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From Hydrophobic to Superhydrophobic and Superhydrophilic Siloxanes by Thermal Treatment Ioannis Karapanagiotis, Panagiotis Manoudis, Andrea Zurba, and Dimitrios Lampakis Langmuir, Just Accepted Manuscript • Publication Date (Web): 14 Oct 2014 Downloaded from http://pubs.acs.org on October 15, 2014
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From Hydrophobic to Superhydrophobic and Superhydrophilic Siloxanes by Thermal Treatment
Ioannis Karapanagiotis*, Panagiotis N. Manoudis, Andrea Zurba and Dimitrios Lampakis
University Ecclesiastical Academy of Thessaloniki, Department of Management and Conservation of Ecclesiastical Cultural Heritage Objects, Thessaloniki 54250, Greece
Abstract The cross-influence effects of treatment temperature and time on the wettability of a siloxane elastomer is investigated in detail, through static and tilt contact angle measurements. The material is heated at 400, 500, 600, 650, 700 and 800oC for various periods, ranging from 1 to 300 s. The siloxane surface is subjected to multiple wettability transitions with treatment time: from intrinsic hydrophobicity to superhydrophobicity (and water repellency) and then through intermediate stages -hydrophobicity and hydrophilicity- to superhydrophilicity. For the time scale used herein (1 - 300 s) this scenario is recorded for treatment at 650, 700 and 800oC. For treatment at lower temperatures (400, 500 and 600oC) only the first transition, from intrinsic hydrophobicity to superhydrophobicity, is recorded. Scanning electron microscopy, micro-FTIR and micro-Raman spectroscopies are employed to correlate the aforementioned wettability transitions with structural and chemical changes of the siloxane surface, developed during thermal treatment. It is shown that the first transition from intrinsic hydrophobicity to superhydrophobicity is accompanied by a severe surface-structure evolution which increases surface roughness. Once superhydrophobicity is achieved, the surface structure reaches a saturation point and it is not subjected to any other change with further thermal treatment. FTIR spectroscopy shows that the intensity of the O-H/C-H peaks increases/decreases with treatment time and Raman measurements show that the C-Si-C vibrations gradually disappear with treatment time. ACS Paragon Plus Environment
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The evaporation of a droplet resting on a superhydrophobic, water repellent siloxane surface, which was produced after appropriate thermal treatment, is monitored. It is shown that droplet evaporation follows initially the constant contact area mode. At later evaporation stages a transition to the constant contact angle mode is recorded. Finally, it is demonstrated that the superydrophobic and water repellent siloxane surfaces exhibit self cleaning properties, good durability and furthermore do not affect practically the optical transparency of glass substrates.
Introduction Progress in technology and engineering is highly based on siloxane materials, as these have numerous applications such as, for instance, in electronics, packaging, automobile, aerospace, biomedical and coatings industry, in building constructions as well as monument protection and conservation.1-3 Significant research over the past decade has focused on the wettability of siloxane surfaces (i.e. their interactions with water) which usually are intrinsically hydrophobic materials, implying that the Young contact angle,4 measured on an atomically smooth surface, ranges from 90o to 150o. Several strategies have been recently developed, to control and modify the interaction of siloxane surfaces with water, inducing either superhydrophobicity or superhydrophilicity, thus changing the static contact angle to >150o and 150o) but it shortly decreases rapidly to < 5o. Further annealing of the siloxane material does not have any effect on θs which remains < 5o. This type of variation of θs with t is illustrated in Figure 1a, which corresponds to siloxane surfaces treated at 800, 700 and 650 oC. Wetting regimes are identified in the plot of Figure 1a, which shows that the wettability of the siloxane surface, treated at the aforementioned temperatures, is subjected to several transitions: the intrinsic hydrophobic fresh siloxane (no thermal treatment) obtains superhydrophobic properties, after short annealing. Further thermal treatment of the film results in the rapid transition to the superhydrophilic regime. During this transition the superhydrophobic (θs > 150o) character of the treated film surface changes quickly to hydrophobic (150o > θ s> 90o), then to hydrophilic (90o > θs > 5o) and finally to superhydrophilic (θs < 5o). The transition is faster for higher treatment temperatures as emphasized by an arrow drawn in Figure 1a. When a low temperature is used to treat the siloxane film, a different variation of θs with t is recorded, according to the plot of Figure 1b: initially θs increases with t and reaches a maximum value (> 150o) which is constant, as it is not affected by further annealing. In Figure 1b this behavior is clearly visualized for treatment at 600 and 500oC. For treatment at 400oC, a gradual increase of θs is recorded with t, reaching the superhydrophobic regime (> 150o) at t = 300 s. A single wettability transition, from hydrophobicity to superhydrophobicty, is reported by the data of Figure 1b. The transition is faster for higher treatment temperatures as illustrated by an arrow drawn in the figure. It is noted that the superhydrophobic character of the siloxane film heated at 600oC was retained even after 1800 s of treatment, which is far beyond the upper limit of the x-axis of Figure 1b.
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Structural and chemical modifications. The surface-structure evolution of the siloxane surface with t is, an effect of melting and, is revealed in Figures 2 and 3. The former (Figure 2) shows SEM images collected within the first 30 s of treatment at 800oC, thus corresponding to the first wettability
transition
reported
in
Figure
1a,
from
hydrophobicity
to
non-wetting
(superhydrophobicity). Other wetting regimes, recorded in Figure 1a after longer thermal treatment, are discussed later in Figure 3. The surface of the untreated siloxane (Figure 2a) appears to be relatively smooth with some scattered defects. After heating at 800oC for only 10 s surface structures are formed (Figure 2b) increasing thus surface roughness. Further annealing results in a densely rough structure as shown in Figures 2c and 2d, corresponding to θs = 161o and θs = 163o, respectively i.e. non wetting state (θs > 150o). Consequently, the first wettability transition reported in Figure 1a, from hydrophobicity to superhydrophobicity, is accompanied by a severe surfacestructure evolution. However, once superhydrophobicity is achieved, the surface structure reaches a saturation point. Surface morphological changes were not observed during the wettability transition from superhydrophobicity to (hydrophobicity, hydrophilicity and finally to) superhydrophilicity, described in Figure 1a. The structures of the superhydrophobic surfaces shown in Figures 2c and 2d are similar with the surface morphologies presented in Figures 3a and 3b. The former (Figure 3a) corresponds to a hydrophobic state captured during the transition from superhydrophobicity to superhydrophilicity. Figure 3b shows the surface of a superhydrophilic film (complete wetting). Figures 2c, 2d, 3a and 3b show similar structures thus implying that the wettability transitions reported in Figure 1a from superhydrophobicity to hydrophobicity, hydrophilicity and then to superhydrophilicity are not accompanied by any significant structural change of the surface. Furthermore, these four structures (Figures 2c, 2d, 3a and 3b) recorded after treatment at 800oC for various t, are not different from the surface morphology of the superhydrophobic film, produced after heating the siloxane film at a low temperature (600oC) for long t (= 150 s). This surface is shown in Figure 3c.
