pubs.acs.org/Langmuir © 2009 American Chemical Society
Design of Surface Hierarchy for Extreme Hydrophobicity Yongjoo Kwon,†,§ Neelesh Patankar,‡ Junkyu Choi,† and Junghoon Lee*,† †
School of Mechanical and Aerospace Engineering, Seoul National University, San 56-1, Sillim, Gwanak, Seoul, Korea 151-742, and ‡Department of Mechanical Engineering, Northwestern University, Evanston, Illinois. § Current address: Nano Systems Institute - National Core Research Center, Seoul National University Received October 3, 2008. Revised Manuscript Received November 22, 2008
An extreme water-repellent surface is designed and fabricated with a hierarchical integration of nano- and microscale textures. We combined the two readily accessible etching techniques, a standard deep silicon etching, and a gas phase isotropic etching (XeF2) for the uniform formation of double roughness on a silicon surface. The fabricated synthetic surface shows the hallmarks of the Lotus effect: durable super water repellency (contact angle > 173°) and the sole existence of the Cassie state even with a very large spacing between roughness structures (>1:7.5). We directly demonstrate the absence of the Wenzel’s or wetted state through a series of experiments. When a water droplet is squeezed or dropped on the fabricated surface, the contact angle hardly changes and the released droplet instantly springs back without remaining wetted on the surface. We also show that a ball of water droplet keeps bouncing on the surface. Furthermore, the droplet shows very small contact angle hysteresis which can be further used in applications such as super-repellent coating and low-drag microfludics. These properties are attributed to the nano/micro surface texture designed to keep the nonwetting state energetically favorable.
1. Introduction The wetting property is amplified on a roughened surface. Thus, the contact angle increases if an intrinsically hydrophobic surface (with contact angle θi > 90°) is roughened. However, different surface energies and contact angles may result even with the same roughness texture depending on how the liquid is configured on the surface.1 When a liquid completely fills all the space in roughness, Wenzel’s model can be used to describe the contact angle θw r as a function of the ratio of roughened area to projected area.2 cos θwr ¼ W cos θi
ð1Þ
where W is the ratio of the roughened area to the projected area and θi is the intrinsic contact angle. Cassie’s model, on the other hand, describes the case when the liquid droplet sits on a composite surface.3 For a water droplet on an air-solid composite surface in the case of a complete lift-up, Cassie’s model is represented by the contact angle θcr in terms of the area fraction of liquid-solid contact.1 cos θcr ¼ C cos θi -ð1 -CÞ
ð2Þ
where C is the area fraction of liquid-solid contact. If the roughness is realized with a square pillar array as in Scheme 1, then W in eq 1 and C in eq 2 can be expressed as W = 1 + 4C/(a/ H) and C = 1/(b/a + 1)2, where a is the pillar width, b is the pillar spacing, and H is the pillar height.4,5 Generally, a very small contact angle hysteresis is observed when the liquid contacts the surface under the Cassie condition. *To whom correspondence should be addressed. E-mail:
[email protected]. kr. (1) Lafuma, A.; Quere, D. Nat. Mater. 2003, 2(7), 457–460. (2) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988–994. (3) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546–551. (4) Patankar, N. A. Langmuir 2003, 19(4), 1249–1253. (5) Lee, J.; He, B.; Patankar, N. A. J. Micromech. Microeng. 2005, 15(3), 591–600.
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Also, a remarkable reduction of fluid drag has been demonstrated in the presence of the Cassie condition due to a very small area contacting the fluid.6-8 The main goal of this work is to design and fabricate a surface that prefers the Cassie condition along with a large contact angle.
2. Theoretical Basis and Conceptual Idea The physical configuration, such as contact angle, of a liquid droplet on a roughened surface is determined at an energy minimum point. We have previously shown that such kind of energy minimum could exist according to the Wenzel and the Cassie criteria, thus leading to two different possible contact angles on the same surface.4 Furthermore, it has also been predicted by calculations and verified by experiments that there would be an energy barrier between those two states which can be overcome by external disturbances.5 Transition to the global energy minimum (lowest energy) occurs if there is an external disturbance to the local energy minimum with energy enough to overcome the energy barrier. A critical problem is that the contact angle representing the global energy minimum is always the lower one between the two possible values of the contact angles.9 It has been shown that the maximum stable contact angle can be realized by designing the roughness structure with geometries such that the Wenzel and the Cassie states have the same energy minimum at a given intrinsic contact angle.9 In this case, however, the Wenzel and the Cassie conditions have the same chance of existence, and thus, this approach is not desirable if one wants to form a Cassie surface that offers a small hysteresis. We suggest that the above issues can be circumvented by creating a roughness hierarchy with a small scale roughness superposed on a larger scale one. In this case, we can define the intrinsic contact angle as the one amplified due to the small scale roughness with vanishing dimensions compared with the (6) He, B.; Lee, J.; Patankar, N. A. Colloids Surf., A 2004, 248(1-3), 101–104. (7) Nosonovsky, M. Langmuir 2007, 23(6), 3157–3161. (8) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Perry, C. C. Langmuir 2005, 21 (3), 937–943. (9) He, B.; Patankar, N. A.; Lee, J. Langmuir 2003, 19(12), 4999–5003.
