Transparent ultra-hydrophobic surfaces

Transcription

1 J. Adhesion Sci. Technol., Vol. 21, No. 5 6, pp (2007) VSP Also available online - Transparent ultra-hydrophobic surfaces P. F. RIOS 1,2, H. DODIUK 1,S.KENIG 1,,S.McCARTHY 2 and A. DOTAN 1 1 Department of Plastics Engineering, Shenkar College of Engineering and Design, 12 Anna Frank St, Ramat-Gan 52526, Israel 2 Department of Plastics Engineering, University of Massachusetts at Lowell, 883 Broadway Street, Lowell, MA , USA Received in final form 29 January 2007 Abstract Self-cleaning surfaces have received a great deal of attention, both in research studies and commercial applications. Both transparent and non-transparent self-cleaning surfaces are highly desired, as they offer many advantages and their potential applications are endless. As in many other cases, also in the case of self-cleaning surfaces, nature found a solution before man. The Lotus flower is a symbol of purity in Asian cultures, even when rising from muddy waters it stays clean and untouched by dirt, organisms and pollutants. The Lotus leaf self-cleaning surface is hydrophobic and rough, showing a two-layer morphology. While hydrophobicity produces a high contact angle, the two-layer morphology reduces the adhesion of dirt and water drops to the surface. Because of this low adhesion, water drops easily slide across the leaf surface carrying the dirt particles with them. In the present work the Lotus leaf morphology was mimicked using hydrophobic chemistry and a twolayer topography, with a base layer of silica and a top layer of intrinsically nanostructured polyhedral oligomeric silsesquioxanes (POSS) particles. Results have indicated that, thus, a transparent ultrahydrophobic coating can be obtained. When these materials were mixed and used as a single layer the hydrophobicity deceased significantly. The contact angle and sliding angle measurements were supported by AFM micrographs. Keywords: Nanoroughness; Lotus effect; contact angle; sliding angle; hydrophobic surfaces; interfacial adhesion strength; coatings; POSS. 1. INTRODUCTION The thermodynamics between a liquid and a solid were first described by Young in 1805 [1]. The so-called Young s equation relates the surface tensions of the liquid, the solid and the gas surrounding them to the contact angle formed between the liquid and the solid substrate. The contact angle is related to the generally used term wetting. Adamson [2] has defined wetting as a phenomenon where To whom correspondence should be addressed: Tel.: (972-3) ; Fax: (972-3) ; samkenig@shenkar.ac.il

2 400 P. F. Rios et al. the contact angle between the liquid drop and the solid surface approaches zero; while non-wetting means that the contact angle is greater than 90. Generally, when the water contact angle is less than 90 the surface is called hydrophilic; when the contact angle is greater than 90 the surface is hydrophobic. A surface having a water contact angle greater than 150 is usually classified as ultra-hydrophobic, i.e., a water-repellent surface (Fig. 1). Among the known high-contact angle substances are alkane, silicone and fluorinebased materials. Reported values for water contact angles on polypropylene (PP), poly(dimethyl siloxane) (PDMS) and poly(tetrafluoroethylene) (PTFE) are 96.4, 95.8 and 104, respectively [3]. Zisman and co-workers [4, 5] have shown that fluorinated surfaces render the lowest polymer surface energies (thus the higher contact angles). Nishino and co-workers [6] demonstrated that the lowest surface energy of any flat solid surface is obtained by regularly aligned and close-packed CF 3 groups. For this surface (and therefore the highest contact angle a smooth surface can reach) the contact angle is 120. A higher contact angle, thus an ultrahydrophobic surface, can be obtained only if the hydrophobic surface is roughened. In the case of a transparent substrate, roughening its surface may cause a reduction in transparency. Thus ultra-hydrophobicity and transparency are competitive properties. However, transparency can be preserved provided that surface roughness is fine enough so it does not disturb the passage of light. This can be achieved when the roughness size is smaller than the visible light wavelength ( nm). The effect of roughness on the contact angle was first considered by Wenzel [7, 8]. He recognized the importance of surface roughness and proposed a modification to the Young s equation, which included a roughness factor defined as the ratio between the actual rough surface area and the geometric projected area. According to Wenzel s equation, a solid substrate with wetting tendency (θ < 90 ) will wet more easily if its surface is rough, but, on the other hand, a solid substrate with water repelling tendency (θ >90 ) will repel more when having a rough surface. Young and Wenzel considered chemically homogeneous surfaces. Cassie and Baxter [9, 10] extended Wenzel s work to non-homogeneous and to porous surfaces. Cassie and Baxter equations can also be used for rough hydrophobic surfaces. On a hydrophobic rough surface, the liquid repellency prevents the liquid from fully Figure 1. Schematic representations of hydrophilic, hydrophobic and ultra-hydrophobic surfaces.