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SEM images, presented in Figures 2 and 3 were useful to correlate the wettability transitions of Figure 1 with structural changes of the siloxane surface. FTIR and Raman studies, presented in Figures 4 and 5, respectively, are used to monitor chemical modifications of the siloxane material during thermal treatment. The evolution of the IR spectra with treatment time (t) at 800oC is shown in Figure 4. Wettabilities and surface structures, discussed previously in Figures 2 and 3, are noted in Figure 4. The bands at around 2900 cm-1 are assigned to the C-H modes of the methylene groups.25,26 These bands appear to be very strong for the untreated siloxane (top spectrum). However, the intensities of these C-H peaks are gradually reduced with t. In the spectrum of the superhydrophilic siloxane, the bands at 2900 cm-1 are hardly detected (bottom spectrum). Furthermore, Figure 4 shows the emergence of the hydroxyl group vibrations, which produce the broad band at ~3400 cm-1 and the sharp bands in the ~3700-3500 cm-l energy region, at t = 20 s. Conversely to what happens with the methylene mode peaks, the intensity of the O-H vibrations increases gradually with t and the corresponding bands become well-defined in the spectra for long t. Raman measurements of the siloxane surfaces treated at 800oC for various periods are shown in Figure 5. The two low energy modes at ~157 and 188 cm-1 are attributed to the C-Si-C twisting and scissoring vibrations, respectively, the strong band at ~487 cm-1 arises from the symmetric stretching vibrations of the Si-O bond, the peaks centered at ~708 and 786 cm-1 are assigned to the symmetric and asymmetric C-Si-C stretches, respectively, the weak modes at ~684 and 860 cm-1 are associated with the CH3 rocking vibrations and, finally, the high energy band at ~1260 cm-1 is due to the symmetric bending vibrations of the C-H bond, while the one at ~1409 cm-1 is assigned to the corresponding asymmetric CH3 deformation.27 The first effect observed in Figure 5 is the significant linewidth increment of all modes with t. Another interesting result is that the absolute Raman signal decreases abruptly with t. For the 70 s treated siloxane, is almost impossible to detect any of the Raman lines (bottom spectrum). Interestingly, the modes at ~157, 188, 708 and 786 cm-1, which involve C-Si-C vibrations, disappear for treatment periods longer than 30 s, while the
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emergence of a broad band at 400-450 cm-1 (marked with an asterisk in Figure 5) is recorded, which is probably due to Si-O vibrations. Combining the above, one may deduce the occurrence of a major structural disorder with heating, which leads to a nearly amorphous state. In summary, Figures 4 and 5 describe the chemical modifications that affect the wettability of the siloxane surface with t (at 800 oC) showing that thermal treatment changes the relative abundance of polar and apolar groups6. In particular, hydrophobic (apolar) carbon-based functional groups, which are inherently present to the siloxane material, degrade (e.g. oxidatively cleaved28) and are replaced by oxygen-based hydrophilic (polar) groups. This chemical, calcination17 process is slower than the structural change occurring at the siloxane surface with t (Figures 2, 3a and 3b). Consequently, hydrophobic functional groups still dominate the siloxane surface after short t, thus inducing superhydrophobicity to the structured surface. For longer treatment, hydrophobic groups are completely replaced by hydrophilic groups thus inducing superhydrophilicity to the structured surfaces. In summary, the rapidly formed structured-rough surface show wettability, superhydrophobic to superhydrophilic switching because of corresponding chemical switching, occurring at the surface. The spectra included in Figures 4 and 5 correspond to treatment at 800oC (high temperature) which results to multiple wettability transitions of the siloxane surface, according to Figure 1a, up to superhydrophilicity. Detailed spectroscopic measurements were furthermore carried out in samples heated at 400oC where the wettability of the siloxane surface barely changes up to superhydrophobicity after extensive treatment (t = 300 s) according to Figure 1b. These results are shown in the Supporting information and offer support to the discussions of Figures 4 and 5 (for treatment at 800oC), considering that at the low temperature of 400oC, chemical modifications occur slowly and furthermore they correspond to a single wettability transition, from hydrophobicity to superhydrophobicty, according to the data of Figure 1b. It is shown that the presence of hydrophobic groups at the structured surfaces of the sample treated at 400oC is dominant even after extensive heating. Consequently, the treated surface retains its superhydrophobic character, as
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shown in Figure 1b. Finally, some spectroscopic measurements collected for treatment at 650oC are included in the Supporting information, thus providing further evidence/support about the surface chemical modifications discussed above.
Tilt contact angle. The static contact angle, which describes the contact of a resting droplet at equilibrium with a horizontal solid surface, does not adequately describe the wettability of the solid surface. For example, the surface of the red rose petal corresponds to θs > 150o and yet at the same a droplet cannot roll off even when the petal is turned upside down, implying thus high droplet adhesion.29 Therefore, contact angle hysteresis or tilt contact angle measurements, discussed herein, should be included in a study which focuses on the wettability of a solid surface. Figures 6a and 6b show the variation of the tilt contact angle (θt) with t, for siloxane surfaces treated at 700 and 600oC, respectively. Static contact angles, presented in Figures 1a (for 700oC) and 1b (for 600oC) are included to assist the discussion. Figure 6a shows that θt decreases rapidly within the first seconds of treatment, obtaining a minimum, low value (< 5o) which corresponds to small drop adhesion i.e. water repellency. In this state the siloxane surface exhibits simultaneously superhydrophobic properties as it is evidenced by the high θs (> 150o). Further annealing results in an increase of θt as the surface obtains hydrophobic and then hydrophilic properties. Finally, at the superhydrophilic regime the water droplet collapses to a continuous film. A simpler variation of θt is recorded for treatment at low temperature, as it is evidenced by the data of Figure 6b which corresponds to 600oC. In this case, θt initially decreases, but with a lower rate compared to the faster decrease reported in Figure 6a for 700oC. As the surface falls to the superhydrophobic (θs > 150o) regime, θt obtains a minimum, low value (< 5o) which corresponds to small droplet adhesion i.e. water repellency. More θt measurements vs t are provided in Figure 7 for various treatment temperatures. It is seen that for t = 20 s, siloxane surfaces treated at very high temperatures (800 and 700oC) have clearly obtained water repellent properties, as they correspond to very low tilt angles (< 5o). Droplets on
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the surface treated at 650oC need a higher θt to roll off, which however is < 10o, thus implying relatively small droplet adhesion. Siloxane films treated at lower temperatures such 600, 500 and 400oC correspond to θt > 10o i.e. high droplet adhesion. For t = 40 s, siloxane surfaces treated at 800 and 700oC are still in the water repellent regime, which now includes siloxane films treated at 650 and 600oC (θt < 5o). Films treated at lower temperatures (500 and 400oC) are still hydrophobic corresponding to a high θt (> 10o). After annealing for 60 s, the film surface treated at 800oC ceases to display droplet sliding behaviour. The wettability of the surface corresponds to the complete wetting regime (superhydrophilicity) and therefore θt could not be measured. Siloxane films treated at 700 and 650oC are in the transition (hydrophobic/hydrophilic) regime towards superhydrophilicity, thus corresponding to high θt. The 600 and 500oC treated films are both in the water repellent, superhydrophobic regime, while the film treated at 400oC, exhibits hydrophobic properties. At t = 120 s the surfaces of the films treated at 800, 700 and 650oC correspond to complete wetting. Films treated at 500 and 600oC are still in the water repellent regime. Similarly, the wettability of the film treated at 400oC did not change as the film still exhibits hydrophobic properties with θt > 10o. According to the results of Figure 1b, for the very low temperature of 400oC, extensive thermal treatment (t = 300 s) is necessary to change the hydrophobic character of the film surface and to induce superhydrophobicity.
Wettability transitions: temperature-time dependence. The wettability transitions of the siloxane surface are summarized in Figure 8, which presents a treatment temperature-time plot. The line borders of the wettability regimes were calculated using the data of Figure 1. The (i) first transition from intrinsic hydrophobicity to superhydrophobicity corresponds to θs = 150o, (ii) the second transition from superhydrophobicity to hydrophobicity again to θs = 150o and (iii) the transition from hydrophlicity to superhydrophilicity to θs = 5o. Apparently (ii) and (iii) were applicable only for the data of Figure 1a which corresponds to high treatment temperatures. The intermediate
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hydrophobic and hydrophilic regimes recorded during the transition from superhydrophobicity to superhydrophilicity, are depicted in the plot of Figure 8 as one area, defined as the “transition” area. The area of hydrophobicity where the intrinsic wettability of the siloxane surface is not affected by the thermal treatment, exists at low temperatures and short t. Increasing treatment temperature and/or time, superhydrophobicity is achieved. Extensive annealing may lead to superhydrophilicity. For the time scale (300 s) of the experiment superhydrophilicity was obtained for treatment at 650oC or higher and was not recorded at 600oC and lower temperatures. The plot of Figure 8 clearly demonstrates the cross influence effects of treatment temperature and time on the wettability of the siloxane surface. As described in the Experimental, the thickness of the siloxane films studied herein was 4 µm, as measured right after deposition on glass and curing. In the Supporting information θs and θt are measured on a 8 µm thick film heated at 800oC as a function of treatment time. It is shown that film thickness (ranging from 4 to 8µm) does not have any noticeable effect on the wettability transitions of the siloxane surface. This result enlarges the impact of Figure 8.
