Sliding Droplets on Superomniphobic Zinc Oxide Nanostructures

ARTICLE pubs.acs.org/Langmuir

Sliding Droplets on Superomniphobic Zinc Oxide Nanostructures Guillaume Perry,†,‡ Yannick Coffinier,‡ Vincent Thomy,*,† and Rabah Boukherroub‡ †

Institut d’Electronique, de Micro
electronique et de Nanotechnologie (IEMN, UMR CNRS 8520), Universit
e Lille 1, Cit
e Scientifique, Avenue Poincar
e BP 60069, 59652 Villeneuve d’Ascq, France ‡ Institut de Recherche Interdisciplinaire (IRI, CNRS-USR 3078), Universit
e Lille 1, Parc de la Haute Borne, 50 Avenue de Halley, BP 70478, 59658 Villeneuve d’Ascq, France

bS Supporting Information ABSTRACT: This study reports on liquid-repellency of zinc oxide nanostructures (ZnO NS). The ZnO NS are synthesized by an easy and fast chemical bath deposition technique. Three different nanostructured surfaces consisting of nanorods, flowers, and particles are prepared, depending on the deposition time and the presence of ethanolamine in the reaction mixture. Chemical functionalization of the ZnO NS with 1H,1H,2H,2Hperfluorodecyltrichlorosilane (PFTS) in liquid (PFTS L) and vapor phase (PFTS V) or through octafluorobutane (C4F8) plasma deposition led to the formation of superomniphobic surfaces. A comprehensive characterization of the wetting properties (static contact angle and contact angle hysteresis) has been performed using liquids composed of deionized water and various concentrations of ethanol (surface tension between 35 and 72.6 mN/m). Depending on the nanostructures morphology, coating nature and liquid employed, high static apparent contact angles θ ≈ 150 160
, and low contact angle hysteresis Δθ ≈ 0
are obtained. The different ZnO NS are characterized using scanning electron microscopy (SEM) and contact angle measurements. The results reported in this work permit preparation of sliding omniphobic surfaces using a simple and low cost technique.

’ INTRODUCTION Zinc oxide (ZnO) thin layers are very promising because of their various useful properties. The wide band gap (3.3 eV) and high exciton binding energy (60 meV) of ZnO make it a good candidate for optoelectronic devices such as lasers, light-emitting diodes, or thin film transistors.1 Another characteristic of ZnO is its high electromechanical coupling coefficient, which is used for the preparation of piezoelectric devices.1 Nonetheless, the bulk-material properties can be enhanced by the realization of micro- or nanostructuration. Zinc oxide nanostructuration gives rise to enhanced surface roughness, improved specific optical, electrical,2 and wetting properties that could find various applications such as gas and liquid sensors,1 3 catalysis,4 oil-fouling, and self-cleaning coatings.5 In contrast to ZnO thin layers, zinc oxide nanostructures (ZnO NS) can be synthesized very easily using various chemical techniques. Different chemical methods to grow ZnO NS have been described, but the most used ones are low temperature (T e 140
) hydrothermal deposition4,6 11 and electrodeposition.12 14 Since the pioneering works of Wenzel15 and Cassie Baxter,16 wetting on rough surfaces was largely studied especially by Bico et al.17 and Herminghaus.18 Briefly, even if a water droplet placed on a rough surface exhibits a high contact angle (g150
), it can adopt two extreme states: impalement inside the roughness r 2011 American Chemical Society

(Wenzel state) or sitting on the top of the structure (Cassie Baxter state). In the first case, the liquid/surface interaction is enhanced compared to a planar surface, leading to an important hysteresis (g40
). In the second case, air pockets are confined under the water droplet; the liquid/surface interaction is minimum, leading to a quasi-null hysteresis (also depicted as a rolling ball state). However, these assertions are limited to aqueous-based liquids of high surface tension (around 72 mN/m) The wetting properties of ZnO nanorods have been investigated in several reports.11,12,19,20 Even though high static contact angle values (>150
) are reported, no indication was given regarding the contact angle hysteresis in all these reports. Zhang et al.13 observed water-repellency on electrodeposited ZnO nanocolumns whose length and diameter are 1 μm and 125 nm, respectively, upon vapor phase modification with a fluorinated silane. The surface exhibited a water contact angle of 150
and a contact angle hysteresis (CAH) = 8
. Sakai et al.9 investigated the effect of the nanorod diameter in ZnO NR film, prepared by hydrothermal process, coated with octadecyltrimethoxysilane on water-repellency. They found that, for a Received: September 6, 2011 Revised: November 6, 2011 Published: November 07, 2011 389