3 Transparent ultra-hydrophobic surfaces 401 penetrating into the depressions of the roughness morphology. Penetration of pores will occur spontaneously only for θ < 90 [11]. According to Youngblood and McCarthy [12] for a hydrophobic rough surface a pressure greater than 3 m of water is needed to force the liquid into pores of micrometer in size. The Cassie and Baxter contact area fraction f C between the liquid and the rough hydrophobic surface is given in equation (1) where θ is the contact angle for the smooth solid surface and θ is the contact angle for the same roughened surface [13]: f = f C = cos θ + 1 cos θ + 1. (1) While Cassie s equation applies to a surface composed of well-separated and distinct domains, further work by Israelachvili and Gee s [14] dealt with chemical heterogeneities of atomic or molecular scale. Based on Israelachvili and Gee s work a novel relationship was proposed by Rios and co-workers [15] as defined in equation (2), where f R is the contact fraction between the liquid and the nanorough hydrophobic surface, θ is the contact angle for the nano-roughened surface and θ is the contact angle for the original smooth solid surface: f = f R = ( cos θ ) (2) cos θ + 1 From equations (1) and (2) it can be seen that a high degree of roughness is needed to achieve ultra-hydrophobicity. For instance, for a starting hydrophobic smooth surface with θ = 120, to increase the contact angle to 150, f C will be 0.27 while f R will be 0.07, i.e., the liquid drop makes contact with only 7% of the solid surface. From a practical point of view, the contact angle is not the only significant parameter for defining hydrophobicity. For self-cleaning surfaces, a low level of water drop adhesion to the surface is important. The adhesion of a water drop to a surface can be characterized by the critical tilting angle of the surface at which a liquid drop, with a certain weight, begins to slide down the tilted plane. Rios and co-workers [15, 16] have shown that a high contact-angle surface does not necessarily show a low sliding angle. They found that poly(tetrafluoroethylene) (PTFE) with a contact angle of 112 possessed much higher sliding angles than silicone with a lower contact angle of 103. Similar results were previously reported by a number of authors [3, 17, 18]. From a self-cleaning point of view, it does not matter how high the contact angle is, provided that the water drop slides off the surface. Consequently, a more comprehensive definition of an ultra-hydrophobic surface should include the effects of both the contact and the sliding angle. Equation (3), proposed by Rios and co-workers [15], relates the sliding angle for a rough surface α, the contact angle as measured on the rough surface θ and the contact area fraction f. K R is the interfacial adhesion parameter and depends on the solid surface chemistry, g is the gravitational acceleration, ρ is the density of

