pubs.acs.org/Langmuir © 2010 American Chemical Society
Water-Collecting Capability of Radial-Wettability Gradient Surfaces Generated by Controlled Surface Reactions Daewha Hong, Woo Kyung Cho, Bokyung Kong, and Insung S. Choi* Molecular-Level Interface Research Center, Department of Chemistry, KAIST, Daejeon 305-701, Korea Received June 11, 2010. Revised Manuscript Received August 11, 2010 In this work, we developed a controlled oxidation reaction of vinyl-terminated self-assembled monolayers (SAMs) to carboxylic acid-terminated ones to generate radially inward wettability gradient surfaces. The hydrophobicity was introduced on a silicon wafer by SAMs of 10-undecenyltrichlorosilane, and after the initial drop in oxidation, followed by the dilution-by-dropping method, radial-wettability gradient surfaces having hydrophilic centers and hydrophobic exteriors were generated. This direct drop reaction on the SAMs did not require an elastomeric stamp to be fabricated, which allowed for facile tuning of the gradients in terms of sizes and shapes. The fabricated wettability gradient surfaces possessed a water-collecting capability toward the hydrophilic center, which was inactive on previous linear wettability gradient surfaces.
Introduction The wettability gradient, from hydrophobic to hydrophilic states or vice versa, at surfaces finds its applications in various areas including microfluidic devices, nanometrology, and fluid transport.1-6 In particular, the spontaneous transport of water droplets has been attempted on wettability gradient surfaces with the aim of developing energy-free transportation systems.3-6 A number of methods for generating wettability gradients have been reported so far, such as vapor-phase diffusion,3,4,7,8 photodegradation,5 gradual immersion,9,10 and microcontact printing.11,12 The reported gradients were generally restricted to the linear form,2,3,5,6,8-10 thus the only 1D linear movement of a liquid droplet was observed. A tighter control of fluid transport, however, would be realized by fabricating 2D radial-wettability gradients that are more complex than the 1D linear gradients. The 2D radial gradients are either radially outward or inward. The radially outward wettability gradient has a maximum hydrophobicity at its center, and water droplets would move outward; the radially inward one has a reversed wettability, and water droplets would move inward and be collected at the center. There have been a few reports on radial-wettability gradients, mainly based on the formation of self-assembled monolayers (SAMs).4,7,11,12 Chaudhury generated radially outward and inward wettability gradients by forming the SAMs of alkyltrichlorosilane via vaporphase diffusion.7 The radial chemical gradients of SAMs were also fabricated by a sophisticatedly modified, practically demand*Corresponding author. E-mail:
[email protected].
(1) Grunze, M. Science 1999, 283, 41. (2) Julthongpiput, D.; Fasolka, M. J.; Zhang, W.; Nguyen, T.; Amis, E. J. Nano Lett. 2005, 5, 1535. (3) Chaudhury, M. K.; Whitesides, G. M. Science 1992, 256, 1539. (4) Daniel, S.; Chaudhury, M. K.; Chen, J. C. Science 2001, 291, 633. (5) Ito, Y.; Heydari, M.; Hashimoto, A.; Konno, T.; Hirasawa, A.; Hori, S.; Kurita, K.; Nakajima, A. Langmuir 2007, 23, 1845. (6) Zhang, J.; Han, Y. Langmuir 2007, 23, 6136. (7) Chaudhury, M. K.; Daniel, S.; Callow, M. E.; Callow, J. A.; Finlay, J. A. Biointerphases 2006, 1, 18. (8) Suda, H.; Yamada, S. Langmuir 2003, 19, 529. (9) Morgenthaler, S.; Lee, S.; Zrcher, S.; Spencer, N. D. Langmuir 2003, 19, 10459. (10) Yu, X.; Wang, Z.; Jiang, Y.; Zhang, X. Langmuir 2006, 22, 4483. (11) Choi, S.-H.; Newby, B.-m. Z. Langmuir 2003, 19, 7427. (12) Kraus, T.; Stutz, R.; Balmer, T. E.; Schmid, H.; Malaquin, L.; Spencer, N. D.; Wolf, H. Langmuir 2005, 21, 7796.