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Langmuir precursor concentration (zinc nitrate hexahydrate) between 6.25 and 50 mM, the ZnO NR diameter varies from 10 to 100 nm, leading to a CA higher than 156
. The Cassie Baxter state was revealed by a tilting angle lower than 11
for a 5 μL water droplet. Pauport
e et al.21 studied intensively the wetting transition from Cassie Baxter to Wenzel state according to the height and density of 140 nm wide well aligned and vertical ZnO nanowires (electrodeposited on fluorine-doped tin oxide substrate) modified with stearic acid. For long ZnO nanowires (5 μm) with a high density, a Cassie Baxter state was reached (CAH e 6
), while for shorter nanowires (2 μm) with a low density a Wenzel state was obtained (CAH g 20
). From these results, it is clear that ZnO NS substrates attain a Cassie Baxter state for water droplets if they are coated with an adequate hydrophobic layer. But the use of liquids of high surface tension limits the application of these surfaces. As a matter of fact, using low surface tension liquids leads to a much more important liquid/surface interaction compared to water; the molecules tend to be more attracted by the surface (adhesives forces) than by the liquid molecules (cohesive forces). To obtain nonwetting surfaces with liquids of low surface tension, superomniphobic surfaces are sought after. These surfaces require both geometric criteria and specific surface coating exhibiting an omniphobic character on a planar surface. The geometric criterion introduced by Tuteja et al.22 is based on “re-entrant” structures to avoid wetting with low surface tension liquids. While this pioneering work was performed on structures prepared using silicon-based technology, some superomniphobic surfaces using the same criteria have also been reported on PDMS substrates.23,24 These surfaces, with characteristic lengths of about tens of micrometers, require the use of classical photolithography techniques with more or less specific dry or wet etching23 and lead to hysteresis of about 40
, synonymous of high contact angle (150
) but sticky droplets with liquids of surface tension as low as 27 mN/m. Few other reports deal with nanotextured surfaces exhibiting omniphobic character due to their double scale roughness. They consist of a microstructured Ti surface covered by TiO2 nanotubes arrays,25 assembly of silica and porous polystyrene nanoparticles,26 structured silicon surfaces and cellulose nanocrystals,27 or a double layer made of silicon nanowires grown by the VLS growth technique.28 In all these papers, the omniphobic character was assigned to the presence of a “re-entrant” shape at nanoscopic scale. To our best knowledge, only one example reported so far in the literature deals with superomniphobic ZnO nanoparticles.29 The authors employed a spray casting method of ZnO nanoparticles fluoropolymer suspensions, leading to a hierarchical surface roughness. Depending on the concentration of these elements, a contact angle of 154
with a contact angle hysteresis as low as 6
with mineral oil (DTE-11M, Mobil, γ ≈ 32 mN/m) can be achieved. It is to be noted that we cannot directly compare these results to those published on the wetting properties of ZnO nanorods. As a matter of fact, the ZnO particles are completely covered and hidden by the polymer layer leading to a hierarchical surface, very different to usual shapes obtained by electro- or hydrothermal deposition. In continuation of our recent work aimed toward the preparation and characterization of ZnO nanostructures,4,6,7,10 we report herein an extensive investigation of the wetting properties of various ZnO NS substrates prepared by chemical bath deposition. Depending on the process conditions, ZnO nanostructures with three different morphologies have been synthesized. The resulting ZnO NS surfaces were modified

ARTICLE

through silanization with 1H,1H,2H,2H-perfluorodecyltrichlorosilane (PFTS) or coating with C4F8 to enhance their liquid repellency. Finally, to fully characterize the wetting properties of these surfaces, we have used seven different water/ethanol mixtures with surface tensions varying from 35 to 72.6 mN/m. Static contact angle and contact angle hysteresis have been measured for all surfaces and liquids. SEM images of a NOA droplet on the nanotextured surfaces are also presented to ascribe the liquid/surface interaction to Wenzel or Cassie Baxter state.