Superhydrophobic siloxane surfaces: droplet evaporation. The evaporation of a droplet resting on the surface of Figure 2d, was monitored. Two distinct modes have been described in the literature for the evaporation of a droplet on a superhydrophobic surface: (i) the constant contact area mode, where the droplet is pinned accompanied by gradual decrease of θs during evaporation and (ii) the constant contact angle mode, where a gradual decrease of the droplet-surface contact area is recorded.21-24 Figure 9a shows the evolution of θs and diameter (d) of the droplet-siloxane contact area, during evaporation and suggests that both distinctive modes appear. However, it is interesting to note that θs is >150o for the total duration of the experiment, in which the droplet shrunk from 6µL (initial volume) to 2µL (last measurement). Figure 9a is in agreement with the results presented by Choi et al, who showed that the evaporation on most superhydrophobic surfaces is dominated by both
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modes:24 at the initial stages of evaporation, d is constant and a decrease of θs is observed (constant contact area mode); at later evaporation stages a transition to the constant contact angle mode is usually recorded.24 This scenario is confirmed by the results of Figure 9a. Systematic studies on patterned superhydrophobic surfaces, where the wettability can be controlled by the geometrical dimensions and distances of the surface structures, which can be fabricated and measured with high accuracy, have shown that the relative durations of the two evaporation modes are highly affected by the solid fraction (fs) of the substrate, defined by the Cassie-Bazter equation.11 It was shown that the constant contact angle evaporation mode is extended/shortened for surfaces corresponding to small/large fs.22,24 The morphology of the superhydrophobic surfaces studied herein is random, as shown for instance in the SEM image of Figure 2d. Consequently, the droplet-substrate contact area cannot be measured with accuracy. However, the dense structure of Figure 2d, suggests that the droplet should be in contact with the substrate only through a small fraction (small fs) thus implying that the constant contact angle mode should occupy the largest portion of the total evaporation lifetime, as supported by the results of Figure 9a. The variation of the mass (m) of the droplet to the 2/3 power with evaporation time (tev) is revealed in Figures 2b and 2c for the two modes. The mass is normalized to its initial (mo) value. For both evaporation modes it is seen that a linear relationship between (m/mo)2/3 with tev predicts the mass transfer rate, as suggested by Choi et al.24
Properties of superhydrophobic siloxanes. In the following we demonstrate that the produced superydrophobic and water repellent siloxane surfaces induce self cleaning properties to the treated glass, exhibit good durability and furthermore do not affect practically the optical transparency of glass. Consequently, the suggested method can be used to induce superhydrophobicity and water repellency on glass surfaces used in buildings, automobiles and other applications. The self-cleaning property of the water repellent siloxane films is demonstrated in the video which is provided as Supporting information. Glass coated with a water repellent siloxane film was
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intentionally contaminated with dust of red ochre. The pigment is easily removed by water droplets, as shown in the video. The durability of the superhydrophobic siloxane surfaces is demonstrated with three experiments. First, contact angle measurements were carried out our over a wide range of the pH of the droplet. Figure 10 depicts θs and θt versus the pH of droplets which were placed on a siloxane surface treated at 600oC for 240 s. It is shown that both angles are stable over a wide range of pH, thus suggesting that the superhydrophobic and water repellent character of the film is stable for acidic and basic aqueous droplets. Second, θs and θt measurements were carried out on siloxane surfaces which were stored in the laboratory for two years and compared with the results obtained right after thermal treatment (fresh siloxanes). We report that the superhydrophobic and water repellent character of the treated siloxanes remained practically unaffected after two years of storage. Specific contact angle measurements for fresh and stored superhydrophobic siloxane surfaces are provided in the Supporting information. Third, a video is provided in Supporting information showing that the water repellent character of a treated glass is retained even after extensive exposure to a copious flow of water. Finally, colorimetric measurements carried out on untreated-bare glass and glasses coated by superhydrophobic and superhydrophilic siloxane films, showed that the application of the siloxane material had practically no effect on the aesthetic appearance and therefore the transparency of glass, as described in detail in the Supporting information and demonstrated in the photographs of Figures 3d and 3e.
Conclusions Treatment of a siloxane elastomer at very high temperatures results in surface structural changes, evolved because of melting and surface chemical modifications (Figures 2 - 5), which affect dramatically the wettability of the siloxane surface. For treatment at 650oC or higher, surface wettability is subjected to several transitions with treatment time as follows (Figures 1a, 6a, 7 and
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8): the intrinsic hydrophobic siloxane surface obtains superhydrophobic, non-sticking properties and
then
through
intermediate
stages
(hydrophobicity
and
hydrophilicity)
becomes
superhydrpophilic where the water drop collapses to a continuous film. For the time scale used herein (1 - 300 s) this scenario was recorded for treatment at 650, 700 and 800oC. For treatment at lower temperatures (400, 500 and 600oC) only the first transition, from intrinsic hydrophobicity to superhydrophobicity, was recorded (Figures 1b, 6b, 7 and 8). The evaporation of a droplet resting on a superhydrophobic, water repellent siloxane surface, which was produced after appropriate thermal treatment, was monitored. The results are provided in Figure 9 and show that droplet evaporation follows initially the constant contact area mode. At later evaporation stages a transition to the constant contact angle mode is recorded. Finally, it is shown the produced superydrophobic and water repellent siloxane surfaces induce self cleaning properties to the treated glass, exhibit good durability and furthermore do not affect practically the transparency of glass.
Acknowledgements I. Karapanagiotis is grateful for the support of Thales (Intermonu, 68/1117) and ERC-13 (ILGradMech-ASM, 88257) programs. The authors would like to thank the personnel of the Laboratory of Mechanics and Materials (Aristotle University of Thessaloniki) for useful discussions and Dr. Dimitrios Christofilos for his assistance in the FTIR measurements.
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13. Budunoglu, H.; Yildirim, A.; Guler, M. O.; Bayindir M. Highly Transparent, Flexible, and Thermally Stable Superhydrophobic ORMOSIL Aerogel Thin Films. ACS Appl. Mater. Interfaces 2011, 3, 539-545. 14. Wohl, C. J.; Belcher, M.A.; Chen, L.; Connell, J.W. Laser Ablative Patterning of Copoly(imide siloxane)s Generating Superhydrophobic Surfaces. Langmuir 2010, 26,11469-11478. 15. Peng, Y. -T.; Lo, K. -F.; Juang, Y. -J. Constructing a Superhydrophobic Surface on Polydimethylsiloxane via Spin Coating and Vapor−Liquid Sol−Gel Process. Langmuir 2010, 26, 5167-5171. 16. Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Perry, C. C. Intrinsically Superhydrophobic Organosilica Sol−Gel Foams. Langmuir 2003, 19, 5626-5631. 17. Lin, Y. -H.; Su, K. -L.; Tsai, P. -S.; Chuang, F. -L.; Yang, Y. -M. Fabrication and Characterization of Transparent Superhydrophilic/Superhydrophobic Silica Nanoparticulate Thin Films. Thin Solid Films 2011, 519, 5450-5455. 18. Rao, V. A.; Kulkarni, M. M. Hydrophobic Properties of TMOS/TMES-Based Silica Aerogels. Mater. Res. Bull. 2002, 37, 1667-1677. 19. Cha, S. -C.; Her, E. K.; Ko, T. -J.; Kim, S. J.; Roh, H.; Lee, K. -R.; Oh, K. H.; Moon, M.-W. Thermal Stability of Superhydrophobic, Nanostructured Surfaces. J. Colloid Interf. Sci. 2013, 391, 152-157. 20. Laine, R. M.; Rahn, J. A.; Youngdahl, K. A.; Babonneau, F. Hoppe, M. L., Zhang, Z. F.; Harrod, J. F. Synthesis and High-Temperature Chemistry of Methylsilsesquioxane Polymers Produced by Titanium-Catalyzed Redistribution of Methylhydridooligo- and -polysiloxanes. Chem. Mater. 1990, 2, 464-472. 21. Zhang, X.; Tan. S.; Zhao, N.; Guo, X.; Zhang, X.; Zhang, Y.; Xu, J. Evaporation of Sessile Water Droplets on Superhydrophobic Natural Lotus and Biomimetic Polymer Surfaces. Chemphyschem 2006, 7, 2067-2070.
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22. Gelderblom, H.; Marín, Á. G.; Nair, H.; van Houselt, A.; Lefferts, L.; Snoeijer, J. H.; Lohse, D. How Water Droplets Evaporate on a Superhydrophobic Substrate. Phys. Rev. E 2011, 83, 026306. 23. Xu W.; Choi, C. -H. From Sticky to Slippery Droplets: Dynamics of Contact Line Depinning on Superhydrophobic Surfaces. Phys. Rev. Lett. 2012, 109, 024504. 24. Xu, W.; Leeladhar, R.; Kang, Y. T.; Choi, C. -H. Evaporation Kinetics of Sessile Water Droplets on Micropillared Superhydrophobic Surfaces. Langmuir 2013, 29, 6032-6041. 25. Nagai, H. Infrared Spectra of Stereoregular Polymethyl Methacrylate. J. Appl. Polym. Sci. 1963, 7, 1697-1714. 26. Domingo, C.; de Buergo, M. A.; Sánchez-Cortés, S.; Fort, R.; Garćia-Ramos, J. V.; GomezHeras, M. Possibilities of Monitoring the Polymerization Process of Silicon-Based Water Repellents and Consolidants in Stones Through Infrared and Raman Spectroscopy. Prog. Org. Coat. 2008, 63, 5-12. 27. Jayes, L.; Hard, A.P.; Séné, C.; Parker, S. F.; Jayasooriya U. A. Vibrational Spectroscopic Analysis of Silicones: A Fourier Transform-Raman and Inelastic Neutron Scattering Investigation. Anal. Chem. 2003, 75, 742-746. 28. Shirtcliffe, N. J.; McHale, G.; Newton, M. I. ; Perry C. C.; Roach, P. Porous Materials Show Superhydrophobic to Superhydrophilic Switching. Chem. Commun. 2005, 3135-3137. 29. Feng, L.; Zhang, Y.; Xi, J.; Zhu, Y.; Wang, N.; Xia, F.; Jiang, L. Petal Effect: A Superhydrophobic State with High Adhesive Force. Langmuir 2008, 24, 4114-4119.
