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Electrowetting of Superhydrophobic ZnO Nanorods Jos Laurie Campbell,*,†,‡ Michael Breedon,‡ Kay Latham,§ and Kourosh Kalantar-zadeh† School of Electrical and Computer Engineering, and Departments of Biology and Biotechnology, and Applied Chemistry, School of Applied Sciences, RMIT UniVersity, Melbourne, Victoria 3001, Australia ReceiVed October 2, 2007. In Final Form: NoVember 29, 2007 This paper reports the electrowetting properties of ZnO nanorods. These nanorods were grown on indium tin oxide (ITO) substrates using different liquid-phase deposition techniques and hydrophobized with sputtered Teflon. The surfaces display superhydrophobic properties. When the applied voltages are less than 35 V, the contact angle change is small and exhibits instant reversibility. For higher voltages, larger contact angle changes were observed. However, the surface was not reversible after removing the applied voltage and required mechanical agitation to return to its initial superhydrophobic state.
Introduction Control of surface hydrophobicity is a highly desirable attribute, and it has become an area of increasing investigation in recent years, due to the wide range of possible applications.1-17 One method for modifying the hydrophobicity of a surface is through the application of an external electric field. Such a phenomenon is referred to as electrowetting, where the surface experiences a change in wettability upon the application of an electric field, which initiates a change in the contact angle between a liquid/ solid system.18-20 Typically, dielectric materials are utilized for the fabrication of electrically wettable surfaces for which the term EWOD (electrowetting-on-dielectric) is used.21-24 †
School of Electrical and Computer Engineering. Department of Biology and Biotechnology, School of Applied Sciences. § Department of Applied Chemistry, School of Applied Sciences. ‡
(1) Quilliet, C.; Berge, B. Curr. Opin. Colloid Interface Sci. 2001, 6 (1), 34-39. (2) Squires, T. M.; Quake, S. R. ReV. Mod. Phys. 2005, 77 (3), 977-1026. (3) Hessel, V.; Lowe, H.; Schonfeld, F. Chem. Eng. Sci. 2005, 60 (8-9), 2479-2501. (4) Mugele, F.; Baret, J. C. J. Phys.: Condens. Matter 2005, 17 (28), R705R774. (5) Crassous, J., et al. In Liquid lens based on electrowetting: a new adaptiVe component for imaging applications in consumer electronics, Proceedings of SPIE, Dec 23, 2004; Jiang, W., Suzuki, Y., Eds.; SPIE Adaptive Optics and Applications III; Vol. 5639. (6) Fair, R. B.; Sriniansan, V.; Ren, H.; Paik, V.; Pamula, K.; Pollack, M. G. Electrowetting-Based On-Chip Sample Processing for Integrated Microfluidics, Electron Devices Meeting, Duke University, Durham, NC, Dec 8-10, 2003. (7) Hoshino, K.; Triteyaprasert, S.; Matsumoto, K.; Shimoyama, I. Sens. Actuators, A 2004, 114 (2-3), 473-477. (8) Mach, P.; Krupenkin, T.; Yang, S.; Rogers, J. A. Appl. Phys. Lett. 2002, 81 (2), 202-204. (9) Pollack, M. G.; Fair, R. B.; Shenderov, A. D. Appl. Phys. Lett. 2000, 77 (11), 1725-1726. (10) Kuo, J. S.; Spicar-Mihalic, P.; Rodriguez, I.; Chiu, D. T. Langmuir 2003, 19 (2), 250-255. (11) Lee, J.; Kim, C. J. J. Microelectromech. Syst. 2000, 9 (2), 171-180. (12) Hsieh, J.; Mach, P.; Cattaneo, F.; Yang, S.; Krupenkin, T.; Baldwin, K.; Rogers, J. A. IEEE Photonics Technol. Lett. 2003, 15 (1), 81-83. (13) Cattaneo, F.; Baldwin, K.; Yang, S.; Krupenkin, T.; Ramachandran, S.; Rogers, J. A. J. Microelectromech. Syst. 2003, 12 (6), 907-912. (14) Gras, S. L.; Mahmud, T.; Rosengarten, G.; Mitchell, A.; Kalantar-zadeh, K. ChemPhysChem 2007, 8 (14), 2036-2050. (15) Berthier, J.; Dubois, P.; Clementz, P.; Claustre, P.; Peponnet, C.; Fouillet, V. Sens. Actuators, A 2007, 134 (2), 471-479. (16) Chang, Y.-W.; Kwok, D. Y. Biomed. MicrodeVices 2006, 8 (3), 215225. (17) Chen, C. Y.; Fabrizio, E. F.; Nadim, A.; Sterling, J. D. Electrowettingbased Microfluidic DeVices: Design Issues, Summer Bioengineering Conference, Sonesta Beach Resort, Key Biscayne, FL, June 25-29, 2003; Keck Graduate Institute of Applied Life Sciences: Claremont, CA, 2003. (18) Kang, K. H. Langmuir 2002, 18 (26), 10318-10322. (19) Verheijen, H. J. J.; Prins, M. W. J. Langmuir 1999, 15 (20), 6616-6620.