’ EXPERIMENTAL SECTION Materials and Reagents. Silicon samples (Æ100æ, n-doped, resistivity: 5 10 Ω 3 cm 1) are provided by Siltronix. Potassium permanganate (KMnO4), tert-butanol, ethanol, zinc sulfate heptahydrate (ZnSO4 3 7H2O), zinc nitrate hexahydrate (Zn(NO3)2 3 6H2O), monoethanolamine (MEA), triethanolamine (TEA), ammonium hydroxide (NH4OH), acetone, isopropanol, and hexane were obtained from Sigma-Aldrich and used without further purification. 1H,1H,2H,2H-Perfluorodecyltrichlorosilane (PFTS) was purchased from ABCR GmbH, Germany. Preparation of ZnO Nanostructures (ZnO NS). The different ZnO nanostructures investigated in this work are deposited on silicon wafer and prepared through a chemical bath deposition method developed by Kokotov and Hodes.8 The technique is fast, simple, and takes place at low-temperature. First, silicon samples are cleaned in successive baths of acetone and isopropyl alcohol in an ultrasonic bath. Before the deposition, a surface pretreatment is realized in PFTE vials filled with 20 mL of deionized water (DW) and 12.7 mM potassium permanganate (KMnO4) aqueous solution containing 50 μL of tert-butanol. Then, the vials are closed and placed in an oil bath at 84
C for 20 min. The resulting substrates are intensively rinsed with DW and sonicated 10 min in ultrasonic bath. Then, the samples are dipped in 34 mM zinc sulfate or 50 mM zinc nitrate aqueous solution (17.5 mL of DW) containing 2 mL of monoethanolamine (MEA) or triethanolamine (TEA) and 500 μL of ammonium hydroxide (NH4OH). The closed vials are put in an oil bath at 96
C for 40 or 80 min. After the deposition, the samples are rinsed with DW and dried in oven at 100
C during 1 h. The three different ZnO NS prepared in this work are produced using different deposition conditions. ZnO nanorods and nanoflowers are synthesized using zinc sulfate precursor, MEA, and a deposition time of 40 or 80 min, respectively. ZnO nanoparticles are prepared using zinc nitrate, TEA, and a deposition time of 40 min. Functionalization of the ZnO Nanostructures (ZnO NS). The resulting surfaces were modified through silanization with 1H,1H, 2H,2H-perfluorodecyltrichlorosilane (PFTS) or coating with C4F8. The silanization reaction was performed in either liquid or gas phase: (i) Liquid phase silanization (PFTS L) is achieved by immersion of the ZnO nanostructured substrates in a PFTS (50 μL) solution in 50 mL n-hexane at 10
C for 4 h. The resulting surfaces are rinsed three times in fresh dichloromethane. (ii) Vapor phase silanization (PFTS V) is achieved by evaporation of 50 μL of PFTS on the ZnO surfaces in a desiccator under vacuum for 4 h. C4F8 coating on ZnO NS samples was performed using inductively coupled radio frequency plasma (Surface Technology Systems, England). The carrier gas used was octafluorocyclobutane (C4F8). The following conditions were used: power = 1000 W, gas flow rate = 220 sccm, radiofrequency = 13.56 MHz, temperature = 10
C, and t = 10 s. The deposition rate was 237 nm/min. Characterization. Scanning Electron Microscopy (SEM). SEM images were obtained using an electron microscope ULTRA 55 (Zeiss) equipped with a thermal field emission emitter and three 390

