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Fabrication and characterization of superhydrophobic surface by electroplating regular rough micro-structures of metal nickel
Microelectronic Engineering 95 (2012) 130–134
Contents lists available at SciVerse ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Fabrication and characterization of superhydrophobic surface by electroplating regular rough micro-structures of metal nickel q Li Guang-yang a,b,1, Li Xue-ping a,b,1, Wang Hong a,⇑, Yang Zhuo-qing a, Yao Jin-yuan a, Ding Gui-fu a a National Key Laboratory of Science and Technology on Micro/Nano Fabrication, Research Institute of Nano/Micro Science and Technology, Shanghai Jiao Tong University, Shanghai 200240, China b Shanghai Aircraft Customer Service Co., Ltd, Shanghai 200241, China
a r t i c l e
i n f o
Article history: Received 11 January 2011 Received in revised form 15 August 2011 Accepted 29 December 2011 Available online 10 January 2012 Keywords: Superhydrophobicity Micromachining technology Hydrophilic nickel Cassie’s model
a b s t r a c t The superhydrophobic behavior has attracted many researchers’ attention. The conventional method of obtaining superhydrophobicity involves the combination of the coating of low-surface-energy material and constructing roughness on smooth surface. According to Cassie–Baxter law, this paper proposes a method that the superhydrophobicity could be obtained on structured hydrophilic surface without any coating of low-surface-energy material. Based on the surface micromachining technology, the superhydrophobic behavior has been achieved by the fabrication of micro-nickel cylinder array. It has been proved by the experiments that the micro-cylinder arrays exhibit superhydrophobic behavior; the maximum contact angle (CA) between the array and the water droplet can be up to 155°, while the intrinsic CA of smooth nickel surface is 82°. By controlling the parameters of cylinders (diameter/height/spacing), we investigated the relationship between the hydrophobicity and the above parameters. It proved that the spacing height ratio contributed to the transition from Cassie’s model to Wenzel’s, which was the main cause of disappearance of superhydrophobicity. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Superhydrophobic surface, on which the water droplet beads up with a contact angle larger than 150°, have many applications in electrical and mechanical industry [1]. Basically, it follows two rules to obtain the superhydrophobicity, either by enhancing surface roughness or by coating of low surface energy materials [2]. It has been shown that with the combination of constructing rough surface structures and low energy material coating, super-hydrophobic surfaces could be easily prepared [3–9]. Many processes have been proposed to fabricate super-hydrophobic surface: such as micro-pattern, template, nanostructure manufacturing, superhydrophobic coating [10–12]. The electrical, chemical, mechanical characters of these surfaces have also been discussed. Superhydrophobic phenomenon could be classically explained by two distinct hypotheses, the Wenzel and Cassie models. In the Wenzel model
q The project was supported by the Nano Special Program of Science and Technology Commission of Shanghai Municipality, under the Grant No. 1052nm02200 and The Program of Science and Technology Commission of Shanghai Municipality under the Grant No. 11DZ2290203. ⇑ Corresponding author. E-mail address: [email protected] (H. Wang). 1 These authors contributed equally to this work.
0167-9317/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2011.12.012
[13] the liquid completely fills the cavity of rough surface, while in the Cassie’ case [14] air is trapped underneath the liquid, which result in a composite surface. Although the validity and feasibility of the two models have been argued in recent years [15,16], the models play key roles in guiding the design of superhydrophobic surfaces. There are many ways to make rough surfaces such as mechanical stretching, laser/plasma/chemical etching [17,18], lithography [19], sol–gel processing and self-assembly [20], layer-by-layer and colloidal assembly [21]. It had been reported that the hydrophobic silicon surface was obtained with coating of OTS (octadecyltrichlorosilane) by the ICP (Inductively Coupled Plasma) etching on hydrophilic silicon surface [22]. The superhydrophobicity was obtained by the coating of low surface material on structured surfaces. However, it was notable that the adhesion force between the coated hydrophobic membrane and substrate was weak, and the coated membrane were generally electrically insulated. These flaws have confined the application of the superhydrophobic surfaces. In this paper, we proposed an alternative way based on the surface micromachining technology. The nickel array consisted of micro cylinders on copper film was manufactured. It proved that without coating of low-surface-energy material like OTS, the array exhibited superhydrophobicity. The relationship between the parameters of microarray and hydrophobicity was investigated. And mechanism of the transition from Cassie’s model to Wenzel’s model was also discussed.