Typically, an electrowettable surface consists of a conductive lower layer with a dielectric layer sandwiched between the lower electrode and an upper hydrophobic material, such as fluoropolymer layers, which lowers the surface energy and confers a larger initial hydrophobicity (Figure 1). There are many factors which affect the performance of an electrowettable surface, such as surface energies and tensions of the materials.1-4,25-27 However, one of the limiting factors demonstrated is the initial contact angle prior to application of the electric field. Most experiments confirm that having the largest possible initial contact angle, in the absence of an electric field, has a considerable effect on the achievable performance of the surface. Consequently, the higher the initial contact angle, the greater the possible change in the actuated contact angle before the electric charge saturation in the dielectric material occurs. It is also observed that minimizing the hysteresis that occurs after actuation is of major importance.18,21 It has been observed that a hydrophobic upper coating of the fluoropolymer Teflon increases the initial contact angle to a maximum of 110-120° on a smooth planar surface. One method to increase the initial hydrophobicity to the realms of “superhydrophobicity” is through the incorporation of nanostructured arrays similar to those responsible for the lotus effect.28,29 By dramatically increasing surface roughness while decreasing the surface energy, contact angles in excess of 150° can be produced. Exploiting these principles, electrowettable surfaces can also be fabricated that exhibit superhydrophobicity. There are few reports in the literature demonstrating electrowetting on superhydrophobic nanostructured surfaces.30-35 (20) Welters, W. J. J.; Fokkink, L. G. J. Langmuir 1998, 14 (7), 1535-1538. (21) Moon, H.; Cho, S. K.; Garrell, R. L.; Kim, C. J. J. Appl. Phys. 2002, 92 (7), 4080-4087. (22) Lee, J.; Moon, H.; Fowler, J.; Schoellhammer, T.; Kim, C. J. Sens. Actuators, A 2002, 95 (2-3), 259-268. (23) Ralston, J. Aust. J. Chem. 2005, 58 (9), 644-654. (24) Schneemilch, M.; Welters, W. J. J.; Hayes, R. A.; Ralston, J. Langmuir 2000, 16 (6), 2924-2927. (25) Saeki, F.; Baum, J.; Moon, H.; Yoon, J. Y.; Kim, C. J.; Garrell, R. L. Electrowetting on dielectrics: Reducing Voltage requirements for microfluidics. Abstr. Pap. Am. Chem. Soc. 2001, 222, U341-U341. (26) Shapiro, B.; Moon, H.; Garrell, R.; Kim, C.-J. MEMS 2003 2003, 201205. (27) Shapiro, B.; Moon, H.; Garrell, R. L.; Kim, C. J. J. Appl. Phys. 2003, 93 (9), 5794-5811. (28) Zhu, L. B.; Xiu, Y.; Xu, J.; Tamirisa, P. A.; Hess, D. W.; Wong, C.-P. Langmuir 2005, 21 (24), 11208-11212. (29) Rios, P. F.; Dodiuk, H.; Kenig, S.; McCarthy, S.; Dotan, A. J. Adhes. Sci. Technol. 2007, 21 (5-6), 399-408.