15080 DOI: 10.1021/la102379s
ing version of microcontact printing (μCP).11,12 On the basis of our previous studies of chemical reactions on SAMs and wettability control,13-15 we in this work suggest a surface chemistry approach to generating a radially inward wettability gradient that does not require an elastomeric stamp to be fabricated and is simple, scalable, and capable of simultaneously generating the gradients at multiple points on the surface. The spontaneous movement of a water droplet was observed on both flat and tilted surfaces of the fabricated radial-wettability gradient, and the gradient surface had an ability to collect water droplets toward the hydrophilic center.
Experimental Section Materials. 10-Undecenyltrichlorosilane (CH2dCH(CH2)9SiCl3, Gelest, Inc.), toluene (99.9%, J. T. Baker), potassium permanganate (KMnO4, 99þ%, Aldrich), sodium periodate (NaIO4, 99%, Aldrich), potassium carbonate (K2CO3, 99þ%, Aldrich), sodium hydrogensulfite (NaHSO3, ACS reagent, Aldrich), hydrochloric acid (HCl, 35%, Junsei), and absolute ethanol (EtOH, 99.9%, J. T. Baker) were used as received. Ultrapure water (18.3 MΩ 3 cm) from the Human Ultra Pure System (Human Corp., Korea) was used. The polished silicon (100) wafers were purchased from Tasco (Korea). Experimental Procedures. Silicon substrates were rinsed with H2O and EtOH and dried in a stream of argon. The cleaned Si/SiO2 substrates were oxidized by an oxygen plasma cleaner (Harrick PDC-002, medium setting) for 1 min to maximize hydroxyl groups on the surfaces. The SAMs were formed by immersing freshly cleaned Si/SiO2 substrates in a 0.1% (v/v) toluene solution of 10-undecenyltrichlorosilane for 30 min. After the formation of the SAMs, the silicon substrates were washed carefully with toluene several times. Stock solutions of KMnO4 (5 mM), NaIO4 (195 mM), and K2CO3 (18 mM) in water were prepared.16 OS-100 was made by combining 5 mL of each solution with 15 mL of water, and OS-20 and OS-2 were obtained by dilution. Silicon substrates coated with vinyl SAMs were (13) Cho, W. K.; Choi, I. S. Adv. Funct. Mater. 2008, 18, 1089. (14) Cho, W. K.; Kang, S. M.; Kim, D. J.; Yang, S. H.; Choi, I. S. Langmuir 2006, 22, 11208. (15) Cho, W. K.; Park, S. J.; Jon, S.; Choi, I. S. Nanotechnology 2007, 18, 395602. (16) Wasserman, S. R.; Tao, Y.-T.; Whitesides, G. M. Langmuir 1989, 5, 1074.
Published on Web 08/24/2010
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placed in 30 mL of OS-100. The samples were removed from the oxidant solution after 4 min and rinsed sequentially with 0.2 M NaHSO3, H2O, 0.1 M HCl, H2O, EtOH, and H2O. The water contact angle and thickness of the samples were measured after the washing process, and the samples were placed back in the oxidant solution. The washing and measuring steps were done every 4 min, and the total time duration was 20 min. The same procedures were applied for OS-20 and OS-2. Radial gradient surfaces were generated by tuning the extent of oxidation. On a silicon wafer coated with 10-undecenyltrichlorosilane (1.2 cm 1.2 cm) was placed a 4 μL drop of OS-100, and after 1 min, a predetermined volume of distilled water was added every 1 min (2, 2, 4, 4, 6, 6,..., 20, and 20 μL). The total reaction time was 21 min (including the 1 min reaction after the last drop at 20 min). The resulting substrates were washed carefully with 0.2 M NaHSO3, H2O, 0.1 M HCl, H2O, EtOH, and H2O. Characterizations. Contact angle measurements were performed using a Phoenix 300 goniometer (Surface Electro Optics Co., Ltd., Korea) on the basis of the sessile drop method. The thickness of the SAMs of 10-undecenyltrichlorosilane was measured with a Gaertner L116s ellipsometer (Gaertner Scientific Corporation, IL) equipped with a He-Ne laser (632.8 nm) at a 70° angle of incidence. A refractive index of 1.46 was used. Humidity was generated with a ultrasonic humidifier (H-656P, LG), and the movement of water droplets was videotaped in real time using a high-definition camcorder (SC-HMX20C, Samsung).