Published on Web 2/13/2009
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Scheme 1. Graphical Illustration of a Micropillar Array
lager one. If the intrinsic contact angle can be amplified in this way, there exists an opportunity to achieve the high contact angle and the Cassie state simultaneously. Figure 1 shows the calculated contact angle on a surface with a square pillar roughness structure according to eqs 1 and 2.4 The x, y, and z axes are the intrinsic contact angle, the ratio of pillar spacing to pillar width, and the cosine value of the amplified contact angle, respectively. With the amplified intrinsic contact angle (e.g., 104° vs 156° in the figure), the Wenzel’s curve exists beyond 180° (below “-1,” unrealizable) throughout the broader range of b/a values (e.g., up to 0.6 vs up to 8.2, respectively) (Supporting Information Figure 1). The following equation (eq 3) is derived by setting the Wenzel equation (eq 1) equal to -1 and can be used to calculate the marginal values of b/a below which the Wenzel state is physically unfavorable. sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 4cos θi -1 b=a ¼ ðcos θi þ 1Þða=HÞ
ð3Þ
It is worth noting that the Wenzel state exists at the border minimum of 180° if the b/a value exceeds the value calculated by eq 3.10 It would be energetically costly to have the meniscus further wet through the roughness, and thus, the surface will prefer the Cassie state with a large contact angle (e.g., over 170°) without any additional causes such as pressure and inertial force. These characteristics, that is, the dominance of the Cassie state and the large contact angle, are the key characteristics of the lotus effect. The resulting surface should have an extreme superhydrophobic characteristic together with a very low contact angle hysteresis. Even when a droplet is squeezed and forced to wet through the roughness pattern, the contact angle will remain large, and the droplet will spring back to the lifted state when released from the squeezing. A surface that realizes these characteristics will enable very low drag suitable to applications such as self-cleaning and various microfluidics. A hierarchical integration of a nanoscale texture on microscale pillarlike structures offers the very advantage of using the amplified intrinsic contact angles (Scheme 2). This kind of double-roughness-based superhydrophobic surface has been theoretically predicted, inspired by the hierarchical approach adapted by a lotus leaf.7,8,11-14 There were a number of (10) Marmur, A. Langmuir 2003, 19(20), 8343–8348. (11) Cao, L.; Hu, H. H.; Gao, D. Langmuir 2007, 23(8), 4310–4314. (12) Patankar, N. A. Langmuir 2004, 20(19), 8209–8213. (13) Feng, L.; Li, S.; Li, Y.; Li, H.; Zhang, L.; Zhai, J.; Song, Y.; Liu, B.; Jiang, L.; Zhu, D. Adv. Mater. 2002, 14(24), 1857–1860. (14) Herminghaus, S. Europhys. Lett. 2000, 52(2), 165–170.
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experimental demonstrations for such possibilities, but they were limited in detailed analysis and systematic design process. For example, nanoscale roughness was lithographically patterned on microscale structures, but only at the top surface.15-17 Also, the side walls were slanted, formed in a rather uncontrolled manner, degrading the degree of superhydrophobicity against external disturbances such as squeezing.18 The surface morphology has been duplicated by direct molding on a lotus leaf.19 However, the materials for molding were limited, and the fidelity of duplicating a living plant was questioned. Chemical and electrochemical methods were introduced with the materials limited to metals. 8,20-22 Most importantly, no systematic test or design process has been carried out to show that the prepared surface could create the stable Cassie state.8,11,13,15,22-25 In this paper, we developed a design-guided fabrication process to produce a conformal texture of nanoroughness on micropillars. Also, a series of experiments were carried out to verify the effectiveness of our design and fabrication processes combined.
