荷叶效应

cien n Ji ysic nce 6; a line In the present article, stable biomimetic superhydrophobic surfaces on aluminum alloy are obtained by wet chemical etching following modi- fication with crosslinked silicone elastomer, perfluorononane (C9F20), and perfluoropolyether (PFPE), respectively. The formation and structure of superhydrophobic surfaces were characterized by means of scanning electron microscopy (SEM), water contact angle measurement, Fourier transform infrared spectroscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The effects of surface roughness resulted from the etch- ing time, and the concentration of NaOH aqueous solution on the superhydrophobicity of the surfaces have been discussed in detail. The optimal surface roughness of starting material is about 0.05–0.5 µm and the resulting surface roughness should be controlled between 2.7 and 5.8 µm in order to realize the superhydrophobicity on aluminum alloy; if the concentration of NaOH aqueous solution is about 4 wt%, the best treatment time is between 2 and 4 h to form a surface roughness changing from 2.7 to 5.8 µm. The trapped air with the binary structure plays a key role in fabricating superhydrophobic surface on aluminum alloy. In other words, the unusual structure on the surface, which has a binary structure consisted of microprotrusions and nanoparticles, plays a very vital role in constructing of the stable biomimetic superhydrophobic surface on aluminum alloy. © 2006 Elsevier Inc. All rights reserved. Keywords: Aluminum alloy; Lotus effect; Superhydrophobicity; Surface roughness; System parameters; Contact angle 1. Introduction Although over the past decade considerable progress has been made in the development of artificial superhydropho- bic surfaces based on lotus leaf, their ideal formation mech- anism has remained elusive both experimentally and theoret- ically [1–16]. The lotus leaf is held as a symbol of purity in many countries, especially in part regions of Asia, due to its ability to remain clean when emerging from murky ponds. Non- wetting water droplets can take away undesirable particulates. Botanists first claimed that the natural cleaning mechanism originated from their microscopic structure and surface chem- istry [17–20]. This is known as Lotus Effect [7,11,21]. The lotus effect (water contact angle larger than 150◦ and sliding angle less than 10◦), is now sure to be a result of a binary structure (micro- and nanoscale), as well as the wax layer presents on * Corresponding author. Fax: +86 931 8277088. E-mail address: wmliu@lzb.ac.cn (W. Liu). the leaf surface [8,13–16,22]. Some nanotechnologists are de- veloping methods to make paints, roof tiles, fabrics and other surfaces that can stay dry and clean themselves similar to lotus leaf [6–9,11,13–16]. Several researchers have successfully fabricated superhy- drophobicity with some specific metallic substrates recently [23–28], such as Au, Cu, and Al. Zhang et al. [24] fabricated su- perhydrophobic coatings on gold threads by combining Layer- by-Layer (LbL) technique and electrochemical deposition of dendritic gold aggregates. In another literature [25], they fabri- cated a pH switchable surface, by combining a fractal-like gold surface and a mixed thiol self-assembled monolayer, show- ing superhydrophobicity for acid and superhydrophilicity for base, respectively. Both of them take advantage of the assem- bly chemistry on Au. But the restriction of the expensive metal in industrial applications accelerates easier ways to make su- perhydrophobic engineering materials. Ren et al. [27] prepared a superhydrophobic surface with evaporated aluminum deco- rated with stearic acid monolayer. Qian and Shen [28] reported the fabrication of superhydrophobic surfaces on three polycrys- Journal of Colloid and Interface S Effects of system parameters o Zhiguang Guo a,b, Feng Zhou a, a State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Ph b Graduate School of Chinese Academy of Scie Received 5 March 200 Available on Abstract 0021-9797/$ – see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2006.06.067 ce 303 (2006) 298–305 www.elsevier.com/locate/jcis making aluminum alloy lotus ngcheng Hao a, Weimin Liu a,∗ s, Chinese Academy of Sciences, Lanzhou 730000, People’s Republic of China s, Beijing 100039, People’s Republic of China ccepted 24 June 2006 8 July 2006 In our previous paper [29] an aluminum and its alloy sur- faces are made superhydrophobic by immersing them in sodium hydroxide for several hours followed by spin coating a layer of perfluorononane or poly(dimethysiloxane) vinyl terminated (PDMSVT). This treatment increases the water contact angle from about 67◦ to above 160◦ and this effect can be explained by Cassie’s law [30]. Electron microscopy shows that the alu- minum or its alloy surface resembles that of a lotus surface with a porous microstructure that can trap a large amount of air. The present paper deals with the effect of system parameters on the superhydrophobicity of aluminum alloy (2024Al). We will particularly focus on the preparation conditions and the correla- tion of the surface structures with superhydrophobicity, which might serve as a direction for its actual applications and provide a detailed data for the research of superhydrophobic formation mechanism further. 