Fabrication of superhydrophobic surface with discarded silicone under arc exposure

Abstract

Super-hydrophobic surfaces can be fabricated based on discarded silicone after arc exposure. Hydrophobicity, microstructural development, chemical composition, corrosive liquid resistance, and the microscopic process of contaminant elimination of the surface are presented. High temperature of the arc discharge plays a substantial role in the preservation of surface hydrophobicity.

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Super-hydrophobic surfaces with extreme wettability have been the subject of academic study for many decades.^1,2 Presently, super-hydrophobic surfaces are considered as excellent measures to decrease the adhesion of water droplets and the accumulation of contaminants on surfaces by the lotus effect.^3–5 Various approaches have been developed to prepare super-hydrophobic surfaces such as the sol–gel process,⁶ micro-phase separation,⁷ templating,⁸ and nanoparticle assembly.^9,10 Despite the success in the preparation of various super-hydrophobic surfaces with high static contact angle and low contact angle hysteresis, silicone materials have always been the primary choice in the preparation of super-hydrophobic surfaces for use in particular applications.^11,12

Silicone with super-hydrophobic surfaces not only ensures good dielectric properties, and hydrophobicity recovery capability,¹³ but also enhances the anti-wetting and self-cleaning performance of silicone used in harsh environments.¹⁴ Zhang et al. reported the preparation of super-hydrophobic room temperature vulcanizing (RTV) silicone coatings via RTV/fluoric nanoparticle composites.⁵ The super-hydrophobic silicone exhibited excellent contaminant and tracking resistance. Hopmann et al. prepared super-hydrophobic silicone surfaces by mimicking the lotus surface using micro-structured injection molds with varying diameters and heights.¹⁵ A water contact angle of 150° was achieved regardless of the parameters of the molding processes. Varner et al. prepared super-hydrophobic coatings based on liquid silicone rubber (LSR) modified with fluorohydrocarbons and mineral nano-fillers.¹⁶ The endurance of the super-hydrophobic surface was evaluated to be greater than 2000 h during energized salt fog tests.

Owing to its good thermal resistance, mechanical strength and gas barrier properties,¹⁷ silicon oxide has been chosen as reinforcing fillers for silicone,¹⁸ fillers in protective coatings¹⁹ and so on.²⁰ Mostly, silicon oxide is prepared by surface oxygen plasma, CVD, laser irradiation and UV-ozone methods.²¹ However, reports on fabricating superhydrophobic surface by directly oxidizing silicone materials into hydrophobic silicone oxide utilizing arc discharge has been seldom reported. We have prepared the transparent superhydrophobic coating by arc-induced deposition.²² Herein, we fabricated the superhydrophobic surface on the silicone surface, which will make full use of discarded silicone.

This paper presents the fabrication of super-hydrophobic surface by arc treatment of discarded silicone. We characterized the hydrophobicity, chemical composition, microstructure, and surface roughness of the silicone surface after arc discharge exposure. Consideration of the capillary pressure verifies that nanostructures formed on the modified surface play a significant role in its marked anti-wettability. The super-hydrophobicity of the modified material and its resistance to corrosive liquids and the accumulation of contaminants are also demonstrated. The mechanism of super-hydrophobic surface formation under arc discharges is proposed. The results demonstrate the efficacy of fabricating super-hydrophobic surface by arc discharges treatment.

The silicone was prepared by spraying a mixture of hydroxyl terminated poly(dimethylsiloxane) elastomer (molecular weight: 16 000) and dibutyl phthalate at a weight ratio of 10 : 1 on a glass plate. The size of the glass plate is 26 mm in width, 76 mm in length and 1 mm in depth. A 150 μm thick silicone film formed after thermal curing at 110 °C for 30 min. The silicone surface was treated by home-made arc discharge generator for 5 s. When the arc discharge happens between the two wire electrodes, the glass slide coated with silicone was about 0.5 cm below the arc. The surface of silicone was turned into super-hydrophobic coating around 5 s. The arc discharge is generated by high voltage imposed between two electrodes with a spacing of 1 cm. Five samples of the silicone coating were locally exposed to two electrodes with a current of 160 ± 47 mA at an applied voltage of 10.1 ± 1 kV. When the arc forms, the current and voltage changes to 4.1 ± 0.3 A and 20 ± 5 V.