3. Results and Discussion 3.1. Fabrication. A surface is known to be roughened on the nanoscale with various approaches including chemical etching, polymeric deposition, electrochemical corrosion, and laser etching.18,20-27 In our approach, a gas phase etching of Si with XeF2 vapor resulted in the desired nanoscale texture as shown in Figure 2. A similar effect, known as black silicon, was achieved by other methods such as SF6 reactive ion etching, but it was found that XeF2 etching provides the most uniform etching results over a topography such as our pillar structure. The etched surface was coated with a heptadecafluoro-1,1,2, 2-tetrahydrodecyl trichlorosilane (HDFS, Gelest #SIH5841.0) self-assembled monolayer (SAM) for a hydrophobic coating. Figure 2c shows the water droplet placed on the nanoroughness. As shown, the contact angle was measured as 156°. This value alone is not large enough to compete with other methods such as nanoturfs.28 However, this approach provides an effective way of achieving the intrinsic contact angle large enough to realize the Cassie superhydrophobicity with an extreme contact angle when combined with the microscale structures fabricated according to design process suggested in this work. In the fabrication process for the double roughness, microscale pillars were first etched by deep reactive ion etching by the Bosch process (Plasma Therm). Since aspect ratio and spacing were not very demanding, this process was readily performed with standard photolithography and etching. After fabrication of the pillars, the (15) Cortese, B.; D’Amone, S.; Manca, M.; Viola, I.; Cingolani, R.; Gigli, G. Langmuir 2008, 24(6), 2712–2718. (16) Thangawng, A. L.; Lee, J. 2004 ASME International Mechanical Engineering Congress, Proceedings of IMECE04, Anaheim, CA, 2004, pp 13-20. (17) Jeong, H. E.; Lee, S. H.; Kim, J. K.; Suh, K. Y. Langmuir 2006, 22(4), 1640– 1645. (18) Xiu, Y.; Zhu, L.; Hess, D. W.; Wong, C. P. Nano Lett. 2007, 7(11), 3388– 3393. (19) Sun, M.; Luo, C.; Xu, L.; Ji, H.; Ouyang, Q.; Yu, D.; Chen, Y. Langmuir 2005, 21(19), 8978–8981. (20) Larmour, I. A.; Bell, S. E. J.; Saunders, G. C. Angew. Chem., Int. Ed. 2007, 46, 1710–1712. (21) Wang, S.; Feng, L.; Jiang, L. Adv. Mater. 2006, 18(6), 767–770. (22) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Chabrol, G.; Perry, C. C. Adv. Mater. 2004, 16(21), 1929–1932. (23) Wang, M. F.; Raghunathan, N.; Ziaie, B. Langmuir 2007, 23(5), 2300–2303. (24) Gao, X.; Yao, X.; Jiang, L. Langmuir 2007, 23(9), 4886–4891. (25) Bormashenko, E.; Stein, T.; Whyman, G.; Bormashenko, Y.; Pogreb, R. Langmuir 2006, 22(24), 9982–9985. (26) Gao, L.; McCarthy, T. J. J. Am. Ceram. Soc. 2006, 128(28), 9052–9053. (27) Shibuichi, S.; Onda, T.; Satoh, N.; Tsujii, K. J. Phys. Chem. 1996, 100(19), 512. (28) Choi, C. H.; Kim, C. J. Phys. Rev. Lett. 2006, 96(6), 66001.
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Figure 1. Contact angles calculated according to Wenzel and Cassie conditions. Supporting Information Figure 1 shows the cross section views. The apparent contact angle is determined by the surface morphology and the intrinsic contact angle. An array of square pillars was used to calculate the contact angles in this graph. According to the Cassie condition, the apparent contact angle always remains less than 180°. Under the Wenzel condition, however, the apparent contact angle becomes a real value only if the spacing between pillars becomes larger than a certain value (e.g., b/a = 0.6 and 8.2). This means that there is some unrealizable interval for the Wenzel state that becomes larger as the intrinsic angle increases (e.g., 104° vs 156°). Scheme 2. Schematic Concept of the Roughness Hierarchy: (a) Intrinsic Contact Angle, θi, Formed on a Normal Flat Surface and Single Microroughness, and (b) Intrinsic Contact Angle Amplified by Nanoroughness and Double Roughness
Figure 2. Water droplet (5 μL) on the XeF2 etched surface. The intrinsic contact angle depends on the nanoroughness pattern. For example, if a pillar pattern were used to obtain the 156° contact angle in the Cassie state, the following conditions would be needed: b/a = 2 and a/H = 0.14. Our AFM study (a), however, does not clearly show if the equivalent roughness is realized by the XeF2 etching. The equivalent spacing and the size of the roughness do not resemble the condition given above. It is believed that the AFM profile does not correctly reflect the roughness pattern related to the hydrophobicity amplification. The SEM profile (b) shows that the nanoroughness has a hidden pattern (mushroom-like profile) that would promote the increase of the contact angle. We thus simply rely on the observed result of the measurement in determining the intrinsic contact angle.