2. Experimental 2.1. Materials and sample preparation Aluminum alloy (2024Al) (aluminum is about 92.8 wt%, copper is about 5.5 wt%, and the remaining about 1.7 wt% Mg, Fe, and Mn, Φ: 24 × 8 mm2) was used as substrate. Poly(dimethysiloxane) vinyl terminated (PDMSVT) (struc- tural formula is shown in Scheme 1), and perfluorononane (C9F20) were purchased from Sigma–Aldrich, USA. 184 curing agent, one part of commercial precursor of silicone elastomer, was obtained from Dow Corning Company. Perfluoropolyether (PFPE) (structural formula is shown in Scheme 1) was obtained from Great Wall Lubricating Oil Company, Sinopec of China. Deionized water was used to clean the aluminum alloy sub- strate and for contact angle measurements. The aluminum alloy substrates were polished mechanically with different type dia- mond pastes to obtain different surface roughness with Ra of changing from 0.05 to 0.5 to 1.5 µm and were then sonicated with acetone for 15 min to remove polishing debris and oleo- materials. Finally, the polished samples with different surface roughness were then dried in a stream of dry nitrogen gas and preserved in a desiccator to store for use in next. The employed methods, here, to fabricate the superhydrophobic surfaces on the aluminum alloy with different surface roughness are as fol- lows: blocks of the aluminum alloy (Ra = 0.05, 0.5, and 1.5 µm, respectively) were immersed into NaOH aqueous solution with concentration of 1, 2, and 4 wt% for about 2, 4, and 6 h, re- spectively, in order to discuss the effects of surface roughness, Scheme 1. Chemical struc treated surfaces were modified with PDMSVT and 1 wt% 184 curing agent as the crosslinker, C9F20 and PFPE, respectively. The modified films were obtained by spin-coating at a speed of 3000 rpm for 30 s, then cured in a heater at about 120 ◦C for PDMSVT, PFPE films, and at about 80 ◦C for C9F20 film for 2 h, respectively. 2.2. Surface characterization The materials before and after use were examined by FT- IR spectrometry (on a bio-Rad FTS-165 IR spectrometer). By using transmission mode, the spectrum was collected for 500 scans at a resolution of 4 cm−1. To eliminate or decrease the interfering of H2O and CO2. The sample chamber and op- tical chamber were vacuumed to 3 mbar. The freshly cleaned KBr wafer was used as the reference. A PHI-multifunctional X-ray photoelectron spectroscope (Physical Eletronics, USA) operating with MgKα irradiation (hν = 1253.6 eV) was per- formed to analyze the chemical states of some typical elements distributed on the superhydrophobic surfaces, with the bind- ing energy of contaminated carbon (C1s: 284.6 eV) as the ref- erence at the photoelectron take off angle of 20◦. The XPS spectra were collected in a constant analyzer energy mode, at a chamber pressure of 10−8 Pa and pass energy of 29.4 eV at 0.125 eV/step. The resolution for the measurement of the binding energy is about ±0.3 eV. The aluminum alloy and the final surface were determined on an X’pert-MRD X-ray diffractometer (Philip Corp., The Netherlands) operating with CuKα radiation at a continuous scanning mode and omega an- gle of 1.0◦. The morphologies of the aluminum alloy before and after treatment were observed on a JSM-5600LV scanning electron microscope (SEM) at 20 kV, and the corresponding element distributions on the surface were determined by an en- ergy dispersive X-ray spectroscopy (EDX). The sessile drop method was used for water contact angle measurements with a CA–A contact angle meter (Kyowa Scientific Company, Ltd., Japan) at ambient temperature. Water droplets (about 8 mg) were dropped carefully onto the surfaces. The average CA value was determined by image analysis software, which is base on a numerical integration of the Laplace equation of capillar- ity and a fitting strategy to experimental drop profiles at five different positions of the same sample. The resolution of this technique is typically ±0.8◦ and their images were captured with a traditional digital camera (Sony). The measurements of Ra mean surface roughness of aluminum alloy before and after treatment were obtained by 2206 surface roughness apparatus Z. Guo et al. / Journal of Colloid and Interface Science 303 (2006) 298–305 299 talline metals, namely aluminum, copper, and zinc with a sim- ple chemical etching. These results undoubtedly all open up a new research field on superhydrophobic engineering materials with imaginably potential important applications. treatment time and the concentration of NaOH aqueous solu- tion on the final superhydrophobicity of the substrates, rinsed with deionized water again to remove the remaining base on the treated surface and then annealed at 120 ◦C for about 1 h. The ture of the modifiers. Int 300 Z. Guo et al. / Journal of Colloid and at three different positions of the same sample with a resolution of 5.0%. 