FIGURE CAPTIONS
Figure 1. Static contact angle (θs) vs t, for treatment at (a) 800, 700 and 650oC - high temperatures and (b) 600, 500 and 400oC - low temperatures. Arrows that emphasize the effect of treatment
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temperature were drawn to guide the eye. Photographs of water drops corresponding to different wetting regimes are included. Distinct wetting regimes are divided by dotted lines. Figure 2. Surface-structure evolution of siloxane after treatment at high temperature (800oC). Contact angle (θs), treatment time (t) and temperature are shown in the upper left corner of each SEM image; the untreated film is shown in (a) which corresponds to t = 0. Thermal treatment, results in surface structure evolution which is accompanied by a wettability transition from hydrohpobicity (a and b) to superhydrophobicity (c and d). Figure 3. SEM images of (a) hydrophobic, (b) superhydrophilic and (c) superhydrophobic surfaces. Contact angle (θs), treatment time (t) and temperature are shown in the upper left corner of each SEM image. The hydrophobic surface in (a) was captured during the transition from superhydrophobicity to superhydrophilicity (Figure 1a). The superhydrophobic surface in (c) was produced after extensive treatment of the siloxane film at a relatively low temperature (600oC). The three surfaces included in the figure have similar structures and furthermore display similar morphologies with the superhydrophobic surfaces developed after heating the siloxane at a high temperature (800oC) for short periods (Figures 2c and 2d). Water on (d) superhydrophilic and (e) superhydrophobic siloxane films on glass are shown. The photographs demonstrate the transparency of the siloxane films. Figure 4. FTIR spectra of siloxane films treated at 800oC for various periods (t). The wettabilities of the siloxane surfaces are described according to the results of Figure 1a. The structures of the siloxane surfaces are shown in Figures 2, 3a and 3b, as noted. Figure 5. Raman spectra of siloxane films treated at 800oC for various periods (t). Asterisks mark the emergence of a new broad peak at 400-450 cm-1. Arrows indicate the peaks which are attributed to the glass substrate and are strongly recorded in some spectra (e.g. film treated at 800oC for t = 50 s). Wettabilities and structures of the siloxane surfaces are described, according to the results reported in Figures 1a, 2, 3a and 3b.
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Figure 6. Tilt contact angle (θt) vs t for treatment at (a) 700oC - high temperature and (b) 600oC low temperature. Measurements of θs and wetting regimes presented and defined in Figures 1a and 1b are reproduced herein to assist the discussion. In the superhydrophilic regime included in (a), θt was not possible to be measured. Figure 7. Tilt contact angle (θt) vs t for various treatment temperatures. The result for the untreated siloxane surface (t = 0) is included. Treatment temperatures are provided on top of the plot bars to guide the eye. Asterisks are used to mark superhydrophilic surfaces where θt could not be experimentally measured and is visualised with the theoretical maximum value (180o). Figure 8. Treatment temperature vs t. The graph shows the various wettability regimes of the siloxane surface. The “transition” regime corresponds to the intermediate, narrow hydrophobic and hydrophilic regimes recorded during the transition from superhydrophobicity to superhydrophilicity (Figure 1a). The exact borders of the transition regime between 650 and 600oC cannot be defined accurately, as these are the temperatures where the change in the wettability behaviour of the siloxane surface was recorded: at 650oC the wettability of the surface is subjected to several transitions
(Figure
1a)
and
at
600oC
only
one
transition
from
hydrophobicity
to
superhydrophobicity (Figure 1b) is recorded at the time scale of the experiment. Consequently this obscured area is marked with gray lines. Figure 9. (a) Contact angle (θs) and contact diameter (d) of a droplet on a superhydrophobic siloxane surface vs evaporation time (tev). The siloxane surface was prepared by thermal treatment at 800oC for 30 s and is shown in Figure 2d. The scale of the diameter d, included in the y-axis of the plot, is smaller than the capillary length which for water at room temperature is 2.7mm. Normalized droplet mass to the 2/3 power vs tev are shown in (b) and (c) for the two distinct evaporation modes. Figure 10. Static (θs) and tilt (θt) contact angle vs the pH of resting droplets, which were placed on the surface of a superhydrophobic and water repellent siloxane film produced after treatment at 600oC for 240 s. Photographs of some droplets are included.
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Superhydrophilic
Hydrophobic (a) θs
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FTIR spectrum
Treatment time (t), wettability, surface structure no treatment, hydrophobic, Fig. 2a
t=10 s, hydrophobic, Fig. 2b
t=20 s, superhydrophobic, Fig. 2c
t=30 s, superhydrophobic, Fig. 2d
t=40 s, superhydrophobic
t=50 s, hydrophobic, Fig. 3a
t=70 s, superhydrophilic, Fig. 3b
Figure 4.
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Raman spectrum
Treatment time, wettability, surface structure
no treatment, hydrophobic, Fig. 2a
t=10 s, hydrophobic, Fig. 2b t=20 s, superhydrophobic, Fig. 2c t=30 s, superhydrophobic, Fig. 2d
t=40 s, superhydrophobic
t=50 s, hydrophobic, Fig. 3a t=70 s, superhydrophilic, Fig. 3b
Figure 5.
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Article
From Hydrophobic to Superhydrophobic and Superhydrophilic Siloxanes by Thermal Treatment Ioannis Karapanagiotis, Panagiotis Manoudis, Andrea Zurba, and Dimitrios Lampakis Langmuir, Just Accepted Manuscript • Publication Date (Web): 14 Oct 2014 Downloaded from http://pubs.acs.org on October 15, 2014
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From Hydrophobic to Superhydrophobic and Superhydrophilic Siloxanes by Thermal Treatment
Ioannis Karapanagiotis*, Panagiotis N. Manoudis, Andrea Zurba and Dimitrios Lampakis
University Ecclesiastical Academy of Thessaloniki, Department of Management and Conservation of Ecclesiastical Cultural Heritage Objects, Thessaloniki 54250, Greece
Abstract The cross-influence effects of treatment temperature and time on the wettability of a siloxane elastomer is investigated in detail, through static and tilt contact angle measurements. The material is heated at 400, 500, 600, 650, 700 and 800oC for various periods, ranging from 1 to 300 s. The siloxane surface is subjected to multiple wettability transitions with treatment time: from intrinsic hydrophobicity to superhydrophobicity (and water repellency) and then through intermediate stages -hydrophobicity and hydrophilicity- to superhydrophilicity. For the time scale used herein (1 - 300 s) this scenario is recorded for treatment at 650, 700 and 800oC. For treatment at lower temperatures (400, 500 and 600oC) only the first transition, from intrinsic hydrophobicity to superhydrophobicity, is recorded. Scanning electron microscopy, micro-FTIR and micro-Raman spectroscopies are employed to correlate the aforementioned wettability transitions with structural and chemical changes of the siloxane surface, developed during thermal treatment. It is shown that the first transition from intrinsic hydrophobicity to superhydrophobicity is accompanied by a severe surface-structure evolution which increases surface roughness. Once superhydrophobicity is achieved, the surface structure reaches a saturation point and it is not subjected to any other change with further thermal treatment. FTIR spectroscopy shows that the intensity of the O-H/C-H peaks increases/decreases with treatment time and Raman measurements show that the C-Si-C vibrations gradually disappear with treatment time. ACS Paragon Plus Environment
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The evaporation of a droplet resting on a superhydrophobic, water repellent siloxane surface, which was produced after appropriate thermal treatment, is monitored. It is shown that droplet evaporation follows initially the constant contact area mode. At later evaporation stages a transition to the constant contact angle mode is recorded. Finally, it is demonstrated that the superydrophobic and water repellent siloxane surfaces exhibit self cleaning properties, good durability and furthermore do not affect practically the optical transparency of glass substrates.