Results and Discussion Our strategy was based on the simple oxidation reaction of vinyl groups to carboxylic acid groups. Vinyl-terminated SAMs were reported to be oxidized to carboxylic acid-terminated ones under oxidizing conditions (KMnO4 and NaIO4) by Whitesides:16 the reported oxidation kinetics showed that the initial water contact angle of vinyl-terminated SAMs decreased exponentially within 20 min and leveled off afterwards, with 0.5 mM KMnO4 and 19.5 mM NaIO4. This leveling off implied that the oxidant concentrations should be controlled on top of the reaction time to tune the extent of oxidation effectively. The simultaneous control of these two parameters, oxidant concentration and reaction time, was achieved by the dilution-by-dropping method in this work (Figure 1). This technique required the nonwettability of a reactant droplet on the substrate surface; a radial pattern could not be maintained on a wettable surface because of the spreading of a droplet. In this respect, the vinyl-terminated SAMs were considered to be suitable because their water contact angle was measured to be 96 ( 2°. The hydrophobicity guaranteed that a droplet of the oxidant solution formed a spherical cap shape. The subsequent dropping of pure water increased the volume (or contact area) of the oxidant droplet and diluted the oxidant solution simultaneously. The dilution caused the newly contacted area to be oxidized more slowly than the previously contacted area. In addition, the reaction time was shorter for the newly contacted area. The reaction time, the amount of solutions (oxidant solution and water), and the initial concentration of the oxidant solution were optimized to obtain a steep wettability gradient in the radial form. The SAMs of 10-undecenyltrichlorosilane were formed on Si/SiO2, and the oxidation kinetics were investigated by water contact angle measurements prior to employment of the dilutionby-dropping process (Figure 2). The concentration of the oxidant solution was varied: the initial oxidant solution contained 0.8 mM KMnO4, 32.5 mM NaIO4, and 3 mM K2CO3 (denoted as the 100% oxidant solution; OS-100), and was diluted 5-fold and 50fold with water. In other words, the relative concentrations of the diluted solutions were 20 and 2%, respectively, compared with the initial 100% oxidant solution (each solution was denoted OS-20 Langmuir 2010, 26(19), 15080–15083
Figure 1. Procedure for the generation of radially inward wettability gradient surfaces by the dilution-by-dropping method. Vinylterminated SAMs were oxidized to carboxylic acid (COOH)terminated SAMs in a controlled way to generate the gradient. The purple droplet indicates the oxidant solution, and the blue droplet indicates pure water.
and OS-2, respectively.) The oxidation rate was monitored by water contact measurements. As expected, the higher concentration resulted in a faster oxidation rate. In the case of OS-100 and OS-20, the sharp decrease in the contact angles was observed within 4 min. In contrast, the contact angle decreased linearly in the case of OS-2. In addition, the contact angles at 20 min were dependent upon the concentration (36 ( 2° for OS-100, 41 ( 2° for OS-20, and 72 ( 4° for OS-2). The kinetic data indicated that the extent of oxidation at and near the center of the radial gradient would be controlled more facilely by the oxidant concentration than by the reaction time. These areas were exposed to the oxidant solution longer (21 min maximum in this study) than were the outer areas; for the generation of a wettability gradient, the oxidant solution should be diluted in such a fashion that full oxidation would not occur within the reaction time. Therefore, we changed the concentration DOI: 10.1021/la102379s
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Figure 3. Movement of a water droplet on the radial-wettability gradient surface: (a) flat surface and (b) tilted surface. The contact angles of the water droplet on the left and right sides were denoted θl and θr, respectively. The scale bar is 1 mm.
