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Wetting on Nanoporous Alumina Surface: Transition between Wenzel and Cassie States Controlled by Surface Structure Chunbo Ran,* Guqiao Ding, Weichang Liu, Yan Deng, and Wentao Hou UnileVer Research China, 3/F Xinmao Building, 99 Tianzhou Road, Shanghai 200233, P. R. China ReceiVed May 12, 2008. ReVised Manuscript ReceiVed July 24, 2008 This paper reports a systematic study on the relationship between surface structure and wetting state of ordered nanoporous alumina surface. The wettability of the porous alumina is dramatically changed from hydrophilicity to hydrophobicity by increasing the hole diameter, while maintaining the hole interval and depth. This phenomenon is attributed to the gradual transition between Wenzel and Cassie states which was proved experimentally by comparing the wetting behavior on these porous alumina surfaces. Furthermore, the relationship between surface wettability and hole depth at a fixed hole interval and diameter was investigated. For those porous alumina with relatively larger holes in diameter, transition between Wenzel and Cassie states was also achieved with increasing hole depth. A capillarypressure balance model was proposed to elucidate the unique structure-induced transition, and the criteria for the design and construction of a Cassie wetting surface was discussed. These structure-induced transitions between Wenzel and Cassie states could provide further insight into the wetting mechanism of roughness-induced wettability and practical guides for the design of variable surfaces with controllable wettability.
Introduction Since Barthlott and Neinhuis revealed the nature of the lotus leaf,1 the wettability of rough solid surfaces has attracted great interests because of both the scientific insight and the practical applications.2-7 It is well-known that the surface roughness plays an important role on the wettability, which can be explained by two main hypotheses attributed to Wenzel and Cassie.8,9 Another derivative model from these two states has been developed for more complicated actual situations.10 In the Wenzel state, the grooves or space between the surface protrusions on the surface is completely wetted with liquid. The surface contact area of liquid with solid is increased by surface roughness. As a result, the hydrophobicity or hydrophilicity of rough surfaces is strengthened, i.e., the apparent contact angle (CA) increases for hydrophobic surfaces but decreases for hydrophilic surfaces. On the other hand, in the Cassie state, the liquid is only in contact with the tips of the asperities, and air pockets exist between the liquid and roughened solid surface. The surface roughness always increases the apparent CA due to the existence of air, regardless of the original wettability of the solid substrate. Although the two well-established theoretical models are frequently employed in explaining different wetting phenomena, the wetting behavior of rough surface is not fully understood, especially in the transition region between Wenzel and Cassie states.11-13 A theoretical study was carried out focusing on the * To whom correspondence should be addressed. E-mail: chun-bo.ran@ unilever.com. (1) Neinhuis, C.; Barthlott, W. Ann. Bot. 1997, 79, 667. (2) Onda, T.; Shibuichi, S.; Satoh, N; Tsujii, K. Langmuir 1996, 12, 2125. (3) Miwa, M.; Nakajima, A.; Fujishima, A.; Hashimoto, K.; Watanabe, T. Langmuir 2000, 16, 5754. (4) Blossey, R. Nat. Mater. 2003, 2, 301. (5) Erbil, H. Y.; Demirel, A. L.; Avci, Y.; Mert, O. Science 2003, 299, 1377. (6) Zhai, L.; Cebeci, F. C.; Cohen, R. E.; Rubner, M. F. Nano Lett. 2004, 4, 1349. (7) Feng, X. J.; Jiang, L. AdV. Mater. 2006, 18, 3063. (8) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988. (9) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546. (10) de Gennes, P. G.; Brochard-Wyart, F.; Que´re´, D. Capillarity and Wetting Phenomena: Drops, Bubbles, Pearls, WaVes; Springer: New York, 2002; p 220. ¨ ner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777. (11) O (12) Yoshimitsu, Z.; Nakajima, A.; Watanabe, T.; Hashimoto, K. Langmuir 2002, 18, 5818.