Introduction Progress in technology and engineering is highly based on siloxane materials, as these have numerous applications such as, for instance, in electronics, packaging, automobile, aerospace, biomedical and coatings industry, in building constructions as well as monument protection and conservation.1-3 Significant research over the past decade has focused on the wettability of siloxane surfaces (i.e. their interactions with water) which usually are intrinsically hydrophobic materials, implying that the Young contact angle,4 measured on an atomically smooth surface, ranges from 90o to 150o. Several strategies have been recently developed, to control and modify the interaction of siloxane surfaces with water, inducing either superhydrophobicity or superhydrophilicity, thus changing the static contact angle to >150o and 150o) but it shortly decreases rapidly to < 5o. Further annealing of the siloxane material does not have any effect on θs which remains < 5o. This type of variation of θs with t is illustrated in Figure 1a, which corresponds to siloxane surfaces treated at 800, 700 and 650 oC. Wetting regimes are identified in the plot of Figure 1a, which shows that the wettability of the siloxane surface, treated at the aforementioned temperatures, is subjected to several transitions: the intrinsic hydrophobic fresh siloxane (no thermal treatment) obtains superhydrophobic properties, after short annealing. Further thermal treatment of the film results in the rapid transition to the superhydrophilic regime. During this transition the superhydrophobic (θs > 150o) character of the treated film surface changes quickly to hydrophobic (150o > θ s> 90o), then to hydrophilic (90o > θs > 5o) and finally to superhydrophilic (θs < 5o). The transition is faster for higher treatment temperatures as emphasized by an arrow drawn in Figure 1a. When a low temperature is used to treat the siloxane film, a different variation of θs with t is recorded, according to the plot of Figure 1b: initially θs increases with t and reaches a maximum value (> 150o) which is constant, as it is not affected by further annealing. In Figure 1b this behavior is clearly visualized for treatment at 600 and 500oC. For treatment at 400oC, a gradual increase of θs is recorded with t, reaching the superhydrophobic regime (> 150o) at t = 300 s. A single wettability transition, from hydrophobicity to superhydrophobicty, is reported by the data of Figure 1b. The transition is faster for higher treatment temperatures as illustrated by an arrow drawn in the figure. It is noted that the superhydrophobic character of the siloxane film heated at 600oC was retained even after 1800 s of treatment, which is far beyond the upper limit of the x-axis of Figure 1b.
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Structural and chemical modifications. The surface-structure evolution of the siloxane surface with t is, an effect of melting and, is revealed in Figures 2 and 3. The former (Figure 2) shows SEM images collected within the first 30 s of treatment at 800oC, thus corresponding to the first wettability
transition
reported
in
Figure
1a,
from
hydrophobicity
to
non-wetting
(superhydrophobicity). Other wetting regimes, recorded in Figure 1a after longer thermal treatment, are discussed later in Figure 3. The surface of the untreated siloxane (Figure 2a) appears to be relatively smooth with some scattered defects. After heating at 800oC for only 10 s surface structures are formed (Figure 2b) increasing thus surface roughness. Further annealing results in a densely rough structure as shown in Figures 2c and 2d, corresponding to θs = 161o and θs = 163o, respectively i.e. non wetting state (θs > 150o). Consequently, the first wettability transition reported in Figure 1a, from hydrophobicity to superhydrophobicity, is accompanied by a severe surfacestructure evolution. However, once superhydrophobicity is achieved, the surface structure reaches a saturation point. Surface morphological changes were not observed during the wettability transition from superhydrophobicity to (hydrophobicity, hydrophilicity and finally to) superhydrophilicity, described in Figure 1a. The structures of the superhydrophobic surfaces shown in Figures 2c and 2d are similar with the surface morphologies presented in Figures 3a and 3b. The former (Figure 3a) corresponds to a hydrophobic state captured during the transition from superhydrophobicity to superhydrophilicity. Figure 3b shows the surface of a superhydrophilic film (complete wetting). Figures 2c, 2d, 3a and 3b show similar structures thus implying that the wettability transitions reported in Figure 1a from superhydrophobicity to hydrophobicity, hydrophilicity and then to superhydrophilicity are not accompanied by any significant structural change of the surface. Furthermore, these four structures (Figures 2c, 2d, 3a and 3b) recorded after treatment at 800oC for various t, are not different from the surface morphology of the superhydrophobic film, produced after heating the siloxane film at a low temperature (600oC) for long t (= 150 s). This surface is shown in Figure 3c.
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SEM images, presented in Figures 2 and 3 were useful to correlate the wettability transitions of Figure 1 with structural changes of the siloxane surface. FTIR and Raman studies, presented in Figures 4 and 5, respectively, are used to monitor chemical modifications of the siloxane material during thermal treatment. The evolution of the IR spectra with treatment time (t) at 800oC is shown in Figure 4. Wettabilities and surface structures, discussed previously in Figures 2 and 3, are noted in Figure 4. The bands at around 2900 cm-1 are assigned to the C-H modes of the methylene groups.25,26 These bands appear to be very strong for the untreated siloxane (top spectrum). However, the intensities of these C-H peaks are gradually reduced with t. In the spectrum of the superhydrophilic siloxane, the bands at 2900 cm-1 are hardly detected (bottom spectrum). Furthermore, Figure 4 shows the emergence of the hydroxyl group vibrations, which produce the broad band at ~3400 cm-1 and the sharp bands in the ~3700-3500 cm-l energy region, at t = 20 s. Conversely to what happens with the methylene mode peaks, the intensity of the O-H vibrations increases gradually with t and the corresponding bands become well-defined in the spectra for long t. Raman measurements of the siloxane surfaces treated at 800oC for various periods are shown in Figure 5. The two low energy modes at ~157 and 188 cm-1 are attributed to the C-Si-C twisting and scissoring vibrations, respectively, the strong band at ~487 cm-1 arises from the symmetric stretching vibrations of the Si-O bond, the peaks centered at ~708 and 786 cm-1 are assigned to the symmetric and asymmetric C-Si-C stretches, respectively, the weak modes at ~684 and 860 cm-1 are associated with the CH3 rocking vibrations and, finally, the high energy band at ~1260 cm-1 is due to the symmetric bending vibrations of the C-H bond, while the one at ~1409 cm-1 is assigned to the corresponding asymmetric CH3 deformation.27 The first effect observed in Figure 5 is the significant linewidth increment of all modes with t. Another interesting result is that the absolute Raman signal decreases abruptly with t. For the 70 s treated siloxane, is almost impossible to detect any of the Raman lines (bottom spectrum). Interestingly, the modes at ~157, 188, 708 and 786 cm-1, which involve C-Si-C vibrations, disappear for treatment periods longer than 30 s, while the
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emergence of a broad band at 400-450 cm-1 (marked with an asterisk in Figure 5) is recorded, which is probably due to Si-O vibrations. Combining the above, one may deduce the occurrence of a major structural disorder with heating, which leads to a nearly amorphous state. In summary, Figures 4 and 5 describe the chemical modifications that affect the wettability of the siloxane surface with t (at 800 oC) showing that thermal treatment changes the relative abundance of polar and apolar groups6. In particular, hydrophobic (apolar) carbon-based functional groups, which are inherently present to the siloxane material, degrade (e.g. oxidatively cleaved28) and are replaced by oxygen-based hydrophilic (polar) groups. This chemical, calcination17 process is slower than the structural change occurring at the siloxane surface with t (Figures 2, 3a and 3b). Consequently, hydrophobic functional groups still dominate the siloxane surface after short t, thus inducing superhydrophobicity to the structured surface. For longer treatment, hydrophobic groups are completely replaced by hydrophilic groups thus inducing superhydrophilicity to the structured surfaces. In summary, the rapidly formed structured-rough surface show wettability, superhydrophobic to superhydrophilic switching because of corresponding chemical switching, occurring at the surface. The spectra included in Figures 4 and 5 correspond to treatment at 800oC (high temperature) which results to multiple wettability transitions of the siloxane surface, according to Figure 1a, up to superhydrophilicity. Detailed spectroscopic measurements were furthermore carried out in samples heated at 400oC where the wettability of the siloxane surface barely changes up to superhydrophobicity after extensive treatment (t = 300 s) according to Figure 1b. These results are shown in the Supporting information and offer support to the discussions of Figures 4 and 5 (for treatment at 800oC), considering that at the low temperature of 400oC, chemical modifications occur slowly and furthermore they correspond to a single wettability transition, from hydrophobicity to superhydrophobicty, according to the data of Figure 1b. It is shown that the presence of hydrophobic groups at the structured surfaces of the sample treated at 400oC is dominant even after extensive heating. Consequently, the treated surface retains its superhydrophobic character, as
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shown in Figure 1b. Finally, some spectroscopic measurements collected for treatment at 650oC are included in the Supporting information, thus providing further evidence/support about the surface chemical modifications discussed above.