G.-y. Li et al. / Microelectronic Engineering 95 (2012) 130–134
2. Basic concept
f ¼
A droplet would have a shape of spherical cap when it dipped on the surface, a curvature profile was obtained and then the contact angle measured. The CA between droplet and smooth surface was called the intrinsic CA. And the CA between the droplet and rough surface was named the apparent CA. The relationship between the intrinsic and apparent CA was described by two different theories, Wenzel [13] law and Cassie [14] law. In the Wenzel’s case, the water fills the gaps of rough surface completely where it contacts with the substrate, depicted in Fig. 1a. The apparent CA hW in Wenzel’s model was given by:
cos h
W
¼ r cos hI
ð1Þ
where hI was the intrinsic CA, and r denoted the roughness ratio which was the ratio of the actual area of rough surface to the projected area. According to Cassie’s law, the interface between the water droplet and rough surface was heterogeneous surface composed of water, air and the substrate. So the apparent CA hC could be predicted by the Eq. (2):
cos hC ¼ f ð1 þ cos hI Þ 1
ð2Þ
where f was the fraction of the solid/liquid interface under the water droplet. In Cassie’s model which was different from Wenzel’s was that the spaces among the rough structure was full of air instead of been filled by water, as shown in Fig. 1b. The regular rough surface packed with cylinder array is defined by cylinder size diameter d, height H, spacing a, as shown in Fig. 1c and d, the r and f in Eqs. (1) and (2) were given as:
r¼
ða þ dÞ2 þ pdH ða þ dÞ2
¼1þ
pdH ða þ dÞ2
ð3Þ
pd2 4ða þ dÞ2
131
ð4Þ
According to Eq. (4), the value of f should be determined by the diameter of cylinder d and the space size of cylinder a, but the height of cylinder H is irrelevant to the f. So, it is possible that the hydrophobicity could be obtained and tuned by the controlling the diameter d and spacing a. 3. Micro-fabrication Surface micromachining technology was widely applied in the manufacturing of MEMS devices [22–26]. In this paper, we fabricate microarray by metal-based surface micromachining technology. The fabrication process was illustrated in the Fig. 2. Copper/ chrome was sputtered on the quartz substrate, followed by the photo-resist spin-coat (Fig. 2a and b). After photolithography process, the photo-resist pattern was formed. Next, electroforming was conducted by using nickel sulfamate solution (Fig. 2c and d) [26]. The photo-resist was removed with acetone (Fig. 2e). After these steps, the substrate packed with cylinders with equal patch size. The prepared microarray was dipped into nickel electrolyte to electrodeposit nickel for 1–2 min to keep the homogeneity of the surface of microarray, so the manufacturing process was accomplished (Fig. 2f). The nickel electrolyte mentioned above consisted of Ni [NH2SO3]2 (600 g/L), H3BO3 (25 g/L), NiCl26H2O (10 g/L). The temperature of the electrolyte was 52 °C, and the pH value was 4.0. The deposition current density was 2 A/dm2. Topology of the sample was imaged by scanning electron microscope (SEM). The wetting ability of microarray was studied by static contact angle (CA) measurement. De-ionized water droplet of 6–8 lL was dropped onto the surface of microarray using a micro syringe. Photos of water drops were recorded with a CCD camera.
Fig. 1. Sketches of two states of water on sough surface and schematic depiction of the surface covered with designed regular array of cylinders with diameter d, height H, spacing a. (a) Wenzel’s model (b) Cassie’s model (c) top view (d) sectional view.