4 402 P. F. Rios et al. the liquid and m is the mass of the liquid drop: sin α = K [ ] Rπf 3 2/3 sin 2 θ m 1/3. (3) g ρπ(2 3cosθ + cos 3 θ ) In the case of hydrophobic surfaces, where the liquid does not penetrate the roughness depressions, small-scale roughness should lead to a reduction in the actual contact area between the drop and the solid surface (like a Fakir bed) and, therefore, to a reduction of the contact area fraction f. From equation (3) sin α is proportional to f ; therefore, the rougher the hydrophobic surface, the smaller f becomes and the lower the sliding angle will be. In can also be shown with equation (3) that for f R (equation (2)) the sliding angle decreases faster than for f C (equation (1)) and, consequently, the lowest sliding angle should be reached when nanoscale roughness is achieved. In addition, it can be shown with equation (3) that the sliding angle depends strongly on the contact angle and falls sharply as the contact angle increases. As in many other cases, also in the case of self-cleaning, non-adhering surfaces, nature found a solution before mankind did. The term Lotus effect is attributed to the botanist Wilhelm Barthlott [19]. The Lotus flower is revered as a symbol of purity in Asian religions, even when rising from muddy waters it stays clean and untouched by dirt, organisms and pollutants. This flower is an example of what nature can do to protect itself from unwanted dirt and contamination. The Lotus leaf self-cleaning surface is hydrophobic and rough. Microscopy reveals that it possesses a two-layer morphology. The lower layer presents a microsized roughness, 5 10 µm high and µm apart. This first layer is covered by a second layer of waxy hydrophobic crystalloids of nanometric diameter. The Lotus reaches contact angles values greater than 150. The mechanism of self-cleaning as demonstrated in nature is characterized by ultra-hydrophobicity, low sliding angle, and removal of dirt particles by the sliding droplet. Thus, according to previous work and nature examples, ultrahydrophobic surfaces with potential self-cleaning properties should be achieved by combining hydrophobic chemistry and proper roughness. Roughness is needed to increase the contact angle and reduce the sliding angle. Moreover, when roughness is in the nanoscale, lower sliding angles and transparent surfaces may be achieved. The objective of the present study was to investigate the process parameters and material composition for obtaining transparent ultra-hydrophobic surface coatings. 2. EXPERIMENTAL Smooth and clear polycarbonate (PC) specimens were coated with one or two layers. In all cases the outer layer contained hydrophobically functionalized polyhedral oligomeric silsesquioxane (POSS) compounds. POSS presents two unique features: it is a hybrid chemical silsesquioxane composition (RSiO 3/2 ), intermediate between

5 Transparent ultra-hydrophobic surfaces 403 silica (SiO 2 ) and silicone (R 2 SiO) and has a cage-like structure of 1.5 nm in size. Thus POSS can be defined as an intrinsically nano-structured organic-inorganic substance [20, 21]. Different types of POSS molecules are available. The basic form is the POSS molecular silica containing a robust SiO core surrounded by non-reactive organic groups which permit the inorganic core to be compatible with an organic matrix. Unlike silica and modified clays, each POSS molecule may contain covalently bonded functionalities. POSS functionalities include alcohols and phenols, amines, halides, acrylates and methacrylates, epoxides, esters, nitriles, olefins, phosphines, thiols and fluoroalkyls [22]. To provide both nanoroughness and hydrophobic surface chemistry fluoro-functionalized POSS (FPOSS) was used. Other authors [23 26] have found that both the length and flexibility of the fluoroalkyl molecule have an effect on the surface energy decrease (and therefore contact angle increase) due to enhanced segregation of the CF 3 chain ends on the surface. Hence, POSS molecules with long pendant fluoroalkyl chains were chosen: trifluoro (3) cyclo-pentyl POSS, C 50 H 93 F 39 O 12 Si 10 (FL0590 Hybridplastics, Hattiesburg, MS, USA, called hereinafter FPOSS1) and fluoro (13) disilanol isobutyl POSS, C 38 H 75 F 13 O 12 Si 8 (FL0569 Hybridplastics, called hereinafter FPOSS2). The two-layer structure was followed with the objective of mimicking the Lotus effect. The inner layer of micrometric roughness was produced using hydrophilic fumed silica (Aerosil 200, Degussa, Frankfurt, Germany) and the outer layer of nanometric roughness was produced using hydrophobic FPOSS. In a second configuration the silica and the FPOSS were mixed together in the same liquid and coated as a single layer onto the PC substrate. Thus, it was possible to compare results of the same composition having one-layer or two-layer configurations. The silica and FPOSS were mixed in various ratios in appropriate solvents according to the material type. Mixing was performed for 2 h using a magnetic stirrer. The PC samples were first cleaned with isopropyl alcohol (IPA), then dipped in the corresponding mixture, dried, cooled and conditioned in controlled laboratory conditions (25 C, 60%RH) for 24 h. In the case of two layers this procedure was adopted for both first and second layers. The static contact angle was measured according to the sessile drop method using a commercial video-based, software-controlled contact angle analyzer (OCA 20, Dataphysics Instruments, Germany). Deionized and ultra-filtered water (0.2 µm filter) was used for the measurements. The sliding angle was measured using a tilting unit (TBU90E, Dataphysics Instruments) incorporated into the contact angle analyzer. A drop was first deposited on the horizontal substrate and after equilibrium the substrate plane was tilted at a rate of 100 degrees/min until the onset of drop motion. The contact and sliding angles were measured using a video-based software (SCA 20, Dataphysics Instruments). 5-µl water drops were used for both contact and sliding angle characterization. To evaluate the coatings transparency, light transmission (LT) and haze were measured using a