Fig. 1(a) shows the ATR-FTIR spectra of the silicone and the coating after arc discharge. Two main bands at 1442 cm⁻¹ and 875 cm⁻¹ can be detected for the silicone surface, whereas they are rather weak for the outside layer and inside bulk of the modified coating. The first absorption band is associated with the C–H bond in the methyl groups. The second band is attributed to Si–CH₃ bonds, which are the side chains of the silicone molecules. The other five peaks observed deceased strongly after exposure to arc discharge. The peaks at 2960 cm⁻¹ and 1257 cm^−l are attributed to the C–H bond in the methyl groups and the Si–CH₃ bonds, respectively. The peaks at 1100 cm⁻¹ and 810 cm⁻¹ are representative of 3D Si–O bonds. The peak at 1011 cm⁻¹ is assigned to the absorption of two-dimensional (2D) Si–O–Si.²³ The decrease in the proportion of 2D Si–O–Si bonds indicates that the silicone molecules are reduced to small silicone chains. Although the absorption intensity of C–H bonds at 1442 cm⁻¹ and Si–CH₃ bonds at 875 cm⁻¹ decreased significantly, the C–H bonds and the Si–CH₃ bonds continue to exhibit some small absorbance. Compared with chemical composition on outside layer of the super-hydrophobic coating, the inside bulk show stronger absorption in 1257 cm⁻¹ due to being oxegen-deficient.²⁴ The results confirm that the silicone surface after arc discharge exposure contained some proportion of Si–CH₃ groups, which are hydrophobic in nature. The silicone molecules in the bulk were decomposed into LMW silicone chains.

Fig. 1 (a) FTIR spectra of silicone, the outside layer and inside bulk after arc exposure; (b) EDX spectra of the outside layer and inside bulk; (c) SEM images of silicone surface before and (d) after arc exposure at low and high magnifications; water contact angle of 153.4° of the surface after arc exposure.

Fig. 1(b) shows EDX spectra of the raw silicone before arc discharge, outside layer and inside bulk of the coating after arc discharge. The carbon, silicon and oxygen elements can be observed on the spectra of the samples. The weight ratios (WR) and atomic ratios (AR) of silicon, oxygen, carbon of the samples are shown in the forms inserted in Fig. 1(b). The atomic ratio of silicon to oxygen of both the outside layer and inside bulk were estimated to be the same, which is nearly 1 : 2. Results demonstrate that the coating generated by arc exposure of silicone is made of silica. The silicone shows greater C/O atomic ratio than that of the coating. The outside layer of the coating shows less carbon contents than that of inside bulk of the coating.

Fig. 1(c) and (d) show SEM images of silicone surfaces before and after exposure to arc discharge. The surface of raw silicone under is flat under low and high magnification. While the surface after arc exposure consists of irregular three-dimensional (3D) microbump with diameters between 20.5 μm and 25 μm. Nanoparticles with diameters in a range of 40 nm and 110 nm are observed on the microbump and the hierarchical structures are observed. The SEM image of the modified silicone surface indicates that the silicone exposed to arc discharge possesses a rough surface.

The water static contact angles of the silicone surface after arc discharge exposure were measured five times at different places on the surface. The insert of Fig. 1(d) presents one of the representative results. The static water contact angle of the arc discharge-treated samples varied from 147.4° to 157.1° with an average value of 153.4°.

A 3D AFM image of the super-hydrophobic surface is shown in Fig. 2(a). The arithmetic mean surface roughness (R_a) and the root mean square surface roughness (R_rms) of the surface were calculated from this topographical image to be 30.1 nm and 43.1 nm, respectively. Fig. 2(b) shows AFM cross-sectional images of the super-hydrophobic surface, which exhibit needle like structures. The surface roughness results are consistent with the SEM observations. Both the hierarchical structures confirmed by SEM and AFM observations and the hydrophobic Si–CH₃ groups on the surface contribute to the super-hydrophobic surface.

Fig. 2 (a) 3D AFM topography images and (b) surface roughness profile of the super-hydrophobic surface.

Fig. 3 illustrates the water repellence of the surface after arc exposure. A 65 μL water droplet was impacted on the superhydrophobic surface at an equivalent speed of 0.73 m s⁻¹. The water droplet rebounded from the super-hydrophobic surface, and the surface remained completely dry.

Fig. 3 High-speed video sequences of a 65 μL water droplet impacting on the superhydrophobic surface at a speed of 0.73 m s⁻¹.