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G.-y. Li et al. / Microelectronic Engineering 95 (2012) 130–134
Fig. 2. Fabrication process of a microarray.
4. Results and discussion Fig. 3 shows the SEM images of prepared microarrays and their apparent CAs. With the help of transilluminator, the transmitted light could be observed between the droplet and the surface, which showed the air was trapped under the droplet, so the Cassie’s law was suitable in this experiment. Eq. (2) suggested that the air was the key factor that contributed to the hydrophobicity. Available air that trapped among the cylinders formed a curtain at the cylinder–water interface, which prevented the water infiltrated into the gaps among the cylinders. That is why the hydrophobicity of Ni film with rough surface was better than the flat ones. So it could be concluded that the hydrophobicity of the microarray should be attributed to the cylinders that trapped the air. It was necessary to investigate the relationship between the parameters of cylinders and hydrophobic behaviors. The obtained surface topography was simplified and depicted in Fig. 1. The designed parameters, predicted CAs and experimental results are listed in Table 1.
Fig. 4 shows the curves of the predicted apparent CAs according to Wenzel and Cassie formulas versus geometric parameter a/d. The experiment result was also included. The height of the arrays in Fig. 4 was 10 lm. According to the experiment result line in Fig. 4, the CAs of the prepared arrays failed to increase continuously with the increase of the a/d which was directly proportional to the fractional solid–gas interface area, but the calculated parameters increased with the increase of the a/d based on Cassie’s model. It was obviously that the prepared array surface would be similar to smooth surface when the a/d decreased to infinitesimal or increased to infinite. As the increase of a/d which could be interpreted as the density of the array, the experiment result was accord with the calculated value based on Wenzel’s model firstly, then were correspondent to Cassie’s, and finally regressed to Wenzel’s model. However, it was impossible to figure out the exact point at which the transition between the Cassie’s state and Wenzel’s occurred. Researchers have proposed several criterions for the design of regular rough surface [15,27–30]. The transition from Cassie’s state to Wenzel’ state could be attributed to the critical height of the de-
Fig. 3. SEM images of prepared cylinder arrays and the corresponding CAs. (a) d = 10 lm, a = 10 lm, H = 10 lm, CA = 135°. (b) d = 10 lm, a = 20 lm, H = 10 lm, CA = 155°. (c) d = 30 lm, a = 60 lm, H = 10 lm, CA = 152°. (d) d = 10 lm, a = 30 lm, H = 20 lm, CA = 155°. (e) d = 10 lm, a = 50 lm, H = 20 lm, CA = 155°. (f) d = 30 lm, a = 45 lm, H = 20 lm, CA = 152°. (g) Sectional view of cylinder (h) magnified sectional view of cylinder.
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G.-y. Li et al. / Microelectronic Engineering 95 (2012) 130–134 Table 1 Geometric parameters of designed cylinders and the CAs of rough surface. Sample
Designed parameters (lm)
Calculated parameters
Calculated CAs (°) C
Experimental CAs (°)
H
d
a
r
f
W
10 10 10 10 10 10 10 10 10 10
10 10 10 10 30 30 30 30 30 30
20 30 50 100 45 50 60 90 100 150
1.35 1.2 1.09 1.03 1.17 1.15 1.12 1.07 1.06 1.03
0.087 0.05 0.022 0.0065 0.126 0.11 0.087 0.05 0.042 0.022
79.2 80.4 81.3 81.8 80.6 80.8 81.0 81.5 81.6 81.8
154.2 160.7 167.2 173.0 148.9 150.9 154.2 160.8 162.3 168.0
153 154 153.6 87.0 152.0 153.0 152.0 155.0 97.0 89.0
20 20 20 20 20 20 20 20 20 20
10 10 10 10 30 30 30 30 30 30
20 30 50 100 45 50 60 90 100 150
1.7 1.39 1.17 1.05 1.34 1.29 1.23 1.13 1.11 1.06
0.087 0.05 0.022 0.0065 0.126 0.11 0.087 0.05 0.042 0.022
76.3 78.8 80.6 81.8 79.3 79.6 80.1 80.9 81.1 81.5
154.2 160.7 167.2 173.0 148.9 150.9 154.2 160.8 162.3 168.0
153.6 155.0 155.0 85.0 152.6 154.6 154.2 153.3 155.7 102.5
h
h
h*
Group 1
Group 2
*
The superscript C represents the CA was calculated based on the Cassie’s theory; the superscript w represents the CA was calculated based on the Wenzel’s theory.