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Figure 1. SEM images at 10 000 magnification of (a) ZnO NF modified with PFTS (liquid phase silanization, PFTS L), (b) ZnO NR modified with PFTS L, (c) ZnO NP modified through C4F8 plasma, and (d) ZnO NP modified by vapor phase silanization (PFTS V).

coatings: vapor and liquid phase silanization (PFTS V and PFTS L, respectively) and C4F8 plasma. The surface covered by ZnO nanoflowers (ZnO NF)-like is prepared by heating the pretreated silicon wafer in an aqueous solution of zinc sulfate (34 mM), monethanolamine (10% v/v), and ammonium hydroxide in an oil bath at 96
C during 80 min. Figure 1a displays a SEM image of ZnO NF modified by liquid phase silanization (PFTS L). These very sharp nanoflowers are assembled like flower petals of 2 μm in diameter. The surface is covered by some bumps of nanoflowers with a maximum height of about 10 μm. This more or less dense layer of nanoflowers is not homogeneous, showing the underlying layer composed of nanorods, leading to a micro/nanotexturation. As shown in Figure 1a, the silanization step does not induce any modification of the surface morphology. Under the same experimental conditions and for shorter deposition times (40 min), the surface consists of a 2 μm thick, homogeneous, and dense layer composed of ZnO nanorods (ZnO NR) of 50 200 nm in diameter. Figure 1b exhibits a SEM image of ZnO NR modified by vapor phase silanization (PFTS V). Spherical ZnO nanoparticles (ZnO NP) are also synthesized by heating the pretreated silicon wafer in an aqueous solution of zinc nitrate, triethanolamine, and ammonium hydroxide in an oil bath at 96
C during 40 min. Their diameters are between 500 nm and 2 μm. Figure 1c and d displays ZnO NP modified by C4F8 plasma and PFTS V, respectively. Compared to the nanoflowers, the nanoparticles covered more homogeneously the surface with a maximum thickness of about 10 μm. Among the three different surfaces, ZnO NP is the unique one clearly exhibiting a re-entrant structure, considering the lower half-part of the nanoparticles. Furthermore SEM images

different detectors (EsB detector with filter grid, high-efficiency in-lens SE detector, Everhart-Thornley secondary electron detector). Contact Angle Measurements. Contact angle measurements were performed using a goniometer (DSA100, Kr€uss GmbH, Germany) placed on a tilting table and its droplet analysis software. The static apparent contact angle (SCA) is measured just after the drop deposition. Contact angle hysteresis (CAH) is calculated as the difference between the advancing and receding angles. The latter are defined just before the contact line depinning during tilting. The tested liquids consist of an 8 μL droplet composed of mixtures of deionized water and various concentrations of ethanol leading to surface tensions comprised between 35 and 72 mN/m (see Table 1 in the Supporting Information). On each surface, four different measurements using one liquid are performed. Standard deviation is also calculated.

’ RESULTS AND DISCUSSION The ZnO nanostructures investigated in this work are prepared using a simple and straightforward technique allowing, for example, iodine doping of the ZnO NS by adding iodic acid (HIO3) in the solution mixture.4 The deposition process relies on potassium permanganate pretreatment to form Mn-(hydroxyl) oxide layer on the substrate.8 This layer acts as an efficient seed layer that allows ZnO NS growth under alkaline conditions. In the first stage of the chemical deposition, nanorods grow preferentially. After saturation of the surface by ZnO NR, nanoflowers’ deposition takes place. The use of TEA in the chemical reaction permits to deposit only ZnO NP according to Weintraub et al.3 Three different ZnO NS are produced using different deposition conditions and subsequently modified with three different 391

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Figure 2. Static contact angle as a function of liquid surface energy measured on (a) flat silicon wafer, (b) ZnO NR, (c) ZnO NF, and (d) ZnO NP coated with C4F8 and PFTS.