10.1021/la7030413 CCC: $40.75 © 2008 American Chemical Society Published on Web 03/29/2008
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Figure 1. Schematic illustration of electrowetting: the solid and dashed lines show the droplet shape prior to and following application of an applied voltage. θ0 is the water contact angle when the applied voltage across the electrodes is zero (solid line), γLG, γSG, and γSL are surface tensions at the liquid-gas (air), solid-gas, and solid-liquid interfaces, respectively, and V is the applied voltage.
Work by Krupenkin et al.31,33 demonstrated nonreversible electrowetting on a surface covered in silicon nanoposts of 350 nm in diameter coated in low surface energy polymer which exhibited superhydrophobicity. Zhu et al.32 also demonstrated electrowetting on superhydrophobic nanostructured surfaces using arrays of aligned carbon nanotubes. Despite the seminal work by Liu36 demonstrating that metal oxides such as ZnO can be used for the fabrication of superhydrophobic surfaces and that large contact angle changes upon exposure to UV can be obtained, other types of energy stimuli, such as electric fields, have not yet been investigated. Despite the promising effect of various nanostructured surfaces, the electrowetting performance of nanostructured metal oxides has been almost neglected. Further investigation is required into the performance and material properties of various dielectrics for use in EWOD devices. This paper presents a comparison between the electrowetting performance of two slightly different nanostructured surfaces made from ZnO. The aqueous growth of ZnO nanostructures from supersaturated Zn2+/NaOH solutions and the hydrothermal decomposition of an equimolar Zn2+/hexamethylenetetramine (HMT) solutions have been used for the controlled growth of nanostructured arrays of ZnO. Such dielectric surfaces are rendered hydrophobic with radio frequency (RF) sputtered Teflon. The overall fabrication method is low cost, simple, and it can be easily scaled to suit larger substrate sizes. Methods and Experimental Procedures Indium tin oxide (ITO) coated glass was chosen as the substrate for growing ZnO and performing electrowetting experiments due to the ease of incorporating the conductive upper layer as the lower electrode. Cleaned 11 mm × 11 mm ITO coated glass substrates (sheet conductivity of approximately 10 Ω/cm) were covered with a 1.2 µm ZnO seed layer deposited by RF sputtering. The sputtering conditions are shown in Table 1. A ZnO seed layer is a necessary prerequisite, as while ZnO nanorods can directly grow onto raw ITO glass, the resulting nanorods (30) Verplanck, N.; Galopin, E.; Camart, J.-C.; Thomy, V.; Coffinier, Y.; Boukherroub, R. Nano Lett. 2007, 7 (3), 813-817. (31) Krupenkin, T.; Taylor, J. A.; Kolodner, P.; Hodes, M. Bell Labs Tech. J. 2005, 10 (3), 161-170. (32) Zhu, L. B.; Xu, J.; Xiu, Y.; Sun, Y.; Hess, D. W.; Wong, C.-P. J. Phys. Chem B 2006, 110 (32), 15945-15950. (33) Krupenkin, T. N.; Taylor, J. A.; Schneider, T. M.; Yang, S. Langmuir 2004, 20 (10), 3824-3827. (34) Dhindsa, M. S.; Smith, N. R.; Heikenfeld, J.; Rack, P. D.; Fowlkes, J. D.; Doktycz, M. J.; Melechko, A. V.; Simpson, M. L. Langmuir 2006, 22 (21), 9030-9034. (35) Wang, Z.; Ou, Y.; Lu, T.-M.; Koratkar, N. J. Phys. Chem. B 2007, 111 (17), 4296-4299. (36) Liu, H.; Feng, L.; Zhai, J.; Jiang, L.; Zhu, D. Langmuir 2004, 20 (14), 5659-5661.