Tilt contact angle. The static contact angle, which describes the contact of a resting droplet at equilibrium with a horizontal solid surface, does not adequately describe the wettability of the solid surface. For example, the surface of the red rose petal corresponds to θs > 150o and yet at the same a droplet cannot roll off even when the petal is turned upside down, implying thus high droplet adhesion.29 Therefore, contact angle hysteresis or tilt contact angle measurements, discussed herein, should be included in a study which focuses on the wettability of a solid surface. Figures 6a and 6b show the variation of the tilt contact angle (θt) with t, for siloxane surfaces treated at 700 and 600oC, respectively. Static contact angles, presented in Figures 1a (for 700oC) and 1b (for 600oC) are included to assist the discussion. Figure 6a shows that θt decreases rapidly within the first seconds of treatment, obtaining a minimum, low value (< 5o) which corresponds to small drop adhesion i.e. water repellency. In this state the siloxane surface exhibits simultaneously superhydrophobic properties as it is evidenced by the high θs (> 150o). Further annealing results in an increase of θt as the surface obtains hydrophobic and then hydrophilic properties. Finally, at the superhydrophilic regime the water droplet collapses to a continuous film. A simpler variation of θt is recorded for treatment at low temperature, as it is evidenced by the data of Figure 6b which corresponds to 600oC. In this case, θt initially decreases, but with a lower rate compared to the faster decrease reported in Figure 6a for 700oC. As the surface falls to the superhydrophobic (θs > 150o) regime, θt obtains a minimum, low value (< 5o) which corresponds to small droplet adhesion i.e. water repellency. More θt measurements vs t are provided in Figure 7 for various treatment temperatures. It is seen that for t = 20 s, siloxane surfaces treated at very high temperatures (800 and 700oC) have clearly obtained water repellent properties, as they correspond to very low tilt angles (< 5o). Droplets on
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the surface treated at 650oC need a higher θt to roll off, which however is < 10o, thus implying relatively small droplet adhesion. Siloxane films treated at lower temperatures such 600, 500 and 400oC correspond to θt > 10o i.e. high droplet adhesion. For t = 40 s, siloxane surfaces treated at 800 and 700oC are still in the water repellent regime, which now includes siloxane films treated at 650 and 600oC (θt < 5o). Films treated at lower temperatures (500 and 400oC) are still hydrophobic corresponding to a high θt (> 10o). After annealing for 60 s, the film surface treated at 800oC ceases to display droplet sliding behaviour. The wettability of the surface corresponds to the complete wetting regime (superhydrophilicity) and therefore θt could not be measured. Siloxane films treated at 700 and 650oC are in the transition (hydrophobic/hydrophilic) regime towards superhydrophilicity, thus corresponding to high θt. The 600 and 500oC treated films are both in the water repellent, superhydrophobic regime, while the film treated at 400oC, exhibits hydrophobic properties. At t = 120 s the surfaces of the films treated at 800, 700 and 650oC correspond to complete wetting. Films treated at 500 and 600oC are still in the water repellent regime. Similarly, the wettability of the film treated at 400oC did not change as the film still exhibits hydrophobic properties with θt > 10o. According to the results of Figure 1b, for the very low temperature of 400oC, extensive thermal treatment (t = 300 s) is necessary to change the hydrophobic character of the film surface and to induce superhydrophobicity.
Wettability transitions: temperature-time dependence. The wettability transitions of the siloxane surface are summarized in Figure 8, which presents a treatment temperature-time plot. The line borders of the wettability regimes were calculated using the data of Figure 1. The (i) first transition from intrinsic hydrophobicity to superhydrophobicity corresponds to θs = 150o, (ii) the second transition from superhydrophobicity to hydrophobicity again to θs = 150o and (iii) the transition from hydrophlicity to superhydrophilicity to θs = 5o. Apparently (ii) and (iii) were applicable only for the data of Figure 1a which corresponds to high treatment temperatures. The intermediate
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hydrophobic and hydrophilic regimes recorded during the transition from superhydrophobicity to superhydrophilicity, are depicted in the plot of Figure 8 as one area, defined as the “transition” area. The area of hydrophobicity where the intrinsic wettability of the siloxane surface is not affected by the thermal treatment, exists at low temperatures and short t. Increasing treatment temperature and/or time, superhydrophobicity is achieved. Extensive annealing may lead to superhydrophilicity. For the time scale (300 s) of the experiment superhydrophilicity was obtained for treatment at 650oC or higher and was not recorded at 600oC and lower temperatures. The plot of Figure 8 clearly demonstrates the cross influence effects of treatment temperature and time on the wettability of the siloxane surface. As described in the Experimental, the thickness of the siloxane films studied herein was 4 µm, as measured right after deposition on glass and curing. In the Supporting information θs and θt are measured on a 8 µm thick film heated at 800oC as a function of treatment time. It is shown that film thickness (ranging from 4 to 8µm) does not have any noticeable effect on the wettability transitions of the siloxane surface. This result enlarges the impact of Figure 8.
Superhydrophobic siloxane surfaces: droplet evaporation. The evaporation of a droplet resting on the surface of Figure 2d, was monitored. Two distinct modes have been described in the literature for the evaporation of a droplet on a superhydrophobic surface: (i) the constant contact area mode, where the droplet is pinned accompanied by gradual decrease of θs during evaporation and (ii) the constant contact angle mode, where a gradual decrease of the droplet-surface contact area is recorded.21-24 Figure 9a shows the evolution of θs and diameter (d) of the droplet-siloxane contact area, during evaporation and suggests that both distinctive modes appear. However, it is interesting to note that θs is >150o for the total duration of the experiment, in which the droplet shrunk from 6µL (initial volume) to 2µL (last measurement). Figure 9a is in agreement with the results presented by Choi et al, who showed that the evaporation on most superhydrophobic surfaces is dominated by both
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modes:24 at the initial stages of evaporation, d is constant and a decrease of θs is observed (constant contact area mode); at later evaporation stages a transition to the constant contact angle mode is usually recorded.24 This scenario is confirmed by the results of Figure 9a. Systematic studies on patterned superhydrophobic surfaces, where the wettability can be controlled by the geometrical dimensions and distances of the surface structures, which can be fabricated and measured with high accuracy, have shown that the relative durations of the two evaporation modes are highly affected by the solid fraction (fs) of the substrate, defined by the Cassie-Bazter equation.11 It was shown that the constant contact angle evaporation mode is extended/shortened for surfaces corresponding to small/large fs.22,24 The morphology of the superhydrophobic surfaces studied herein is random, as shown for instance in the SEM image of Figure 2d. Consequently, the droplet-substrate contact area cannot be measured with accuracy. However, the dense structure of Figure 2d, suggests that the droplet should be in contact with the substrate only through a small fraction (small fs) thus implying that the constant contact angle mode should occupy the largest portion of the total evaporation lifetime, as supported by the results of Figure 9a. The variation of the mass (m) of the droplet to the 2/3 power with evaporation time (tev) is revealed in Figures 2b and 2c for the two modes. The mass is normalized to its initial (mo) value. For both evaporation modes it is seen that a linear relationship between (m/mo)2/3 with tev predicts the mass transfer rate, as suggested by Choi et al.24
Properties of superhydrophobic siloxanes. In the following we demonstrate that the produced superydrophobic and water repellent siloxane surfaces induce self cleaning properties to the treated glass, exhibit good durability and furthermore do not affect practically the optical transparency of glass. Consequently, the suggested method can be used to induce superhydrophobicity and water repellency on glass surfaces used in buildings, automobiles and other applications. The self-cleaning property of the water repellent siloxane films is demonstrated in the video which is provided as Supporting information. Glass coated with a water repellent siloxane film was
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intentionally contaminated with dust of red ochre. The pigment is easily removed by water droplets, as shown in the video. The durability of the superhydrophobic siloxane surfaces is demonstrated with three experiments. First, contact angle measurements were carried out our over a wide range of the pH of the droplet. Figure 10 depicts θs and θt versus the pH of droplets which were placed on a siloxane surface treated at 600oC for 240 s. It is shown that both angles are stable over a wide range of pH, thus suggesting that the superhydrophobic and water repellent character of the film is stable for acidic and basic aqueous droplets. Second, θs and θt measurements were carried out on siloxane surfaces which were stored in the laboratory for two years and compared with the results obtained right after thermal treatment (fresh siloxanes). We report that the superhydrophobic and water repellent character of the treated siloxanes remained practically unaffected after two years of storage. Specific contact angle measurements for fresh and stored superhydrophobic siloxane surfaces are provided in the Supporting information. Third, a video is provided in Supporting information showing that the water repellent character of a treated glass is retained even after extensive exposure to a copious flow of water. Finally, colorimetric measurements carried out on untreated-bare glass and glasses coated by superhydrophobic and superhydrophilic siloxane films, showed that the application of the siloxane material had practically no effect on the aesthetic appearance and therefore the transparency of glass, as described in detail in the Supporting information and demonstrated in the photographs of Figures 3d and 3e.