6 404 P. F. Rios et al. Hazemeter (BYK Gardner, Germany). The surface morphology was investigated with an atomic force microscope (AFM, Veeco, USA) in the non-contact mode. Surface region analysis was performed and surface roughness was characterized quantitatively in terms of the root mean square roughness R rms and qualitatively by AFM topography images. 3. RESULTS AND DISCUSSION To meet the objective of the current investigation a variety of process parameters and materials were studied. Table 1 shows the optical and surface properties for selected samples. Sample 1 is the uncoated PC reference sample. Sample 2 was produced by coating the PC substrate with a single layer of 3 wt% FPOSS1 in α, α, α-trifluorotoluene (TFT). The PC substrate was dipped for 10 s and dried at room temperature. Samples 3, 4 and 5 were produced by double coating the PC substrate first with a layer of silica in IPA and then with a second layer of FPOSS2 also in IPA. The PC substrate was dipped in the silica mixture for 1 min, dried in a drying oven for 1hat120 C and conditioned for 24 h. The procedure was repeated for the FPOSS2 mixture. Sample 6 was produced by mixing both silica and FPOSS2 together in IPA and coating them as a single layer onto the PC substrate. Component concentrations of each sample are shown in Table 1. Different types of one-layer approaches were attempted. Best results were obtained for Sample 2 (3 wt% FPOSS1 in TFT). This sample showed good transparency, increased contact angle and reduced sliding angle compared to the untreated PC. However, the improvement is limited and the sample does not exhibit ultra-hydrophobic characteristics. These results were presented by Rios and coworkers in previous work [15]. Other samples of this type may show higher Table 1. Light Transmission (LT), Haze, contact angle (θ) and sliding angle (α) for different samples Sample Coating LT Haze θ (5 µl) α (5 µl) (%) (%) (degree) (degree) 1 Uncoated PC wt% FPOSS1 in TFT (one-layer) wt% silica + 3 wt% FPOSS2 in IPA >165 <1 (two-layer) wt% silica wt% FPOSS2 in IPA >165 <1 (two-layer) wt% silica wt% FPOSS2 in IPA >90 (two-layer) wt% silica wt% FPOSS in IPA (mixed one-layer)

7 Transparent ultra-hydrophobic surfaces 405 hydrophobicity but lower transparency. Sample 2 showed the optimal balance of these two properties. Ultra-hydrophobic surfaces with potential self-cleaning applications were obtained when silica and FPOSS2 were applied in a two-layer configuration using IPA as solvent (Samples 3 and 4). It was noticed that silica and FPOSS in high concentrations tended to agglomerate reducing transparency. Consequently, for transparent surfaces the concentrations of both components were reduced to the minimum requirement (Sample 4). Further reduction of concentration decreased the ultrahydrophobic effect. Sample 4 (0.5% silica inner layer and a 1.5% FPOSS outer layer) was found to present the optimal balance of properties. At higher concentrations, ultra-hydrophobicity was obtained at the expense of transparency (Sample 3). In these ultra-hydrophobic samples the contact angle is very high and the sliding angle very low. It is extremely difficult to measure the contact angle, since water drops hardly stand still and slide from the horizontal solid surface at the slightest disturbance. Water drops will slide before the tilting unit reaches 1 degree. The measured contact angle is at least higher than 165. When the particle concentrations were lowered more, the sample lost its ultra-hydrophobicity (Sample 5). Sample 5, while still hydrophobic, shows a significantly lower contact angle and a 5-µl water drop will not slide from its surface even when the surface is completely vertical (α = 90 ). For comparison Table 1 also describes the results for a single-layer treatment having the same optimal concentration as the ultra-hydrophobic Sample 4 (Sample 6). As shown, the mixed one-layer coating is hydrophobic but not ultrahydrophobic and a significantly higher sliding angle is observed (64 ). It was concluded that the two-layer Lotus-like morphology configuration was the optimal one. Figure 2 shows the contact angles of 5 µl water sessile drops on (Fig. 2a) Sample 1 (uncoated PC), (Fig. 2b) Sample 6 (0.5 wt% silica wt% FPOSS2 mixed onelayer) and (Fig. 2c) Sample 4 (0.5 wt% silica wt% FPOSS2 as two layers). Figure 3 shows AFM topographic images of Sample 1 (uncoated PC), Sample 2 (single layer of 3 wt% FPOSS1 in TFT) and Sample 4 (0.5 wt% silica wt% Figure 2. Sessile drops for static contact angle measurements on (a) uncoated PC and PC coated with (b) 0.5 wt% silica wt% FPOSS (mixed) and (c) 0.5 wt% silica wt% FPOSS (two-layer coating).