Table 1. Sputtering Conditions for the Deposition of ZnO and Teflon Layers ZnO seed layer target to substrate distance sputtering power process gas temperature
Teflon
7.5 cm
7.5 cm
100 W 60% N2/40% O2 220 °C
80 W 100% Ar ambient temperature
Table 2. ZnO Nanorod Growth Conditions reaction temperature reaction time substrate dimensions HMT solution mix NaOH solution mix
80 °C 18 h 11 mm × 11 mm 10 mM Zn(NO3)2·6H2O + 10 mM C6H12N4 (HMT) 10 mM Zn(NO3)2·6H2O + 0.4 M NaOH
suffer from poor substrate coverage which manifests as sporadic growth. The addition of the sputtered ZnO seed layer improves the uniformity of the nanorods over the whole substrate, providing an excellent nanostructured template for electrowetting experimentation. The sputtered substrates were then placed in holders residing within the sealed reaction vessels. The ZnO nanorods were grown via two different chemical approaches as follows: (1) Hydrothermal decomposition of hexamethylenetetramine (HMT)/Zn(NO3)2‚6H2O solutions. This growth was described by Vayssieres et al.37(2) Epitaxial chemical growth with solutions of NaOH and Zn(NO3)2‚ 6H2O. This growth method was based on the paper by Peterson et al.38 During deposition, ZnO seed coated substrates were held inverted and submerged below the waterline of the reaction vessel. This serves to prevent the disruption of the nanostructured thin film with debris, such as precipitates generated during growth. Sealed reaction vessels were employed to avoid the evaporation of the growth solution, negating potential complications from solubility related instabilities of the aqueous zinc species. Reaction conditions for both methods are presented in Table 2. The reaction vessels were placed in a standard laboratory grade oven for 18 h, after which the substrates were removed, washed in DI water, and dried in a stream of dry nitrogen. Figures 2-4 highlight the differences in structure produced by the two different chemical deposition approaches, observed via transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Table 3 provides a comparison between the two different chemical methods. The nanostructured arrays produced by HMT solution had an approximate diameter of 100 nm, an average length of 1 µm, and a tip diameter of 10 nm, while the ZnO structures grown from NaOH solution had an average diameter of 200-250 nm, a length of 2 µm, and a tip diameter of 100 nm. (37) Vayssieres, L. AdV. Mater. 2003, 15 (5), 464-466. (38) Peterson, R. B.; Fields, C. L.; Gregg, B. A. Langmuir 2004, 20 (12), 5114-5118.
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Figure 3. TEM pictures of the ZnO nanorod tips. Figure 2. TEM images of ZnO nanorods. In order to infer the desired initial hydrophobicity to the surfaces, the nanorods were coated in an 85 nm thick layer of Teflon via RF sputtering. The sputtering conditions for the deposition of Teflon are shown in Table 1. Typically, NaOH derived ZnO nanorods are significantly larger than those grown from HMT solutions as seen in Table 3 and Figures 2-4. Notably, ZnO nanorods grown from either solution have an aspect ratio of approximately 10:1. X-ray diffractometry (XRD) patterns for the two ZnO growth methods and the ZnO seed layer on ITO glass were collected at room temperature on a Bruker D8 ADVANCE X-ray diffractometer, fitted with a graphite-monochromated copper tube source (Cu KR radiation, λ ) 1.5406 Å) and a scintillation counter detector. The X-ray diffractograms (Figure 6) of all three samples have a number of common peaks and show well-defined peaks conducive to the formation of highly crystalline surface species. A search of the International Centre for Powder Diffraction Data (ICDD) database identified the presence of ZnO in all cases, with excellent pattern matching to [36-1451], Zincite, syn, ZnO. The ITO peaks of the
substrate were also identified and matched against ICDD No. [880773] (indium tin oxide, In4Sn3O12). The main peaks of these patterns are labeled in Figure 6. The main differences between the XRD spectra are observed for the HMT directed ZnO growth. In comparison to the other spectra, and the database pattern [36-1451], the (002) reflection (∼34.5° 2θ) is enhanced. This corresponds to a higher proportion of c-axis oriented growth in the HMT directed ZnO samples in comparison to the NaOH directed ZnO samples. This is also confirmed by the SEM image in Figure 4b. However, a consequence of this higher crystallinity, and more oriented growth, is that the ITO substrate is more exposed in the HMT directed ZnO samples. Thus, the X-ray beam is able to more easily penetrate through the ZnO layer to the ITO beneath, and hence, ITO peaks are observed with greater intensity in the HMT directed sample than in the NaOH directed sample, and some minor peaks (e.g., ITO reflection at 32.5° 2θ) come into view for the first time. With ZnO and ITO identified, one peak remains to be assigned. This peak is only present in the NaOH directed sample and occurs at approximately 29.5° 2θ. On close examination of the SEM images of this sample, a number of plateletlike structures are observed:
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Figure 5. SEM image of the platelets that appear randomly on different regions of the NaOH directed surface. neither nitrogen or vanadium has been observed, one can only conclude that a crystalline phase which is isomorphic to either [230782] or [36-0837], and has a high chromium content, may be present in the NaOH directed sample and that further work is needed to confirm its identity.