relationship between the solid surface morphology and the transition between Wenzel and Cassie states.14 Furthermore, the transition from Cassie to Wenzel state has been realized by external stimuli such as pressure,15 vibration,16 electric voltage,17 or evaporation.18 However, they are abrupt and irreversible processes on the same rough surfaces, and the typical sizes of these rough surfaces are in the range of several to tens of micrometers. As far as we know, few experimental studies have been reported on the transition region between Wenzel and Cassie states and the influence of the surface structure or morphology on the wetting state in the nanometer scale, which are crucial for the design of superhydrophobic surface. Fortunately, the typical size of porous alumina is in the nanoscale region.19,20 The pore structures on the surface can be tuned from 20 to 980 nm in diameter, and the depth can be extended to several hundreds of micrometers.21 Such structures are easy to prepare, and the surface morphology can be controlled in a highly ordered manner by changing the anodization conditions. The well-controlled surface morphology provides us an ideal model substrate to investigate the relationship between surface structure and its corresponding wetting state. In this study, we investigate the effect of hole diameter and depth on the wetting behavior of porous anodic alumina (PAA) and the associated transition from Wenzel regime to Cassie regime. This transition was proved, for the first time, through the observation of air bubble exclusion during the CA measurement, a direct proof of the Wenzel state. To understand the wettability transition, a capillary-pressure balance model was proposed to calculate the depth of water penetrating into the hole. Capillary (13) Patankar, N. A. Langmuir 2004, 20, 7097. (14) Marmur, A. Langmuir 2003, 19, 8343. (15) Lafuma, A.; Que´re´, D. Nat. Mater. 2003, 2, 457. (16) Bormashenko, E.; Pogreb, R.; Whyman, G.; Mordehai, E. Langmuir 2007, 23, 6501. (17) Bahadur, V.; Garimella, S. V. Langmuir 2007, 23, 4918. (18) Nosonovsky, M.; Bhushan, B. Nano Lett. 2007, 7, 2633. (19) Rurneaux, R. C.; Rigby, W. R.; Davidson, A. P. Nature 1989, 337, 147. (20) Patermarakis, G.; Moussoutzanis, K. Corros. Sci. 2001, 43, 1433. (21) Chu, S. Z.; Wada, K.; Inoue, S.; Isogai, M.; Katsuta, Y.; Yasumori, A. J. Electrochem. Soc. 2006, 153, B384. (22) Adamson, A. W.; Gast, A. P. Physical Chemistry of Surfaces, 6th ed.; John Wiley & Sons: New York, 1997; Chapter II, Section 4.
10.1021/la801461j CCC: $40.75 2008 American Chemical Society Published on Web 08/15/2008
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Figure 2. CA of water on the porous alumina surface as a function of the hole diameter. The hole interval is fixed 450 nm, and the depth is 4.5 µm. The line connecting the experimental data points is used for illustration only.
Figure 1. SEM images of porous alumina with a hole interval of 450 nm. The corresponding diameters are (a) 85 nm; (b) 180 nm; (c) 290 nm, and (d) 420 nm.
force was regarded as the driving force for water penetration into the hole. The criteria for obtaining a Cassie wetting surface has been discussed, establishing the design rules for creating superhydrophobic surfaces. The study on the structure-induced transition of wetting behaviors between Wenzel and Cassie states provides not only further insight into theoretical understanding of surface wettability but also valuable information for practical applications.
Experimental Section Fabrication of Porous Anodic Alumina. The porous alumina are fabricated using a two-step anodization process described in detail elsewhere.19-21 Prior to anodization, an Al sheet (99.99% purity, 0.25 mm thickness) was degreased by ultrasonication in acetone for 5 min and electropolished in a mixed solution of HClO4 and C2H5OH (1:4, volume ratio) to obtain a smooth surface. Aluminum anodizing was preformed by a regulated dc power supply in a H3PO4-H2O-C2H5OH system following the reported process. Pore opening was conducted in a 5 wt% H3PO4 solution at 30 °C for different etching times depending on the desired hole diameter. Characterization. The surface morphology of the porous alumina was observed using a field emission scanning electron microscope (FE-SEM: JSM-6700F, JEOL). CAs of water were measured using a CA meter (drop shape analysis system 100, Kru¨ss) at room temperature. The volume of water droplet used here for the measurements was ∼3 µL. The CA values were the averages of five independent points.
Results and Discussion Using the two-step technique, the porous alumina can be fabricated in a controllable way. The hole diameter, interval, and the hole depth were tuned by varying anodization time, voltage, and pore opening time. Figure 1 shows the typical SEM images of the porous alumina films fabricated under different conditions. The prepared porous alumina have a uniform hole interval of 450 ( 25 nm and a depth of ca. 4.5 µm. The hole diameters for Figure 1a,b,c, and d are 85 ( 15, 180 ( 25, 290 ( 14, and 420 ( 15 nm, respectively, according to the measurement from the SEM images. CA measurements were conducted on these fabricated porous alumina substrates. As shown in Figure 2, the CA increased monotonically with the hole diameter. In particular, when the hole diameter of the porous alumina increased from 85 to 420 nm, the wettability of the surface changed from
Figure 3. Images of 3 µL water drops deposited on porous alumina surface with hole diameters of 85 nm (a), and 420 nm (b).