(Figure 1; see also Figure S1 in the Supporting Information) clearly show that no morphological changes were induced by the hydrophobic coating, whatever the technique employed. Static Contact angle (SCA) Measurements. The wetting properties of the ZnO NS modified with PFTS (liquid (L) or vapor (V) phase silanization) or coated with C4F8 are investigated and compared to those of a flat silicon surface modified under the same conditions. On a flat silicon surface, the measured SCA corresponds to the Young angle. Whatever the coating used the contact angle of flat silicon surface decreases quite linearly when the liquid surface tension decreases (Figure 2a). The curves corresponding to C4F8 and PFTS V coatings are almost similar and decrease from 105 to 75
for water (72 mN/m) and a mixture of water/ethanol (35 mN/m), respectively. The limit between the wetting and nonwetting liquids is set between 46 and 52 mN/m. Concerning the PFTS L coating, the contact angle decreases more slowly from a maximum value of 120
for water (72 mN/m) to 75
for a water/ethanol mixture (35 mN/ m). In that case, the wetting threshold is about 35 mN/m. Thus, a flat silicon surface coated with PFTS L seems to have the best omniphobic properties. As expected, deposition of ZnO NS on the silicon substrate enhanced the static contact angle (Figure 2b d). Generally, for all tested liquids and different coatings, the SCA is higher than 140
with some key features like a SCA of 160
for ZnO nanoflowers for a liquid of a surface tension of 47 mN/m. More specifically, for ZnO NR substrates (Figure 2b), a comparison of the different coatings indicates that, down to 40 mN/m, the SCA is quite higher for PFTS L and C4F8 compared to PFTS V. With the lowest surface tension liquid (35 mN/m), PFTS L exhibits

the highest contact angle (152
). On ZnO NF (Figure 2c), a clear difference appears below 52 mN/m: here again the PFTS L led to the best behavior, while PFTS V and C4F8 show SCA inferior of about 5
and 10
, respectively. A different behavior was observed on ZnO NP: the three coatings gave the same SCA for each tested liquid (Figure 2d). From these results, we can draw the following conclusions: (i) First and surprisingly, even with a Young angle below 90
(on a flat surface) with liquids of a certain surface tension, as depicted above, apparent contact angles on rough surfaces are always higher than 140
. This contradiction with the classical wetting theoretical model can be ascribed to the locally re-entrant structure of the nanotextured surfaces, as depicted by others.28 (ii) Second, it seems that the surface exhibiting very sharp shapes like ZnO NF led to more important SCA variation compared to ZnO NP. If now we consider a same hydrophobic coating whatever the nanostructure morphology, the same wetting behavior is observed for the C4F8 coating: a linear decrease of the SCA from about 160
to 145
as the liquid surface tension decreases from 72 to 35 mN/ m. The following remark can be made for ZnO NF: at low surface tensions (40 and 35 mN/m), the very sharp texturation leads to a SCA 5 10
lower than those from other nanostructures. Concerning PFTS coating, whatever the technique employed (vapor or liquid phase silanization), the SCA of both ZnO NR and ZnO NF follows the same trend: linear decrease with a 5 10
more for PFTS L, as on smooth surfaces. Nonetheless, on ZnO NP, this difference clearly disappeared, and the SCA is the same for the three different coatings. This point underlines the assumption, in 392

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Figure 3. Contact angle hysteresis as a function of liquid surface energy measured on (a) flat silicon wafer, (b) ZnO NR, (c) ZnO NF, and (d) ZnO NP coated with C4F8 and PFTS.