signed array which was larger than the height of prepared array. It was necessary that the height of array should be larger than the critical height to stop the water protrusion between the cylinders from contacting the substrate. Otherwise the water droplet would collapse. A concise model was established to discuss the critical height of the cylinder in this experiment (Fig. 5). As we know, the large CA could be observed at Cassie’s model, which the air was trapped in the gaps among the cylinders, and the water droplet protruded between the cylinders, depicted in Fig. 5. The protrusion depth h depended on the spacing a and the angle a between the vertical plane of cylinder and the water protrusion curve. An assumption was made that the water protrusion could be described as a segment of a circle with a radius r, and based on geometry knowledge the h could be calculated as:
h¼
Fig. 4. Curves of predicted and measured CAs versus a/d, the height of the array was 10 lm.
a tanða=2Þ 2
ð5Þ
In Extrand’s experiment [29], the h0 was considered as the true contact angle, and the material of his experiment was hydrophobic with an intrinsic CA (h0 > p/2). However, in this research, the sub-
Fig. 5. Sectional view of water drop suspended on regular array. a was angle between the vertical plane of cylinder and the droplet curve, a was the spacing between adjacent cylinders, hA was the apparent angle on the array surface, h was height of the water protrusion which was also the critical height of array.
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G.-y. Li et al. / Microelectronic Engineering 95 (2012) 130–134
strate was hydrophilic nickel with h0 < p/2. The angle between the vertical plane of cylinder and the water curve could not be deduced by h0 directly. Another method to obtained h0 was presented in Wang’s experiment [31], in which the h0 was calculated based on Cassie’s model, and most of the h0 at the edge of the silicon pillars was more than 150°, according to Eq. (2), the critical height would be far greater than the height of arrays. For example, the critical height of the array of 10 lm 10 lm 50 lm (height/diameter/ spacing) in our experiment would be 39.9 lm, which was far larger than the height of the array, but the water droplet suspended on the array successfully and the CA was 153.6°. So, this method was not proper to obtain h0 on hydrophilic material. The critical height could be determined by the diameter and the spacing of cylinder by comparison of the result listed in Table 1. Further research should be carried out to propose a mathematic model to calculate the critical height of the array fabricated by hydrophilic material, which would be a meaningful factor to the design of regular hydrophobic array. 5. Conclusion In this paper, the microarrays consisted of nickel cylinders were fabricated by surface micromachining technology. It proved that hydrophobicity could be obtained by constructing regular rough structure on hydrophilic surface. Without any coating of low-surface-energy material on the fabricated microarray, the maximum contact angle was up to 155°. The super-hydrophobicity was tuned by changing the parameters of cylinders, such as the diameter d, space size a and height H. Based on Cassie’s model, the larger of the fractional the liquid–gas contact area is, the more hydrophobic surface. However, while the distance between adjacent cylinders was too large to suspend the water droplet, the water protrusion would touch the substrate, then the Cassie’s model transmitted to Wenzel’s, and the water drop collapsed. The compromise be-
tween the pursuing superhydrophobicity and the sustainability of Cassie’s state was necessary. The experiment result in this research provided useful guidance to the design of regular hydrophobic array. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
[25]
[26] [27] [28] [29] [30] [31]