Measurements and Discussions
Figure 4. SEM images of ZnO nanorods grown on ZnO sputtered ITO glass. Table 3. ZnO Nanorod Dimensional Comparison
rod end shape rod end diameter nominal rod length nominal rod diameter
ZnO nanorods (NaOH)
ZnO nanorods (HMT)
rounded (hemispherical) ∼100 nm ∼2 µm ∼200-250 nm
tapering to a tip ∼10 nm ∼1 µm ∼100 nm
structures with a completely different morphology from those of the ZnO nanorods (Figure 5). Energy dispersive X-ray (EDX) analysis of these features reveals the presence of chromium and calcium, together with expected levels of indium and zinc, a small amount of silicon, and enhanced levels of oxygen. This “chemistry” is in conflict with the composition of the phases which appear to match this “rogue” peak. A database search identified two possible matches. First, ammonium indium chloride hydrate ((NH4)2(InCl)5‚H2O) [ICDD 23-0782] or, alternatively, indium vanadium oxide (V3InO7) [ICDD 36-0837]. As
The setup in Figure 1 was employed to test the electrowettability of the hydrophobized nanostructured surfaces. A platinum wire was used as the counter electrode which was inserted into a droplet of either DI water or 0.3 M NaCl with a volume of 5 -20 µL. Subsequently, the ITO glass was connected to the negative voltage. Varying voltages were applied between the electrodes, and the behavior of the differing droplet solutions on both the HMT and NaOH directed ZnO surfaces was recorded. In large voltage ranges, nonreversible damage was caused to the surface by breakdown of the Teflon insulating layer, which allowed the current to pass through the system, electrolyzing the water and permanently rendering the surface hydrophilic. In this paper, in all recorded results, the surface was inspected carefully to ensure that the contact angle change had been caused by the electrowetting effect rather than damage. The electrowetting performance of HMT directed nanorods arrays was found to be better than that of NaOH directed nanorods arrays. This is primarily ascribed to the dimensional variation of the nanorods tips. As can be seen in Figure 3, the HMT directed ZnO nanorods taper to a fine 10 nm point whereas the NaOH directed ZnO nanorods have tips of 100 nm. It is believed that that this factor effectively reduces the available contact area on which the surface of liquid can impinge upon, causing enhanced superhydrophobicity in HMT directed ZnO samples.39,40 Due to the poor conformity, surface abnormalities, and microfissures in the sputtered Teflon layer deposited, the surfaces tested had varying performance at different sites across the sample. Figure 7 displays the highest initial contact angles obtained on the structures grown by the two different growth mechanisms. However, these regions on the surface did not exhibit the highest (39) Lau, K. K. S.; Bico, J.; Teo, K. B. K.; Chhowalla, M.; Amaratunga, G. A. J.; Milne, W. I.; McKinley, G. H.; Gleason, K. K. Nano Lett. 2003, 3 (12), 1701-1705. (40) Feng, L.; Li, S. H.; Li, Y. S.; Li, H. J.; Zhang, L. J.; Zhai, J.; Song, Y. L; Liu, B. Q.; Jiang, L.; Zhu, D. B. AdV. Mater. 2002, 14 (24), 1857-1860.