the recent literature, pretending that re-entrant curvatures are responsible for the high contact angles with low surface tension liquids even at the nanometric scale.28 Contact Angle Hysteresis (CAH) Measurements. While SCA gives just part of the information of the wetting behavior, the contact angle hysteresis (CAH) gives a more comprehensive wetting behavior of the nanostructures and liquid-repellency effect.30 On a flat silicon wafer surface, the CAH is higher than 20
independent of the coating and liquid surface energy (Figure 3a). On ZnO NR, the CAH for C4F8 and PFTS L coatings is quasinull for liquids of surface tension higher than 46 mN/m (Figure 3b). It remains lower than 30
at 35 mN/m for C4F8, but increases significantly for PFTS L (up to 80
). The PFTS V coating gives CAH values between 20 and 40
for all tested liquids. On ZnO NF coated with PFTS V and PFTS L, CAH is quasinull for liquids of surface tension higher than 43 mN/m; it increases for liquids of low surface tension up to 30
for PFTS V and 40
for PFTS L (Figure 3c). For C4F8, the CAH is also quasinull for liquids of surface energy higher than 59 mN/m and then it increases rapidly for liquids of lower surface energy. For the C4F8 coating on ZnO NF, it is worth to notice the important values of error bars for liquids of surface tensions equal to 35 and 42 mN/m, corresponding to the highest hysteresis characterized during this work (180
). For both liquids, the droplet does not slide at a tilting angle of 90
. This can be assigned most likely to droplet impregnation within the nanostructures (Wenzel state). On ZnO NP, the values are almost the same for C4F8 and PFTS V with a quasi-null hysteresis for liquids of surface energy

higher than 46 mN/m (Figure 3d). The CAH then increases up to 30
for liquids of lower surface tension. On the contrary, PFTS L leads to a CAH of about 19
with liquids of surface tension lower than 59 mN/m. For the same coating, ZnO NR and ZnO NP surfaces present comparable hysteresis for C4F8. While for PFTS V the CAH is similar for ZnO NF and ZnO NP. From these results, the differences between Cassie and Wenzel states are obvious. For liquids of surface tension superior to 40 mN/m and when the CAH is inferior to 20
, the Cassie Baxter state prevails. For CAH higher than 40
for liquids of low surface tension, the Wenzel state dominates. Between these both extreme and ideal states (completely suspended or impaled droplet), the frontier is not really defined due to the unstable state in which the droplet stays. According to the way the droplet is deposited, the liquid can either sit at the top of the nanotexturation or may be partially impaled.31 These comments are based on observations made at the macroscopic scale. In order to get information at a microscopic scale, an 8 μL droplet of NOA 72 (Norland Optical Adhesive, Norland Inc.) of a surface tension around 40 mN/m32 is deposited on the ZnO NP surface coated with PFTS V. NOA 72 is polymerized by irradiation at 365 nm (UV lamp, 12 mW/cm2) during 10 min. From the results in Figures 2d and 3d, the SCA and CAH are 150 and 20
, respectively. In the SEM picture in Figure 4, we can observe that the droplet does not impregnate the nanostructures, but the triple contact line is partially pinned at the top of the structure.33 This local deformation of the triple line already observed on welldefined microscopic structures is directly linked to CAH.34 This 393

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Telephone: + 33 (0)3 20 19 79 51. Fax: +33 (0)3 20 19 78 98.

’ ACKNOWLEDGMENT We acknowledge the European Regional Development Fund for financial support under the INTERREG IVa FW1.1.9 “PLASMOBIO” project, Maxime Harnois for technical support, and Mr. Christophe Boyaval from SEM imaging. Figure 4. SEM image of an NOA droplet on ZnO NP surface coated with PFTS V.