sample was brought into the XeF2 etching chamber (Tech Bank, Korea) and etched with the same recipe used to obtain the sample in Figure 2 (Supporting Information Scheme 1). The XeF2 etching provided a uniform nanoscale roughening effect everywhere including the top, bottom, and side wall surfaces. This etching process is not demanding and can be possibly performed on a large area, since high aspect ratio structures such as nanograss are not needed,29 and (29) Kim, J.; Kim, C. J. IEEE Conf. MEMS 2002, 479–482.
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can be offered without any masking or lithographic process.18,30,31 Therefore, the overall fabrication process can be readily used in a wide range of applications where silicon is used as a beginning material for processes such as lab on a chip and molding processes. Figure 3 presents the results of fabrication. (30) Krupenkin, T.; Taylor, J. A.; Kolodner, P.; Hodes, M. Bell Labs Tech. J. 2005, 10(3), 161–170. (31) Krupenkin, T. N.; Taylor, J. A.; Schneider, T. M.; Yang, S. Langmuir 2004, 20(10), 3824–3827.
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Figure 3. Droplet on the fabricated nano-microroughened hierarchical surface. (a-c) Nanoscale roughness etched by XeF2 gas that conformally covers the microscale array of pillars fabricated through deep reactive etching. There was some shrinkage of the pillar dimensions as the XeF2 etching was introduced as shown in the discrepancies between the b/a (pillar spacing to width ratio) values of single roughness (no XeF2 etching) and double roughness (XeF2 etching) in Table 1. (d,e) Droplet sitting on the double roughness with a b/a value of 7.5, supported by only several pillars. At this large spacing, the droplet would normally wet through a single roughness. Table 1. Contact Angles for Various b/a (Calculation & Measurement) Values single microroughness (θi = 104°, a/H = 0.5) flat case no. CA (meas) [°] 1 2 3 4 5
104+2 -2 -
CA (calcd)c
nanoroughness a,b,f
CA (meas) [°] 156+2 -3 -
b/a 1.4 2.3 3.4 4.7 5.6
θw r
[°]
125.3 114.8 110.0 107.5 106.6
θcr
[°]
150.3 158.5 163.9 167.6 169.3
double roughness (θi = 156°, a/H = 0.45)
CA (meas)
CA (calcd)
d,g
e
w [°]
n-w [°]
n/d n/d 106+5 -3 116+2 -1 107+1 -2
144+1 -1 159+5 -2 158+5 -3 n/d n/d
θw r
b/a 1.6 3.6 5.1 6.6 7.5
θcr
h
[°]
n/c n/c n/c n/c n/c
CA (meas) [°]
w [°]
n-w [°]
170.8 174.8 176.1 176.8 177.2
n/d n/d n/d n/d n/d
162+4 -4 170+7 -7 172+7 -7 173+6 -7 173+6 -3
a CA: contact angle. b meas: measured value. c calcd: calculated value. d w: wet condition. e n-w: not wet condition. f -: not applicable. g n/d: not w detected. h n/c: not calculable; θw r > 180°or cos θr < -1.
Table 2. Tilting Angles for Various b/a Values
flat case no. TA [°]a-c
Figure 4. Sequential image of a water droplet (10 μL) on the tilted stage. A precision microstage was used to provide small tilting angles. The droplet on the double roughness (b/a = 6.6, case 4 of Table 2) stayed unstable as soon as the tilting started. The droplet started moving in any instance due to minute vibration, but a quick rolling started at 0.6°.
3.2. Contact Angle Measurement. Contact angles were measured and compared with the calculation for the samples with various geometric parameters (five b/a with fixed a/H for etch surface). Table 1 compares the results, showing a fair agreement between calculation and experiment. It also compares the contact angles on the surfaces with single nanoroughness and double structures. The comparison indicates the significant increase of the contact angle, from 156° to 173.3°, as a result of the hierarchical integration. Furthermore, the 6132
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1 2 3 4 5
stuck -
nanoroughness
single microroughness (θi = 104°, a/H = 0.5)
TA [°]
b/a
TA [°]d
10 -
1.4 2.3 3.4 4.7 5.6
4 stuck (wet) stuck (wet) stuck (wet) stuck (wet)
double roughness (θi = 156°, a/H = 0.45) b/a 1.6 3.6 5.1 6.6 7.5
TA [°] 2 1.5 1.4 0.6 173° were achieved together with the very small tilting angle of 0.6°. We directly verified that the Wenzel state was energetically unfavorable with various experiments such as squeezing and dropping tests of the droplets on the surface. Our approach will open up applications in systematic surface treatments with targeting areas including water-repellent coatings, self-cleaning surfaces, and low-drag microfluidics.
