhesive work between the surface and the water droplet can be adjusted from extremely high to very low. The varied effect of the adhesion was ascribed to the transition from microlens to microbowls and from Wenzel to Cassie-Baxter state. Noticeably, the as-prepared superhydrophobic surface with lotus effect has better heat resistance and antifriction property than rose petal effect. Considering the simple strategy of our method and the controllable adhesion, the method in this work might also be applied to any other fiber and resin matrix.

DOI 10.1007/s11998-015-9692-1

J. Sun, J. Wang ([mail])

Jiangsu Key Laboratory of Advanced Metallic Materials, School of Materials Science and Engineering, Southeast University, Nanjing 211189, People's Republic of China

e-mail: wangjigang@seu.edu.cn

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Table 1: Static contact angles and roll-off angles of different samples

Sample [S.sub.1] [S.sub.2]

Static contact angle (°) 152 ± 1.5 119 ± 2.5
Roll-off angle (°) RA > 180 RA > 180

Sample [S.sub.3] [S.sub.4]

Static contact angle (°) 126 ± 3.2 149 ± 0.8
Roll-off angle (°) >90 25 ± 5

Sample [S.sub.5]

Static contact angle (°) 160 ± 1.6
Roll-off angle (°) 9 ± 2


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Date:Nov 1, 2015
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