3. Results and discussion 3.1. Effect of surface roughness It is well known that chemical modification of artificial ma- terials using fluoropolymeric coatings or organisilane layers on a flat hydrophobic surface cannot give water contact an- gles higher than 120◦ [31–33]. For example, the contact an- gles are both about 112◦ and 115◦ for long-chain hydrocarbon and fluorocarbon self-assembled monolayer modified surfaces, respectively [32]. To reach the extreme values of the contact angle more than 150◦, surface roughness is often adjusted to amplify the surface hydrophobicity [11–16,21–25]. An appro- priate surface roughness or structure is an important factor in fabricating the biomimetic superhydrophobic surfaces. In or- der to investigate roughness effect on superhydrophobicity, we have prepared three kinds of aluminum alloy substrates with different surface roughness changing from 1.5 to 0.5 to 0.05 µm, respectively, and their corresponding SEM images were shown in Fig. 1. It is clearly seen that many grooves with diameter of changing from 5 to 30 µm distributed desultorily on the surface, and the water contact angle is about 52◦ (inset of Fig. 1a), indi- cating hydrophilic properties. Similar to Fig. 1a, Fig. 1b shows the SEM image of the substrate with roughness of about 0.5 µm, exhibiting that many grooves with diameter of round 20 µm are parallel and uniformly distributed on it, and the water contact angle is about 92◦ (inset of Fig. 1b), exhibiting hydrophobic properties. When the surface roughness is about 0.05 µm, the surface is exceedingly smooth, which is shown in Fig. 1c, and the water contact angle is about 36◦ (inset of Fig. 1c), show- ing the optimal hydrophilic properties among the three. These results suggest further that the surface roughness can strongly affect surface wettability, thus roughness is a very important factor in fabricating a superhydrophobic surface. About the rea- son that the surface with roughness of 0.5 µm shows hydropho- bic compared to the other two kinds of surfaces, it is still an elusive case for us. It is noting that the sliding angles of water droplet on the three substrates are all larger than 50◦, showing higher adhesion and hysteresis. Fig. 2 shows the relationships between water contact an- gles and the surface roughness of the substrates modified with PDMSVT and C9F20, respectively. It is obviously seen that, compared to the mentioned water contact angles in Fig. 1, the water contact angles all increased whatever the substrates were modified with PDMSVT or C9F20, indicating that the surface energy will further decrease after chemical modification with PDMSVT or C9F20. We can also find clearly that the roughness value of 0.5 µm is the optimal benefit to amplify the hydropho- bicity of the substrate before and after modification compared to the two other kinds of surface roughness values. The SEM images of the aluminum alloy with different sur- face roughness, treated with 4 wt% NaOH aqueous solution for 2 h, were shown in Fig. 3. It is clearly seen that many pro- trusions were uniformly distributed on the substrate with the erface Science 303 (2006) 298–305 Fig. 1. SEM images of aluminum alloy (2024Al) with different surface rough- ness Ra and different water contact angles. (a) 1.5 µm, 52◦; (b) 0.5 µm, 92◦; (c) 0.05 µm, 36◦ . The weight of water droplets are all about 8 mg. Fig. 2. Relationships between water contact angles and the surface roughness of the substrates modified with PDMSVT and C9F20, respectively. The water con- tact angles are 75◦, 102◦ , and 61◦, corresponding to the surface roughness of 1.5 µm, 0.5 µm, and 0.05 µm, respectively, modified with PDMSVT; the water contact angles are 98◦ , 109◦ , and 96◦ , corresponding to the surface roughness of 1.5 µm, 0.5 µm, and 0.05 µm, respectively, modified with C9F20. In Z. Guo et al. / Journal of Colloid and Fig. 3. SEM images of the aluminum alloy (2024Al) with different surface roughness treated with 4 wt% NaOH aqueous solution for 2 h. The surface roughness is changed from (a) 1.5 to 2.5 µm; (b) 0.5 to 2.7 µm; (c) 0.05 to 3.0 µm after immersion, respectively. surface roughness of 1.5, 0.5, and 0.05 µm, respectively, af- ter immersion, and their corresponding surface roughness were changed to 2.5 to 2.7 to 3.0 µm, respectively (Figs. 3a–3c). Interestingly, their corresponding average diameter decreased with the decrease of the origin surface roughness, but the re- sulting surface roughness was just converse. To explain the reason above, we should first introduce a concept about dislo- cation defects [28], which was established appropriately a half century ago. As is well known, there are large numbers dis- location defects, formed