Figure 2. (a) Oxidation kinetics of vinyl-terminated SAMs: OS100 (9), OS-20 (b), and OS-2 (2). (b) Relative concentration of the oxidant solution by the dilution-by-dropping method. The value of the y axis (N) corresponds to OS-N. OS-100 is the oxidant solution composed of 0.8 mM KMnO4, 32.5 mM NaIO4, and 3 mM K2CO3. The x axis is divided into three parts: the oxidation rate depends mainly on the concentration of the oxidant solution (04 min), on both the concentration of the oxidant solution and the reaction time (4-13 min), and on the reaction time (13-20 min).
more dramatically in the early stages than in the later stages. Figure 2b shows the concentration change employed in this work. With 4 μL of OS-100 as an initial quantity, the oxidant solution on the vinyl-terminated surface was diluted by the dilutionby-dropping process. Every 1 min, a specific amount of water was dropped sequentially: the amounts were 2, 2, 4, 4, 6, 6,..., 20, and 20 μL in 20 min. The dropping diluted the oxidant solution profoundly in the early stages, and OS-100 was diluted to OS-25 in the first 4 min; because the extent of oxidation near the center was mostly determined by the concentration, the wettability difference was realized. In contrast, OS-2 (the diluted state at 13 min) was further diluted at the slower pace from 13 to 20 min. Therefore, the reaction time dictated the extent of oxidation in this later stage. Both the reaction time and oxidant concentration were believed to affect the extent of oxidation in the period of 4 to 13 min. All of these combinations of concentration and reaction time proved effective in generating the radial-wettability gradient. To confirm the generation of the radial gradient surface, a water droplet was placed arbitrarily at the outer area of the radial gradient. The water droplet moved toward the hydrophilic center regardless of its initial location. Figure 3 shows a photograph of the moving water droplet on flat and tilted surfaces, respectively, with the water contact angles on each side. The contact angle was measured to change from 94 to 52° over a distance of 5 mm, 15082 DOI: 10.1021/la102379s
indicative of the wettability gradient. The value of 52° implied that the full oxidation to carboxylic acid (contact angle 36 ( 2°) did not occur because of the dilution. The migration of a water droplet was also observed even on the tilted gradient surface (Figure 3b), which means that the intrinsic surface free energy dragged the water droplet against gravity. It is interesting that the differences between θl and θr in each stage were observed to be larger for the tilted surface than for the flat surface, which might be caused by a gravity effect. The radial-wettability gradient was also confirmed by placing two droplets on the tilted surface, one on the left and the other to the right of center. Two droplets moved toward the more hydrophilic center and merged there (Figure 4a). Taken together, these results clearly showed that the radially inward wettability gradient was successfully generated simply by using the dilution-by-dropping method. To demonstrate a potential application of the radial gradient fabricated-a water-harvesting device from humidity-we applied “mist” onto the surface that had a hole at the hydrophilic center with a diameter of 1 mm (Figure 4b).17 Dewdrops were formed by the mist and condensed to form a water droplet on the surface. When the size of the water droplet reached a certain value (∼10 μL), the droplet started moving toward the hydrophilic center and was collected through the hole. In short, the radially inward gradient surface had an ability to gather water droplets at the designated point; this water-collecting capability could be combined with other microfludic devices. The surface chemistry-based fabrication, in principle, would make it easy to tune the diameter and steepness of the radialwettability gradient. On the basis of the oxidation kinetics, dilution-by-dropping was an adjustable approach to the generation of expanded radial-wettability gradient surfaces by controlling the extent of the oxidation. As a proof-of-demonstration, the volume of both the oxidant solution and distilled water was (17) The same procedure was performed except for the fabrication of a pore in the silicon substrate. A milling machine was used to generate a pore with a diameter of 1 mm. After undergoing punching, the silicon wafer was placed in a solution of concentrated H2SO4 and 30% H2O2 and washed with water to remove dirt, followed by the formation of vinyl-terminated SAMs.