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Figure 6. XRD patterns of the sputtered ZnO, and ZnO nanorods deposited using the NaOH and HMT solutions.
Figure 7. Maximum initial contact angles achieved: (a) 5µL droplet of NaCl on the HMT directed nanorods at 0 V and (b) 5 µL droplet of NaCl on the NaOH directed nanorods at 0 V. Initial contact angles were equal for both the DI water and NaCl solutions.
electrowetting performances. It is assumed that the variation of surface performance can be ascribed to the lack of uniformity of the Teflon layers, which may lead to charge leakage arising between the droplet and substrate. Figure 7 shows the initial contact angle for droplets of NaCl solution on HMT and NaOH directed nanostructured layers for a 5 µL droplet. The maximum initial contact angle was measured to be 175° on the HMT directed ZnO surface, while on the NaOH directed ZnO surface the maximum initial contact angle observed was measured at 145°. For 5 µL droplets of NaCl solution, a very small contact angle change of approximately up to 10° was observed for voltages as large as 35 V. Up to this voltage, the hydrophobicity change was completely reversible. Interestingly, voltages greater than this caused a sharp transition between wetting states as can be observed in Figure 8 in which a contact angle change as large as 78° can be seen.
Figure 8. 5 µL droplet of NaCl on ZnO nanostructures grown from HMT solution before (a) and after (b) application of 40 V displaying a contact angle shift from 155° to 77°.
While the surface did not exhibit instant reversibility once the switching threshold was reached, the droplets were able to be drawn off the surface with gentle agitations, returning the surface back to a superhydrophobic state. Although hysteresis (5°-10°) effects occurred after this process, the reactuation of the droplet could be performed and a similar final contact angle was achieved at effectively the same voltage. The sudden change in the contact angle was also seen for the NaOH directed ZnO surfaces, with a smaller contact angle change of less than 20° being observed (Figure 9). The contact angle change when DI water droplets were used was significantly less than that of the NaCl solution. As can be seen in Figure 10, where 20 µL of DI water was actuated at 40 V, a contact angle shift of less than 20° was observed. It is most likely that the poor performance of DI water in comparison to the NaCl solution is due to the lack of charge
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Table 4. Comparison of Developed Nanostructured Surfaces
fabrication process ZnO nanorods
hydrothermal deposition
CNTs raw
CVD/solder reflux
silicon nanowires silicon nanoposts
VLS-CVD lithography-oven driven oxide growth
CNFs
DC-PECVD
CNTs parylene coated
thermal CVD
nanostructural geometry (base, dimension of the base, and length) 10 nm tip diameter, 1 µm length 100 nm tip diameter, 2 µm length 10 nm diameter, 20 nm spacing, 180 µm height 200 nm diameter, 18 µm height 300-500 nm diameter, 4-7 µm height, 1-4 µm spacing 25 nm tip radius, 10 µm height coated in different conformal coatings 10 nm diameter, 50 nm spacing, 400 µm length
carrying ions present in the DI water to allow charge accumulation between the surfaces.41 A comparison of electrowettability for different surfaces made of various nanorods or nanoposts has been demonstrated in Table 4. This table displays the parameters of different nanostructured electrowettable surfaces and their performance, fabrication process, and surface geometrical characteristics. As can be seen in Table 4, materials such as carbon and silicon nanostructures can be fabricated using chemical vapor deposition (CVD) techniques which can result in a homogenously distributed
Figure 9. 5 µL of NaCl actuated at 40 V on a NaOH directed ZnO surface with a contact angle shift from 140° (a) to 122° (b).