in the processing and polishing peri- ods, respectively, in the aluminum alloy with different surface roughness. These dislocation sites, due to possessing relatively higher energy, are prone to destroy, and thus when being im- mersed into NaOH aqueous solution, they would be readily dissolved first [28]. Since many dislocation defects formed in the substrate in processing procedure or when polishing them in order to realize a certain origin surface roughness, more- over the dislocation defect increased with the decrease of the terface Science 303 (2006) 298–305 301 Fig. 4. The relationships between water contact angles and the surface rough- ness of the substrate treated with 4 wt% NaOH aqueous solution for 2 h fol- lowing modification with PDMSVT or C9F20, respectively. The water contact angles are 145◦ , 151◦ , and 161◦ , corresponding to the surface roughness of 2.5 µm, 2.7 µm, and 3.0 µm, respectively modified with PDMSVT; the water contact angles are 151◦ , 153◦ , and 157◦ , corresponding to the surface rough- ness of 2.5 µm, 2.7 µm, and 3.0 µm, respectively modified with C9F20. origin surface roughness, among of them, the surface with the origin surface roughness of about 0.05 µm possessed the high- est energy, and the chemical etching of between NaOH aqueous solution and the substrate was the fastest, forming the surface topography shown in Fig. 3c, and such surface, consisted of binary structure [29], would trap a large of air into its rough surface and decrease surface energy further. Note that the ex- isted oxidation layer on the origin surface also affects the final superhydrophobicity [27,29], and the results in detail will be discussed in elsewhere. Fig. 4 shows the relationships between water contact an- gle and the surface roughness of the substrate treated with 4 wt% NaOH aqueous solution for 2 h following modifica- tion with PDMSVT or C9F20, respectively. The aluminum alloy with origin surface roughness of 1.5 µm, 0.5 µm, and 0.05 µm treated with 4 wt% NaOH aqueous solution and modified with PDMSVT shows a water contact angle of about 145◦, 151◦, and 161◦, respectively, much larger than the measured values on non-etched surfaces having the same other treatment condi- tions. This indicates that the formation of the many protrusions on the surface formed during chemical etching enhances the surface superhydrophobicity. The similar results also come out when being modified with C9F20 with both high contact angles of above 150◦ (shown in inset of Fig. 4) and low sliding angles of about 3◦ for surfaces with the original roughness of 1.5 µm, 0.5 µm, and 0.05 µm. As mentioned above clearly, the surface roughness plays a key role in the fabrication of superhydrophobic surfaces [21–25]. On rough surfaces, the real contact area of water droplets on the surface is limited due to the existence of the dis- persed protrusions of asperities. Therefore, the three-phase con- tact line becomes discontinuous and highly distorted [13,28], which directly leads to superhydrophobic surfaces with low sliding angle. In our system, it is necessary that the resulting surface roughness is larger than 2.5 µm according to the exper- Int 302 Z. Guo et al. / Journal of Colloid and Fig. 5. Relationship between etching time with 4 wt% NaOH aqueous solution and the final surface roughness. iments above. The optimal origin surface roughness is 0.05 µm in fabricating the superhydrophobic surface among the three tested surface roughness in our experiments. Accordingly, we will fix this kind of substrate as the material for further study. 3.2. Effect of treatment time with NaOH To explore the effect of etching time on the wettability, the same aluminum alloy was treated for 2 to 4 to 6 to 8 h with 4 wt% NaOH aqueous solution. Fig. 5 shows the relationship between the resulting surface roughness and the treatment time with 4 wt% NaOH aqueous solution. It is clearly seen that the resulting surface roughness increased with increasing treatment time, and after 6 h, the surface roughness reached a higher value of about 7.2 µm, due to the formation of the deeper and wider protrusions on the surface. Further increasing the etching time no longer can help to enhance the surface roughness and many black fragments were generated, indicating the surface was seriously damaged. Therefore, the treatment time should not exceed 6 h. Fig. 6 shows the relationship between the etching time with 4 wt% NaOH aqueous solution and the water contact angles after modifying with PDMSVT. It is clearly seen that the wa- ter contact angles decreased with the prolonging of immersion time, in other words, the final surface roughness, changing from superhydrophobic state to hydrophobic state. This indicates that the etching time determined the surface roughness, and so the final superhydrophobicity. We should make clear that the sam- ple with 8 h etching was deadly damaged, though contact angle could be measured. As a conclusion