Conclusions Treatment of a siloxane elastomer at very high temperatures results in surface structural changes, evolved because of melting and surface chemical modifications (Figures 2 - 5), which affect dramatically the wettability of the siloxane surface. For treatment at 650oC or higher, surface wettability is subjected to several transitions with treatment time as follows (Figures 1a, 6a, 7 and
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8): the intrinsic hydrophobic siloxane surface obtains superhydrophobic, non-sticking properties and
then
through
intermediate
stages
(hydrophobicity
and
hydrophilicity)
becomes
superhydrpophilic where the water drop collapses to a continuous film. For the time scale used herein (1 - 300 s) this scenario was recorded for treatment at 650, 700 and 800oC. For treatment at lower temperatures (400, 500 and 600oC) only the first transition, from intrinsic hydrophobicity to superhydrophobicity, was recorded (Figures 1b, 6b, 7 and 8). The evaporation of a droplet resting on a superhydrophobic, water repellent siloxane surface, which was produced after appropriate thermal treatment, was monitored. The results are provided in Figure 9 and show that droplet evaporation follows initially the constant contact area mode. At later evaporation stages a transition to the constant contact angle mode is recorded. Finally, it is shown the produced superydrophobic and water repellent siloxane surfaces induce self cleaning properties to the treated glass, exhibit good durability and furthermore do not affect practically the transparency of glass.
Acknowledgements I. Karapanagiotis is grateful for the support of Thales (Intermonu, 68/1117) and ERC-13 (ILGradMech-ASM, 88257) programs. The authors would like to thank the personnel of the Laboratory of Mechanics and Materials (Aristotle University of Thessaloniki) for useful discussions and Dr. Dimitrios Christofilos for his assistance in the FTIR measurements.
References 1. Rahimi, A., Shokrolahi, P. Application of inorganic polymeric materials I. Polysiloxanes. Int. J. Inorg. Mater. 2001, 3, 843-847. 2. de Morais, T. D.; Chaput, F.; Lahlil, K.; Boilot, J. -P. Hybrid Organic–Inorganic Light-Emitting Diodes. Adv. mater. 1999, 11, 107-112.
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3. Manoudis, P. N.; Karapanagiotis, I.; Tsakalof, A.; Zuburtikudis, I.; Kolinkeov, B.; Panayiotou, C. Superhydrophobic Films for the Protection of Outdoor Cultural Heritage Assets. Appl. Phys. A 2009, 97, 351-360. 4. Young T. An Essay on the Cohesion of Fluids. Philos. Trans. R. Soc. Lond. 1805, 95, 65-87. 5. Ayandele, E.; Sarkar, B.; Alexandridis, P. Polyhedral Oligomeric Silsesquioxane (POSS)Containing Polymer Nanocomposites. Nanomaterials 2012, 2, 445-475. 6. Schutzius, T. M.; Bayer I. S.; Jursich, G. M.; Dasa A.; Megaridis, C. M. Superhydrophobic– Superhydrophilic Binary Micropatterns by Localized Thermal Treatment of Polyhedral Oligomeric Silsesquioxane (POSS) - Silica Films. Nanoscale 2012, 4, 5378-5385. 7. Manoudis, P. N.; Karapanagiotis, I.; Tsakalof, A.; Zuburtikudis, I.; Panayiotou C. Superhydrophobic Composite Films Produced on Various Substrates. Langmuir 2008, 24, 1122511232. 8. Manoudis, P. N.; Karapanagiotis, I. Modification of the Wettability of Polymer Surfaces Using Nanoparticles. Prog. Org. Coat. 2014, 77, 331-338. 9. Polizos, G.; Tuncer, E.; Qiu, X.; Aytuǧ, T.; Kidder, M. K.; Messman, J. M.; Sauers, I. Nonfunctionalized Polydimethyl Siloxane Superhydrophobic Surfaces Based on Hydrophobic– Hydrophilic Interactions. Langmuir 2011, 27, 2953-2957. 10. Wenzel, R. N. Resistance of Solid Surfaces to Wetting by Water. Ind. Eng. Chem. 1936, 28, 988-994. 11. Cassie A. B. D.; Baxter, S. Wettability of Porous Surfaces. Trans. Faraday Soc. 1944, 40, 546551. 12. Mahadik S. A.; Mahadik, D. B.; Kavale, M. S.; Parale, V. G.; Wagh, P. B.; Barshilia, H. C.; Gupta, S. C.; Hegde, N. D.; Rao, A. V. Thermally Stable and Transparent Superhydrophobic Sol– Gel Coatings by Spray Method. J. Sol-Gel Sci. Technol. 2012, 63, 580-586.
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13. Budunoglu, H.; Yildirim, A.; Guler, M. O.; Bayindir M. Highly Transparent, Flexible, and Thermally Stable Superhydrophobic ORMOSIL Aerogel Thin Films. ACS Appl. Mater. Interfaces 2011, 3, 539-545. 14. Wohl, C. J.; Belcher, M.A.; Chen, L.; Connell, J.W. Laser Ablative Patterning of Copoly(imide siloxane)s Generating Superhydrophobic Surfaces. Langmuir 2010, 26,11469-11478. 15. Peng, Y. -T.; Lo, K. -F.; Juang, Y. -J. Constructing a Superhydrophobic Surface on Polydimethylsiloxane via Spin Coating and Vapor−Liquid Sol−Gel Process. Langmuir 2010, 26, 5167-5171. 16. Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Perry, C. C. Intrinsically Superhydrophobic Organosilica Sol−Gel Foams. Langmuir 2003, 19, 5626-5631. 17. Lin, Y. -H.; Su, K. -L.; Tsai, P. -S.; Chuang, F. -L.; Yang, Y. -M. Fabrication and Characterization of Transparent Superhydrophilic/Superhydrophobic Silica Nanoparticulate Thin Films. Thin Solid Films 2011, 519, 5450-5455. 18. Rao, V. A.; Kulkarni, M. M. Hydrophobic Properties of TMOS/TMES-Based Silica Aerogels. Mater. Res. Bull. 2002, 37, 1667-1677. 19. Cha, S. -C.; Her, E. K.; Ko, T. -J.; Kim, S. J.; Roh, H.; Lee, K. -R.; Oh, K. H.; Moon, M.-W. Thermal Stability of Superhydrophobic, Nanostructured Surfaces. J. Colloid Interf. Sci. 2013, 391, 152-157. 20. Laine, R. M.; Rahn, J. A.; Youngdahl, K. A.; Babonneau, F. Hoppe, M. L., Zhang, Z. F.; Harrod, J. F. Synthesis and High-Temperature Chemistry of Methylsilsesquioxane Polymers Produced by Titanium-Catalyzed Redistribution of Methylhydridooligo- and -polysiloxanes. Chem. Mater. 1990, 2, 464-472. 21. Zhang, X.; Tan. S.; Zhao, N.; Guo, X.; Zhang, X.; Zhang, Y.; Xu, J. Evaporation of Sessile Water Droplets on Superhydrophobic Natural Lotus and Biomimetic Polymer Surfaces. Chemphyschem 2006, 7, 2067-2070.
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22. Gelderblom, H.; Marín, Á. G.; Nair, H.; van Houselt, A.; Lefferts, L.; Snoeijer, J. H.; Lohse, D. How Water Droplets Evaporate on a Superhydrophobic Substrate. Phys. Rev. E 2011, 83, 026306. 23. Xu W.; Choi, C. -H. From Sticky to Slippery Droplets: Dynamics of Contact Line Depinning on Superhydrophobic Surfaces. Phys. Rev. Lett. 2012, 109, 024504. 24. Xu, W.; Leeladhar, R.; Kang, Y. T.; Choi, C. -H. Evaporation Kinetics of Sessile Water Droplets on Micropillared Superhydrophobic Surfaces. Langmuir 2013, 29, 6032-6041. 25. Nagai, H. Infrared Spectra of Stereoregular Polymethyl Methacrylate. J. Appl. Polym. Sci. 1963, 7, 1697-1714. 26. Domingo, C.; de Buergo, M. A.; Sánchez-Cortés, S.; Fort, R.; Garćia-Ramos, J. V.; GomezHeras, M. Possibilities of Monitoring the Polymerization Process of Silicon-Based Water Repellents and Consolidants in Stones Through Infrared and Raman Spectroscopy. Prog. Org. Coat. 2008, 63, 5-12. 27. Jayes, L.; Hard, A.P.; Séné, C.; Parker, S. F.; Jayasooriya U. A. Vibrational Spectroscopic Analysis of Silicones: A Fourier Transform-Raman and Inelastic Neutron Scattering Investigation. Anal. Chem. 2003, 75, 742-746. 28. Shirtcliffe, N. J.; McHale, G.; Newton, M. I. ; Perry C. C.; Roach, P. Porous Materials Show Superhydrophobic to Superhydrophilic Switching. Chem. Commun. 2005, 3135-3137. 29. Feng, L.; Zhang, Y.; Xi, J.; Zhu, Y.; Wang, N.; Xia, F.; Jiang, L. Petal Effect: A Superhydrophobic State with High Adhesive Force. Langmuir 2008, 24, 4114-4119.