8 406 P. F. Rios et al. Figure 3. AFM topography images of (a) uncoated PC and PC coated with (b) single layer of 3 wt% FPOSS in TFT and (c) 0.5 wt% silica wt% FPOSS (two-layer coating).

9 Transparent ultra-hydrophobic surfaces 407 FPOSS2 as two layers). The R rms roughness calculated for uncoated PC is 1.5 nm, indicating a high degree of smoothness of the starting PC substrate. After coating the PC surface with a single layer of FPOSS1 the R rms roughness increased to 14.5 nm and for the two-layer coating the R rms roughness increased to 60 nm. The R rms roughness values of these three samples correlate to the measured values of light transmission and haze. Since these values are smaller than the light wavelength (400 nm to 700 nm) the surface roughness does not disturb significantly the passage of the light and the samples remain reasonably clear. Sample 2 is hydrophobic due to the fluorinated functionalities of the POSS and its surface is rough in the nanometric range, however this degree of roughness is not enough and ultra-hydrophobicity is not achieved. Sample 4 has a similar surface chemistry but the higher level of nanometric roughness gave rise to ultra-hydrophobicity. Moreover, because of the Fakir-bed -like morphology of this sample as seen in the AFM image, water should not penetrate significantly the surface valleys and hence the effective contact area will comprise of only the nano-roughness crests, leading to a very low contact area fraction f R, thus the sliding angle approaches zero (equation (3)). 4. CONCLUSIONS It is widely accepted that ultra-hydrophobicity can be obtained by a proper combination of surface chemistry and surface roughness. In this work, transparent polycarbonate substrates were coated with one or two layers. One-layer coatings were produced taking advantage of intrinsically nanostructured POSS particles functionalized with hydrophobic fluorine. These coatings exhibited good optical clarity and improved hydrophobicity (increased contact angle and reduced sliding angle) compared to the uncoated polycarbonate sample; however, ultra-hydrophobicity was not achieved with these one-layer POSS coatings. Ultra-hydrophobic and transparent surfaces with potential self-cleaning applications were achieved with a two-layer configuration comprising a base layer of hydrophilic fumed silica and an upper layer of hydrophobic POSS. When the same materials and in the same concentrations were used to coat PC as a mixed single layer, hydrophobicity was reduced. This study concluded that two-layer morphology similar to that seen in the Lotus flower promotes ultra-hydrophobicity. REFERENCES 1. T. Young, Phil. Trans. Royal Soc. London 95, 65 (1805). 2. A. W. Adamson, Physical Chemistry of Surfaces, 5th edn. Wiley, New York, NY (1990). 3. H. Murase and T. Fujibayashi, Prog. Org. Coatings 31, 97 (1997). 4. E.F.Hare,E.G.ShafrinandW.A.Zisman,J. Phys. Chem. 58, 236 (1954). 5. E. G. Shafrin and W. A. Zisman, J. Phys. Chem. 64, 519 (1960). 6. T. Nishino, M. Meguro, K. Nakamae, M. Matsushita and Y. Ueda, Langmuir 15, 4321 (1999). 7. R. N. Wenzel, Ind. Eng. Chem. 28, 988 (1936).

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