hydrophilic (CA ) 70 ( 1°) to hydrophobic (CA ) 132 ( 2°). Clearly, there exists a gradual transition of the porous alumina surface from hydrophilic to hydrophobic as the hole diameter increases. We also conducted the wettability measurement on a clean nonporous alumina flat surface. The CA of a nonporous flat alumina surface is 85 ( 3°. This is less hydrophilic than the porous alumina with a diameter of 85 nm (70 ( 1°), but also less hydrophobic than the one with a diameter of 420 nm (132 ( 2°). According to the two classical theoretical models, the hydrophilicity of a rough surface increases in the Wenzel case for the primary hydrophilic surfaces, i.e., the apparent CA decreases, while, in the Cassie case, the apparent CA of rough surface always increases in the Cassie state due to the existence of air pockets, despite of the wettability of the primary surface. Therefore, the wetting states on the porous alumina with diameters of 85 and 420 nm are likely Wenzel and Cassie states, respectively. Of particular interest is that, when we conducted the wettability measurement, an air bubble emission phenomenon appeared on the porous alumina surface with a diameter of 85 nm, as shown in Figure 3a. After dripping a water droplet onto the porous alumina surface with a diameter of 85 nm, air bubbles were observed to emerge initially at the alumina surface, rise quickly, and remain in the water droplet. Interestingly, the number of bubbles is largely dependent on the hole depth. More bubbles were observed along with the increase of hole depth. In addition, the air bubbles were found over the entire region of the surface covered by the droplet. This special bubble exclusion phenomenon strongly revealed that water penetrated into the 85 nm-in-diameter hole and expelled air, suggesting the wetting state is indeed a full Wenzel or nearWenzel intermediate state. Conversely, no bubbles were observed when the water droplet was dripped on the porous alumina surface with a diameter of 420 nm, as shown in Figure 3b. This indicates that water remained on the substrate and air pockets were trapped underneath the water, revealing that the wetting state is full Cassie or near-Cassie intermediate case.
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Figure 5. CA of water on porous alumina surface with fixed hole diameter of 260 ( 10 nm and hole interval of 400 ( 12 nm, but varying hole depth. The line connecting the data points is for illustration only.
Figure 4. SEM images of porous alumina with hole interval of 400 nm. The corresponding depths are (a) 800 nm, (b) 1.2 µm, (c) 1.7 µm, (d) 3.4 µm, and (e) 9.2 µm.
The apparent CA of water with the porous alumina substrates in the complete Wenzel and Cassie state can be calculated using the following two theoretical expressions:8,9
cos θW ) r cos θS
(1)
cos θCB ) -1 + fS(cos θS + 1)
(2)
where θS is the Young’s CA of a smooth surface (85 ( 3° for alumina surface), θW and θCB are CAs in the Wenzel and Cassie states, respectively, r is the surface roughness factor (the ratio of total surface area to the projected area in the horizontal plane, r g 1), and fS is the fraction of the solid-liquid interface at the base of the droplet (the surface area made up by the solid substrate and air). For the porous alumina substrate with a diameter of 85 nm and a hole interval of 450 nm, the surface roughness factor is calculated to be 6.8 in the complete Wenzel state. Using eq 1, the CA in the Wenzel state is 54°. For the 420 nm-in-diameter porous alumina, the fraction of solid-liquid interface, fS, is 0.21. According to eq 2, the calculated CA is 140°. The CAs of these porous alumina substrates are all in the range between calculated CAs of complete Wenzel state (54°) and Cassie state (140°), indicating a transition region between the Wenzel and Cassie states. In order to further investigate the effect of the hole depth on the wettability of porous alumina substrate, porous alumina substrates with fixed hole diameter and interval but varied hole depth were fabricated. SEM images of the fabricated porous alumina are shown in Figure 4. The hole interval is 400 ( 18 nm, and the diameter is 260 ( 10 nm for all four samples. The corresponding hole depth for the samples shown in Figure 4a, b, c, d, and e are 0.8, 1.2, 1.7, 3.4, and 9.2 µm, respectively. The measured water CAs with the prepared porous alumina samples are shown in Figure 5. Initially, when the hole depth was increased from 0.8 to 1.2 µm, the CA decreased from 81° to 58°. However, further increase in the hole depth to 1.7 µm reversed the trend and resulted in a rapid increase in the CA to
Figure 6. Schematic illustration of water on porous alumina surface: (a) near-Wenzel intermediate state; (b,c) intermediate state where water wets the hole surface partially; (d) near-Cassie intermediate state.