FIGURE CAPTIONS
Figure 1. Static contact angle (θs) vs t, for treatment at (a) 800, 700 and 650oC - high temperatures and (b) 600, 500 and 400oC - low temperatures. Arrows that emphasize the effect of treatment
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temperature were drawn to guide the eye. Photographs of water drops corresponding to different wetting regimes are included. Distinct wetting regimes are divided by dotted lines. Figure 2. Surface-structure evolution of siloxane after treatment at high temperature (800oC). Contact angle (θs), treatment time (t) and temperature are shown in the upper left corner of each SEM image; the untreated film is shown in (a) which corresponds to t = 0. Thermal treatment, results in surface structure evolution which is accompanied by a wettability transition from hydrohpobicity (a and b) to superhydrophobicity (c and d). Figure 3. SEM images of (a) hydrophobic, (b) superhydrophilic and (c) superhydrophobic surfaces. Contact angle (θs), treatment time (t) and temperature are shown in the upper left corner of each SEM image. The hydrophobic surface in (a) was captured during the transition from superhydrophobicity to superhydrophilicity (Figure 1a). The superhydrophobic surface in (c) was produced after extensive treatment of the siloxane film at a relatively low temperature (600oC). The three surfaces included in the figure have similar structures and furthermore display similar morphologies with the superhydrophobic surfaces developed after heating the siloxane at a high temperature (800oC) for short periods (Figures 2c and 2d). Water on (d) superhydrophilic and (e) superhydrophobic siloxane films on glass are shown. The photographs demonstrate the transparency of the siloxane films. Figure 4. FTIR spectra of siloxane films treated at 800oC for various periods (t). The wettabilities of the siloxane surfaces are described according to the results of Figure 1a. The structures of the siloxane surfaces are shown in Figures 2, 3a and 3b, as noted. Figure 5. Raman spectra of siloxane films treated at 800oC for various periods (t). Asterisks mark the emergence of a new broad peak at 400-450 cm-1. Arrows indicate the peaks which are attributed to the glass substrate and are strongly recorded in some spectra (e.g. film treated at 800oC for t = 50 s). Wettabilities and structures of the siloxane surfaces are described, according to the results reported in Figures 1a, 2, 3a and 3b.
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Figure 6. Tilt contact angle (θt) vs t for treatment at (a) 700oC - high temperature and (b) 600oC low temperature. Measurements of θs and wetting regimes presented and defined in Figures 1a and 1b are reproduced herein to assist the discussion. In the superhydrophilic regime included in (a), θt was not possible to be measured. Figure 7. Tilt contact angle (θt) vs t for various treatment temperatures. The result for the untreated siloxane surface (t = 0) is included. Treatment temperatures are provided on top of the plot bars to guide the eye. Asterisks are used to mark superhydrophilic surfaces where θt could not be experimentally measured and is visualised with the theoretical maximum value (180o). Figure 8. Treatment temperature vs t. The graph shows the various wettability regimes of the siloxane surface. The “transition” regime corresponds to the intermediate, narrow hydrophobic and hydrophilic regimes recorded during the transition from superhydrophobicity to superhydrophilicity (Figure 1a). The exact borders of the transition regime between 650 and 600oC cannot be defined accurately, as these are the temperatures where the change in the wettability behaviour of the siloxane surface was recorded: at 650oC the wettability of the surface is subjected to several transitions
(Figure
1a)
and
at
600oC
only
one
transition
from
hydrophobicity
to
superhydrophobicity (Figure 1b) is recorded at the time scale of the experiment. Consequently this obscured area is marked with gray lines. Figure 9. (a) Contact angle (θs) and contact diameter (d) of a droplet on a superhydrophobic siloxane surface vs evaporation time (tev). The siloxane surface was prepared by thermal treatment at 800oC for 30 s and is shown in Figure 2d. The scale of the diameter d, included in the y-axis of the plot, is smaller than the capillary length which for water at room temperature is 2.7mm. Normalized droplet mass to the 2/3 power vs tev are shown in (b) and (c) for the two distinct evaporation modes. Figure 10. Static (θs) and tilt (θt) contact angle vs the pH of resting droplets, which were placed on the surface of a superhydrophobic and water repellent siloxane film produced after treatment at 600oC for 240 s. Photographs of some droplets are included.
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o
800 C o
700 C Superhydrophobicity, θs>150o
o
650 C 160
Hydrophobicity, 150o>θs>90o
S
o
θ ()
120
80 Hydrophilicity, 90o>θs>5o treatment temperature
40
0
Superhydrophophilicity θs150o
o
400 C 160 treatment temperature
Hydrophobicity, 150o>θs>90o
120
S
o
θ ()
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
80
40
0 0
50
100
150 t (sec)
200
250
300
(b) Figure 1.
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Hydrophobic (a)
θs=115o;
Hydrophobic (b) θs=121o; t=10s; 800oC
t=0
5µm
5µm
Superhydrophobic
Superhydrophobic (c) θs
=161o;
t=20s;
800oC
(d) θs=163o; t=30s; 800oC
5µm
5µm
Figure 2.
Superhydrophilic
Hydrophobic (a) θs
=122o;
t=50s;
800oC
(b) θs=0; t=70s;
Superhydrophobic
800oC
(c) θs=163o; t=150s; 600oC
1µm
1µm
Superhydrophilic
Superhydrophobic
(d)
(e)
Figure 3.
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FTIR spectrum
Treatment time (t), wettability, surface structure no treatment, hydrophobic, Fig. 2a
t=10 s, hydrophobic, Fig. 2b
t=20 s, superhydrophobic, Fig. 2c
t=30 s, superhydrophobic, Fig. 2d
t=40 s, superhydrophobic
t=50 s, hydrophobic, Fig. 3a
t=70 s, superhydrophilic, Fig. 3b
Figure 4.
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Raman spectrum
Treatment time, wettability, surface structure
no treatment, hydrophobic, Fig. 2a
t=10 s, hydrophobic, Fig. 2b t=20 s, superhydrophobic, Fig. 2c t=30 s, superhydrophobic, Fig. 2d
t=40 s, superhydrophobic
t=50 s, hydrophobic, Fig. 3a t=70 s, superhydrophilic, Fig. 3b
Figure 5.
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24
Superhydrophophilicity
120
700 oC
S
o
θ and θ ( )
SuperhydroHydroHydrophilicity phobicity phobicity Hydrophobi160 city
θ
80
t
θ 40
S t
Water repellency
0 0
50
100
150
t (s)
(a)
160
Hydrophobicity
Superhydrophobicity
600 oC
120
θ
S
o
θ and θ ( )
80
θ
t
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
S t
40 Water repellency
0 0
50
100
150
200
t (s)
(b) Figure 6.
ACS Paragon Plus Environment
250
300
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25
o o
800 (*)
700 C o
40
800 (*) 700 (*) 650 (*)
800 C
650 C o
600 C 35
o
500 C 700
o
400 C 400
30
400 500
600 500
600
800
5
800 700 650 600
700
10
500
600
untreated
650
15
400
20
400
500
o
t
650
25 θ ()
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
0 0
20
40 t (s)
60
Figure 7.
ACS Paragon Plus Environment
120
Langmuir
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26
o
θ = 150 S
800
o
n itio
700
o
Treatment temperature ( C)
ns tra
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
θ = 150 S
o
θ =5
superhydrophilicity
S
600 supe rhyd roph obic
500
ity
hydrophobicity (intrinsic wettabillty)
400
300 0
50
100
150
200
250
t (sec)
Figure 8.
ACS Paragon Plus Environment
300
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27
180
1,5
160
120
S
1
constant contact angle
d (mm)
constant contact area
o
θ ()
140
100
0,5
80 θ
60
S
d 40
0 0
500
1000
1500
2000
2500
3000
3500
t (s) ev
(a)
2/3
1
0,8
0,8
o
o
(m/m )
2/3
1
(m/m )
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
constant contact area
0,6
constant contact angle
0,6
0,4
0,4 0
200 400 600 800 1000 t (s) ev
0
1000 2000 3000 4000 t (s)
(b)
ev
(c)
Figure 9.
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28
θ
170
S
S
o
θ ()
160 150 140 130 30 120 25 20 t
o
θ ()
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
15
θ
10 5 0 0
2
4
6
pH
8
10
12
Figure 10.
ACS Paragon Plus Environment
14
t
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FOR TABLE OF CONTENTS USE ONLY
o
700 C
Superhydrophobicity, θs>150o
o
600 C 160
Hydrophobicity, 150o>θs>90o 120
S
o
θ ()
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
80
Hydrophilicity, 90o>θs>5o 40
Superhydrophophilicity θs