’ REFERENCES (1) Ozgur, U.; Hofstetter, D.; Morkoc, H. Proc. IEEE 2010, 98, 1255–1268. (2) Ahmad, M.; Zhu, J. J. Mater. Chem. 2011, 21, 599–614. (3) Weintraub, B.; Zhou, Z. Z.; Li, Y. H.; Deng, Y. L. Nanoscale 2010, 2, 1573–1587. (4) Barka-Bouaifel, F.; Sieber, B.; Bezzi, N.; Benner, J.; Roussel, P.; Boussekey, L.; Szunerits, S.; Boukherroub, R. J. Mater. Chem. 2011, 21, 10982–10989. (5) Zhang, J. L.; Pu, G.; Severtson, S. J. ACS Appl. Mater. Interfaces 2010, 2, 2880–2883. (6) Liu, H. Q.; Piret, G.; Sieber, B.; Laureyns, J.; Roussel, P.; Xu, W. G.; Boukherroub, R.; Szunerits, S. Electrochem. Commun. 2009, 11, 945–949. (7) Sieber, B.; Addad, A.; Szunerits, S.; Boukherroub, R. J. Phys. Chem. Lett. 2010, 1, 3033–3038. (8) Kokotov, M.; Hodes, G. J. Mater. Chem. 2009, 19, 3847–3854. (9) Sakai, M.; Kono, H.; Nakajima, A.; Zhang, X. T.; Sakai, H.; Abe, M.; Fujishima, A. Langmuir 2009, 25, 14182–14186. (10) Sieber, B.; Liu, H. Q.; Piret, G.; Laureyns, J.; Roussel, P.; Gelloz, B.; Szunerits, S.; Boukherroub, R. J. Phys. Chem. C 2009, 113, 13643– 13650. (11) Zhu, X. T.; Zhang, Z. Z.; Men, X. H.; Yang, J.; Xu, X. H. Appl. Surf. Sci. 2010, 256, 7619–7622. (12) Badre, C.; Dubot, P.; Lincot, D.; Pauporte, T.; Turmine, M. J. Colloid Interface Sci. 2007, 316, 233–237. (13) Zhang, X. T.; Sato, O.; Fujishima, A. Langmuir 2004, 20, 6065–6067. (14) Xiong, J.; Das, S. N.; Shin, B.; Kar, J. P.; Choi, J. H.; Myoung, J. M. J. Colloid Interface Sci. 2010, 350, 344–347. (15) Wenzel, R. N. J. Phys. Colloid Chem. 1949, 53, 1466–1467. (16) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 0546–0550. (17) Bico, J.; Marzolin, C.; Quere, D. Europhys. Lett. 1999, 47, 220–226. (18) Herminghaus, S. Europhys. Lett. 2000, 52, 165–170. (19) Feng, X. J.; Feng, L.; Jin, M. H.; Zhai, J.; Jiang, L.; Zhu, D. B. J. Am. Chem. Soc. 2004, 126, 62–63. (20) Myint, M. T. Z.; Kitsomboonloha, R.; Baruah, S.; Dutta, J. J. Colloid Interface Sci. 2011, 354, 810–815. (21) Pauport
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image confirms the intermediate state of the droplet between Cassie and Wenzel states. From an applicative point of view, the SCA measurement is not a key characteristic as revealed by the following results: (i) On ZnO NR coated with PFTS L, while the SCA is higher than 150
for all tested liquids, the CAH is quasi-null (rolling ball effect) for liquids of surface tension higher than 35 mN/m and increases to about 80
(sticky droplet) for liquids of higher surface energy. (ii) On ZnO NF coated with C4F8, the SCA is higher than 140
, but the CAH varies from 0
to 140
, that is, from a perfect Cassie Baxter to a Wenzel state. (iii) On ZnO NP, the coating seems to have a negligible influence on the wetting properties, but a slightly higher CAH is observed for PFTS L. Thus, rather than only giving SCA for the water droplet, the wetting characterization of superomniphobic surfaces necessitates the definition of three states (Cassie state leading to a rolling ball effect, intermediate Wenzel state of CAH < 10
, and Wenzel state of CAH > 10
) according to the surface tension of the liquids (see Table 2 in the Supporting Information).

’ CONCLUSION The characterization of ZnO nanostructures of different morphologies coated with various low surface energy molecules has permitted to obtain superomniphobic surfaces presenting liquidrepellency properties. Among the studied parameters, the wetting properties of these different surfaces depend on the surface morphology and hydrophobic coating type. The superomniphobic surfaces are produced by a chemical method that allows fast, easy, and low-cost nanostructure fabrication and deposition of ZnO. They can be integrated in lab-on-chip devices as a superomniphobic top cover and easily coupled with biosensors. The complete study with liquids of a large range of surface tension permits one to determine a critical surface energy, for which a liquid droplet rolls off, moderately sticks, and completely imbibes in the roughness. We think that these results would pave the way to the integration of these surfaces in lab-on-chip devices.35 ’ ASSOCIATED CONTENT

bS

Supporting Information. Additional results and tables. This material is available free of charge via the Internet at http:// pubs.acs.org. 394

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