110°. When the hole became deeper than 1.7 µm, the CA remained essentially constant. For the porous alumina surface with hole depths of 0.8 and 1.2 µm, the CAs are 81° and 58°, respectively, both smaller than the Young’s CA of 85 ( 3° for the smooth alumina surface. Therefore, a water droplet wet on these two surfaces in full Wenzel or near-Wenzel intermediate state. In this regime, the increase in the hole depth expands the contact area and hence reduces the CA. However, when the hole depth of the porous alumina surface is further increased to 1.7 µm and beyond, the CA becomes 110°, which is much greater than the Young’s CA of smooth alumina. This value undoubtedly belongs to a complete or near-Cassie intermediate state. Thus, varying hole depth can also induce the transition between Wenzel and Cassie statse for porous alumina surface. To explain the above-mentioned surface structure-induced transition of wetting states on porous alumina, we propose a capillary-pressure balance mechanism. Since we observed the air bubbles were dispersed from the entire surface covered by the droplet, it is reasonable to assume that water enters all the holes covered by the droplet. On the basis of this assumption, we propose a pressure balance model to further analyze the static balance state of one hole under the water droplet. This approximate analysis is sufficient to provide a simple and convenient approach to predict penetration of the water droplet into each of the pores. In this model, it is assumed that when water wets the porous alumina surface, the air in the hole is not excluded but compressed. The air is deemed as an ideal gas in the present model. Considering the water-air interface in the hole, there are two forces controlling
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the ingress of water into the hole: the capillary force and the gravity force. The capillary force is related to the Young’s CA and is defined as22
Fc ) πγd cos θ
(3)
where γ and d are the surface tension and hole diameter, respectively. Compared with the capillary force, the gravity force is relatively small and can be neglected. The ingression of water into the hole is resisted by the trapped air, and the pressure can be calculated by
Fp ) P0aπd2/4(L - a)
(4)
where P0, a, d, and L are the atmospheric pressure, water displacement into the hole by the compression of air, hole diameter, and hole depth, respectively. The capillary force is balanced by the air resistance, and the displacement of water into the hole can be termed as
a ) f(r) ) 4Lγ cos θ ⁄ (P0d + 4γ cos θ)
(5)
According to eq 5, the calculated penetrated length for hole diameters of 85, 180, 290, and 420 nm are 3.2, 2.3, 1.8, and 1.2 µm, respectively. Figure 6 shows the schematic illustration of different wetting states. For the porous alumina with a diameter of 85 nm, water ingression into the hole is relatively high, ca. 71% of the hole depth, and the wetting is the near-Wenzel intermediate state. On the other hand, for the 420 nm-in-diameter porous alumina, the calculated water displacement into the hole is only 28% of the hole depth, indicating that the wetting state is the near-Cassie intermediate regime. The other two cases are in the intermediate regime during the transition between the two states that have been described as the mixed state in a previous theoretical study.14 The aforementioned mechanism can also be applied to account for the observed hole depth-induced Wenzel-Cassie transition state. For the porous alumina with relatively large hole diameter and small hole depth, water can intrude the hole easily, and air in the hole is expelled. Thus, the Wenzel state was formed on the surface. As the hole depth increases, it becomes more difficult for water to enter the hole. Consequently, the wetting state changes to the Cassie regime.
Conclusions We have studied the surface wettability of nanoporous alumina substrates, and the structure-induced gradual transitions from the Wenzel state wetting to Cassie state wetting were realized as the hole diameter and hole depth changed. A mechanism based on capillary wetting of the hole surfaces has been proposed to explain the observed complex wetting properties. Both the experimental observation and the proposed mechanism of capillary wetting suggest that the transition of wetting states from Wenzel to Cassie induced by surface structure is gradual rather than abrupt. The Wenzel state and Cassie state are two typical states of capillary wetting on porous holes, and the wetting of a porous alumina substrate generally falls into the transition regime between the two extremes. The proposed mechanism predicts that the apparent CA depends on how much water wets the holes as well as how much air is trapped and compressed beneath the water. The capillary wetting of the hole surface can be controlled by two critical factors: the ratio of the hole diameter to the interhole spacing, and the ratio of the hole depth to the hole diameter. If the ratio of the hole diameter to the interhole spacing is small, the wetting state exhibits characteristics more like the Wenzel state. As this value increases while the interhole spacing keeps constant, the capillary wetting of the hole surface decreases and the wetting becomes more like the Cassie state. On the other hand, if the ratio of the hole depth to the hole diameter is small, water can wet the hole surface easily by excluding the air in the hole, and the wetting of the porous alumina is in the Wenzel state. There is a critical value for the ratio of the hole depth to the hole diameter, beyond which exclusion of air from the hole is prevented and water can stand on the groove to form the Cassie state. This study of correlation between wetting states and surface morphology on the nanoscale rough surface provides valuable information for the design of superhydrophobic surfaces. Acknowledgment. The authors would like to acknowledge the helpful discussions with Gang Hu, Jian Cao, and Qiqing Zhang from Unilever Research China, and Michael Butler, Guoping Lian, and Shiping Zhu from Unilever Research Colworth. LA801461J