5. Experimental Section For the fabrication, we first formed microscale textures with various dimensions and spacing (e.g., a = 24 μm, b = 36 μm, and H = 48 μm) on a silicon surface using a deep reactive etching process (Bosch process, Plasma Therm) (Supporting Information Scheme 1b). XeF2 silicon etching (Tech Bank, Korea) was used to produce the nanoscale texture on the surface of the microscale pillars (Supporting Information Scheme 1c). The nanoscale texture is formed at the early phase of the etching process and soon disappears as the etching continues due to the isotropic etching property. Therefore, the etching time has to be carefully controlled for the uniform formation of the nanoscale texture without reduced roughness due to overetching. We set the time, area, and pressure to be 20 s, 54 mm2, and 2 Torr, respectively. The silicon surface is easily oxidized and forms a thin film of native oxide. The oxide film is very hydrophilic (contact angle < 10°) due to oxygen’s affinity with water. In order to obtain an intrinsically hydrophobic surface, we coated the surface with SAM of HDFS (heptadecafluoro-1,1,2,2-tetrahydrodecyl trichlorosilane, Gelest #SIH5841.0) (Supporting Information Scheme 1d).35 The intrinsic contact angle was 104° after the coating. We measured the contact angle using image analysis. A water droplet was placed on the given surface which was well leveled, and images were taken from the side. Before obtaining the image of the water droplet, the effect of gravity had to be minimized. So, we reduced the size of the water droplet but not until the size of the individual roughness was comparable to the droplet size. We set the volume of the droplets to be 5 μL.34 After taking the image, we tried to use standard equipment for measuring contact angles. However, the measurement was not possible in some cases, especially when the contact angle approached 180°, because the microroughness features interfered with the droplet image acquired. Thus, we relied on our own image analysis with the help of CAD (AutoCAD) software. (35) Lee, H.; Jung, G. Y. Jpn. J. Appl. Phys. 2004, 43(12), 8369–8373.
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After the image of the droplet was taken, three points were selected along the contour of the liquid-gas interface close to the triple point to construct a circle that represents the contour. The contact angle was then measured along the contour at the triple point. During the process of determining the contour of the liquidgas interface, we considered the edge blurring effect of the image, which affected the determination of the resulting contact angle. We defined the outer and the inner contour at 10% and 90% reductions of the brightness of white background, respectively, due to the image blurring effect (Supporting Information Figure 2). The minimum value of the contact angle was measured at the outer edge, and the maximum value at the inner edge. Three sets of measurements were taken with the minimum and the maximum values resulting from the image analysis. All these values were averaged and used as the representing contact angle in the paper. Table 1 shows the averaged values together with the minimum and the maximum values. For the squeezing test, a water droplet (2 μL) was pressed using a SAM coated glass against the roughened surface until the gap became 240 μm. For maintaining gap uniformity, we used multilayered tape as a spacer (single layer = 80 μm). For the dropping test, we dropped a water droplet (10 μL) on the fabricated surfaces (height = 4 mm, speed = 1.2 m/s in
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Figure 6; height = 8 mm, speed = 1.5 m/s in Supporting Information video 5). The motion of the droplet was captured using a high-speed camera (MotionPro HS-4) at 5000 frames per second. For the tilting test, the tilting stage (Namil Optics, Korea) was leveled and loaded with the fabricated samples. A water droplet (10 μL) was then placed on the sample surface, and the stage was made to begin tilting controlled by a precision screw. The angle of the stage was recorded at the onset of the slipping.
Acknowledgment. This research was supported by Pioneer R&D Program for Converging Technology and Basic Research Promotion Fund through the Korea Science and Engineering Foundation and Korea Research Foundation both funded by the Ministry of Education, Science and Technology (Grant number: M10711270001-08M1127-00110 and KRF-2007-412J03002 respectively). Supporting Information Available: Additional schemes and figures, details of the derivation of the interfacial energy, and videos showing droplet behavior for the tilting, squeezing, and dropping tests. This material is available free of charge via the Internet at http://pubs.acs.org.
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