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Figure 4. (a) Movement of two water droplets toward the hydrophilic center, indicative of the radial gradient. The scale bar is 2 mm. (b) Photographs of the water collection. Artificial humidity was applied to the radial gradient surface. Dewdrops coalesced and moved toward the hydrophilic center. Water droplets were collected at the center and went through the hole. Two representative water droplets are indicated by the arrows. The scale bar is 2 mm.
decreased to a value similar to the initial one. The observed fast drop movement in the middle implied that the wettability gradient was not linear and was greater in the middle region than in the other regions. In contrast, the 20-mm-wide system showed a relatively constant velocity of a water droplet (Figure 5b): a constant velocity of 0.4 mm/s was observed, which was indicative of the generation of a linear wettability gradient. This result implied that the gradient morphology and the size of the gradient could be controlled by simple surface-based chemistry. In addition, because the initial dropping site determines the center of the radial gradient, in principle it would be possible to generate multipoint radial gradients with different morphologies.
Conclusions We fabricated a radially inward wettability gradient surface by the simple dilution-by-dropping method based on the oxidation of vinyl-terminated SAMs to carboxylic acid-terminated ones. The fabricated radially inward wettability gradient showed the capability to collect water droplets at its hydrophilic center, which we believe would be beneficial in the development of water-harvesting devices. This direct drop reaction on the SAMs was relatively insensitive to humidity variations and did not require an elastomeric stamp to be fabricated, which allowed for the facile tuning of the gradients in terms of sizes and shapes. In addition, functional groups to be investigated would not be limited to vinyl groups, and other functional groups could be employed for organic transformations on SAMs and polymer films to generate gradients. In this respect, we believe that the radial-wettability gradient surfaces, generated by the dilution-by-dropping method, would find many different applications ranging from water collection to cell adhesion.18-22
Figure 5. Distance vs time in the movement of a water droplet (∼5 μL) on a radial-wettability gradient surface with diameters of (a) 10 and (b) 20 mm.
increased five-fold; the other reaction conditions were the same. The increase in the volume increased the gradient diameter to 20 mm. Of more interest, this simple increase in the volume also changed the shape/steepness of the gradient. The 10-mm-wide gradient showed three distinctive ranges of velocity (Figure 5a): the initial velocity of 0.7 mm/s was observed until about 0.8 s, and after that it was greatly increased to 19.6 mm/s. The velocity was then Langmuir 2010, 26(19), 15080–15083
Acknowledgment. This work was supported by an HRHRP grant from KAIST and a National Research Foundation of Korea (NRF) grant funded by the Korean Government (MEST) (2010-0001953). (18) Dertinger, S. K. W.; Jiang, X.; Li, Z.; Murthy, V. N.; Whitesides, G. M. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 12542. (19) Simon, K. A.; Burton, E. A.; Han, Y.; Li, J.; Huang, A.; Luk, Y.-Y. J. Am. Chem. Soc. 2007, 129, 4892. (20) Gallant, N. D.; Lavery, K. A.; Amis, E. J.; Becker, M. L. Adv. Mater. 2007, 19, 965. (21) Song, W.; Veiga, D. D.; Custodio, C. A.; Mano, J. F. Adv. Mater. 2009, 21, 1830. (22) Petty, R. T.; Li, H.-W.; Maduram, J. H.; Ismagilov, R.; Mrksich, M. J. Am. Chem. Soc. 2007, 129, 8966.
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