initial contact angle/ contact angle change/ voltage 155°/77°/40 V
ref this paper
140°/122°/40 V 155°/50°/10 V
32
164°/106°/150 V
30 31
158°/120°/44 V (surface covered in Al2O3) 158°/100°/80 V (surface covered in parylene) 108°/92°/80 V
34
35
and/or well-oriented nanotubes or nanorods. Conversely, ZnO nanorods can be fabricated in an aqueous environment without any need for complex and high cost fabrication techniques. The durability of such surfaces depends on the type of materials used. ZnO is known to be etched by a large number of different acids or bases.42 However, despite the fact that materials such as carbon nanotubes (CNTs) have high mechanical strength, their surface can rapidly adsorb biocontaminants when exposed to the ambient environment, which alters the surface properties.14
Figure 10. 20 µL droplet of DI water on a NaOH directed surface actuating at 40 V with a contact angle shift from 170° (a) to 151° (b).
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Figure 11. Schematic diagram of a droplet being drawn into the interstitial space before (top) and after (bottom) the threshold actuation voltage.
Silicon can also be oxidized easily, whereas ZnO, being an oxide, does not change its surface state when exposed to ambient conditions. The observed threshold switching of the droplet between the Cassie-Baxter (sitting on top of the tips) and Wenzel (being drawn into the interstitial spaces) morphologies occurred at a threshold voltage, and it is likely due to the buildup of surface charge reaching a point where it can overcome the higher local repulsion force at the Teflon tip interfaces (Figure 11). This bistable situation is believed to occur as a result of the largely different surface energies of the two areas of the surface, above the rods and between the rods, interacting with the droplet at different voltages. The surface energy of the upper tips of the nanorods is low in comparison to the surface energy of the interstitial spaces, and as the droplet interacts with the different surface energies in turn, the wetting state alters.4,43 This switching phenomenon would be expected to be observed across all droplet sizes. However, as the size of the droplet increases, a small increase in the contact angle would be observed as the downward force of the droplet mass begins to push out the sides of the droplet. This accommodates the additional volume with the least increase in height, with relation to the maximum droplet side curvature possible depending on the base surface tension of the fluid. This would slightly alter both the advancing (41) Vallet, M.; Vallade, M.; Berge, B. Eur. Phys. J. B 1999, 11 (4), 583-591. (42) Jagadish, C.; Pearton, J. S. Zinc Oxide Bulk, Thin Films and Nanostructures: Processing, Properties, and Applications, 1st ed.; Elsevier Science: Oxford, U.K., 2006. (43) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546.
and receding contact angles of a fluid on the surface. This gravity effect can be observed in Figure 10 as the outer edges of the 20 µL droplet sag down toward the substrate compared to the more uniform spherical edges of the 5 µL droplets. This allows the droplet to be drawn into interstitial spaces between the nanorods. This movement of liquid from sitting on the surface to descending into the interstitial space is highlighted in Figure 10. The 20µL DI water droplet can be seen to suddenly submerge into the substrate surface upon reaching the threshold voltage, where it would remain strongly adhered to the surface unless agitated. After the threshold voltage had been exceeded and the droplet had undergone a gentle actuation, no further change in the contact angle could be obtained with the application of higher voltages, which indicates that the system had reached a point of saturation.44,45
Conclusions Nanorods of ZnO were grown using two different techniques, and subsequently the tips of these nanorods were covered with sputtered Teflon. Superhydrophobic surfaces were obtained with nanostructured arrays of ZnO directed HMT growth (with sharper tips) having a maximum initial contact angle of 175°. The wettabilities of surfaces were tested by the application of external voltages. The wettabilities of surfaces were instantly (44) Peykov, V.; Quinn, A.; Ralston, J. Colloid Polym. Sci. 2000, 278 (8), 789-793. (45) Vallet, M.; Berge, B.; Vovelle, L. Polymer 1996, 37 (12), 2465-2470.
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reversible for voltages less than 35 V, and for larger voltages they required gentle mechanical agitation to regain superhydrophobicity. The contact angle change for nanostructured surfaces with sharper tipped nanorods was larger than that for
Campbell et al.
smooth-end nanorods (NaOH directed growth) with a contact angle change of 78° versus less than 20°. LA7030413