The wetting pressure and anti-wetting pressure of the super-hydrophobic surface were calculated to explain the water-droplet rebounding process. The capillary pressure P_c is influenced by the surface tension σ of the water, the advancing water contact angle θ_a, and the space between the micro/nano-scale structures on the surface r, and is given as follows.²⁵

P_c = −2σ cos(θ_a)/r (1)

The value of σ for the water is about 72 mN m⁻¹, the average value for θ_a is about 115°, and the value of r between the central points of micro-scale structures is between 17.5 μm and 33.78 μm, with an average value of 25.6 μm. The value of r between the central points of nano-scale structures is between 50 nm and 150 nm, with an average value of 100 nm. The value of P_c induced by the micro-scale structure is therefore about 2.3 kPa, while the value of P_c induced by the nano-scale structure is about 608.6 kPa. The water hammer pressure P_w is determined by the density of water ρ at room temperature, the speed of sound in water u, and the speed of the impacting water droplet v on the surface, as follows:

P_w = kρuv (2)

where the k is a coefficient.

The value of ρ is about 1000 kg m⁻³, the value of u is about 1500 m s⁻¹, v was given as 0.73 m s⁻¹, and k is estimated to be 0.04.²⁶ The value of P_w is calculated to be 43.8 kPa. It can therefore be determined that the value of P_c induced by the nano-scale structure is greater than P_w, while the value of P_c induced by the micro-scale structure is much smaller. It is thus indicated that nano-scale structures substantially increase P_c, and result in the water droplet rebounding from the super-hydrophobic surface.

To characterize the microscopic process underlying the self-cleaning of contaminants on the super-hydrophobic surface, we impacted the surface deposited with contaminants with 10 μL water droplets. Fig. 4 illustrates the self-cleaning of sodium chloride (NaCl) contaminants on the super-hydrophobic surface. NaCl particles with diameters in the range of 0.5–1 mm were dropped randomly on the surface, as shown in the dotted circle of Fig. 4(a). A proportion of the deposited NaCl particles coming in contact with the water droplets were dissolved, and, as the water droplet rebounded away from the surface, the dissolved NaCl particles were removed, as shown in Fig. 4(b) and (c). Within a relatively short period, the super-hydrophobic surface becomes partially clean, and remains completely dry.

Fig. 4 Self-cleaning of sodium chloride contaminants with diameters of 0.5–1 mm on superhydrophobic surface by a 10 μL water droplet. The arrows show the movement direction of the water droplet. The dotted circle shows locations of NaCl particles.

Water droplets with pH values in a range of 1 and 14 were used to attack the surface. Fig. 5 shows the contact angles of the corrosive aqueous solutions on the super-hydrophobic surface. As shown in the figure, the average static contact angles of the corrosive liquids on the super-hydrophobic surface remain above 150° after the acid and base attack for all pH values. Thus, the arc discharge-induced super-hydrophobic surface is demonstrated to be resistant to chemical corrosion.

Fig. 5 Contact angles of corrosive liquids with pH values in a range of 1 and 14 on the superhydrophobic surface.

A possible mechanism for the formation of a super-hydrophobic surface by exposing the silicone surface to arc discharge can be explained as follows. High temperature is one of the characteristics of arc, which relies on the thermionic emission of electrons from the electrodes.²⁷ The organic polymer subjected to the discharge will be molten owing to its low fusion point. The polymer surface also becomes rougher at micro- and nano-scales owing to electron bombardment. Most of the methyl groups on the silicone surface are destroyed by the arc discharge, and a proportion of the silicone molecules within the bulk are reduced to LMW silicone chains. LMW silicone chains are capable of migrating from the bulk onto the surface, and restore the hydrophobicity of silicone.²⁸ Moreover, the high temperature can increase the rate of hydrophobicity recovery for silicone.²⁹ The high temperature of the arc substantially increases both the proportion of LMW silicone chains within the bulk and their migration rate onto the surface. Thus, the surface hydrophobicity of silicone can be enhanced under arc discharge.³⁰

Conclusions

The arc discharge can produce superhydrophobic material on silicone surface, and markedly affect the surface wettability. The microscopic process governing the elimination of soluble and insoluble contaminants, and the high contact angle that is maintained for corrosive liquids on the super-hydrophobic surface demonstrate its good self-cleaning capabilities and chemical resistance, respectively. The hierarchical structures induced by the high temperature from arc discharges, and the hydrophobic methyl groups preserved on the rough surface impart the marked anti-wettability to the silicone surface. The high temperature of the discharge substantially increases the proportion of LMW silicone chains within the bulk as well as their rate of migration onto the surface. Results provide a new approach to recycle discarded silicone to fabricate superhydrophobic materials. It is expected that the superhydrophobic materials produced by arc can be expanded to more fields for practical use.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 51321063) and the Project 111 of the Ministry of Education, China (No. B08036).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra21601j

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