Z. Xi et al., J. Mater. Chem. 18 (2008) 621–633. H. Tao et al., Appl. Surf. Sci. 256 (2010) 2400–2404. X. Feng, L. Jiang, Adv. Mater. 18 (23) (2006) 3063–3078. H. Zhang et al., Appl. Phys. Lett. 91 (2007) 254106. J.M. Xi et al., Appl. Phys. Lett. 92 (2008) 053102. C. Dorrer, J. Rühe, Adv. Mater. 5 (2009) 51–61. T. Nakanishi et al., Adv. Mater. 20 (3) (2008) 443–446. A. Lafuma, D. Quéré, Nat. Mater. 2 (7) (2003) 457–460. D. Quéré, Rep. Prog. Phys. 68 (11) (2005) 2495–2532. Y. Zhang et al., J. Mater. Chem. 18 (2008) 4442–4449. J. Pacifico et al., Langmuir 22 (2006) 11072–11076. Y.K. Lai et al., Adv. Mater. 21 (2009) 3799–3803. R.N. Wenzel, Ind. Eng. Chem. 28 (8) (1936) 988–994. A.B.D. Cassie, S. Baxter, Trans. Faraday Soc. 40 (1944) 546. L.C. Gao, T.J. McCarthy, Langmuir 23 (2007) 3762–3765. G. McHale, Langmuir 23 (2007) 8200–8205. H. Zhang et al., Sci. Technol. Adv. Mater. 6 (2005) 236–239. L.M. Lacroix et al., Surf. Sci. 592 (2005) 182–188. R. Furstner et al., Langmuir 21 (2005) 956–961. H. Shang et al., Thin Solid Films 472 (2005) 37–43. L. Zhai et al., Nano Lett. 4 (2004) 1349–1353. Y. Song, Nano-Micro Lett. 2 (2010) 95–100. M.H. Bao, W. Wang, Sens. Actuators, A 56 (1996) 135–147. C. Liu, Micro electromechanical systems (MEMS): technology and future applications in circuits, in: Proceedings of the IEEE Fifth International Conference on Solid-State and Integrated Circuit Technology, 1998. S. Sugiyama, Synchrotron radiation micro lithography and etching (SMILE) for MEMS fabrication, in: Proceedings of the IEEE International Conference on Microprocesses and Nanotechnology, 2001. Y.H. Zhang et al., Mechatronics 17 (2007) 165–171. H. Bo et al., Langmuir 19 (2003) 4999–5003. C.W. Extrand, Langmuir 18 (2002) 7991–7999. C.W. Extrand, Langmuir 20 (2004) 5013–5018. J.D. Wang et al., Langmuir 24 (2008) 10174–10180. J.D. Wang et al., Appl. Phys. Lett. 95 (2009) 084104.
Contents lists available at SciVerse ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Fabrication and characterization of superhydrophobic surface by electroplating regular rough micro-structures of metal nickel q Li Guang-yang a,b,1, Li Xue-ping a,b,1, Wang Hong a,⇑, Yang Zhuo-qing a, Yao Jin-yuan a, Ding Gui-fu a a National Key Laboratory of Science and Technology on Micro/Nano Fabrication, Research Institute of Nano/Micro Science and Technology, Shanghai Jiao Tong University, Shanghai 200240, China b Shanghai Aircraft Customer Service Co., Ltd, Shanghai 200241, China
a r t i c l e
i n f o
Article history: Received 11 January 2011 Received in revised form 15 August 2011 Accepted 29 December 2011 Available online 10 January 2012 Keywords: Superhydrophobicity Micromachining technology Hydrophilic nickel Cassie’s model
a b s t r a c t The superhydrophobic behavior has attracted many researchers’ attention. The conventional method of obtaining superhydrophobicity involves the combination of the coating of low-surface-energy material and constructing roughness on smooth surface. According to Cassie–Baxter law, this paper proposes a method that the superhydrophobicity could be obtained on structured hydrophilic surface without any coating of low-surface-energy material. Based on the surface micromachining technology, the superhydrophobic behavior has been achieved by the fabrication of micro-nickel cylinder array. It has been proved by the experiments that the micro-cylinder arrays exhibit superhydrophobic behavior; the maximum contact angle (CA) between the array and the water droplet can be up to 155°, while the intrinsic CA of smooth nickel surface is 82°. By controlling the parameters of cylinders (diameter/height/spacing), we investigated the relationship between the hydrophobicity and the above parameters. It proved that the spacing height ratio contributed to the transition from Cassie’s model to Wenzel’s, which was the main cause of disappearance of superhydrophobicity. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Superhydrophobic surface, on which the water droplet beads up with a contact angle larger than 150°, have many applications in electrical and mechanical industry [1]. Basically, it follows two rules to obtain the superhydrophobicity, either by enhancing surface roughness or by coating of low surface energy materials [2]. It has been shown that with the combination of constructing rough surface structures and low energy material coating, super-hydrophobic surfaces could be easily prepared [3–9]. Many processes have been proposed to fabricate super-hydrophobic surface: such as micro-pattern, template, nanostructure manufacturing, superhydrophobic coating [10–12]. The electrical, chemical, mechanical characters of these surfaces have also been discussed. Superhydrophobic phenomenon could be classically explained by two distinct hypotheses, the Wenzel and Cassie models. In the Wenzel model
q The project was supported by the Nano Special Program of Science and Technology Commission of Shanghai Municipality, under the Grant No. 1052nm02200 and The Program of Science and Technology Commission of Shanghai Municipality under the Grant No. 11DZ2290203. ⇑ Corresponding author. E-mail address: [email protected] (H. Wang). 1 These authors contributed equally to this work.
0167-9317/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2011.12.012
[13] the liquid completely fills the cavity of rough surface, while in the Cassie’ case [14] air is trapped underneath the liquid, which result in a composite surface. Although the validity and feasibility of the two models have been argued in recent years [15,16], the models play key roles in guiding the design of superhydrophobic surfaces. There are many ways to make rough surfaces such as mechanical stretching, laser/plasma/chemical etching [17,18], lithography [19], sol–gel processing and self-assembly [20], layer-by-layer and colloidal assembly [21]. It had been reported that the hydrophobic silicon surface was obtained with coating of OTS (octadecyltrichlorosilane) by the ICP (Inductively Coupled Plasma) etching on hydrophilic silicon surface [22]. The superhydrophobicity was obtained by the coating of low surface material on structured surfaces. However, it was notable that the adhesion force between the coated hydrophobic membrane and substrate was weak, and the coated membrane were generally electrically insulated. These flaws have confined the application of the superhydrophobic surfaces. In this paper, we proposed an alternative way based on the surface micromachining technology. The nickel array consisted of micro cylinders on copper film was manufactured. It proved that without coating of low-surface-energy material like OTS, the array exhibited superhydrophobicity. The relationship between the parameters of microarray and hydrophobicity was investigated. And mechanism of the transition from Cassie’s model to Wenzel’s model was also discussed.
G.-y. Li et al. / Microelectronic Engineering 95 (2012) 130–134
2. Basic concept
f ¼
A droplet would have a shape of spherical cap when it dipped on the surface, a curvature profile was obtained and then the contact angle measured. The CA between droplet and smooth surface was called the intrinsic CA. And the CA between the droplet and rough surface was named the apparent CA. The relationship between the intrinsic and apparent CA was described by two different theories, Wenzel [13] law and Cassie [14] law. In the Wenzel’s case, the water fills the gaps of rough surface completely where it contacts with the substrate, depicted in Fig. 1a. The apparent CA hW in Wenzel’s model was given by:
cos h
W
¼ r cos hI
ð1Þ
where hI was the intrinsic CA, and r denoted the roughness ratio which was the ratio of the actual area of rough surface to the projected area. According to Cassie’s law, the interface between the water droplet and rough surface was heterogeneous surface composed of water, air and the substrate. So the apparent CA hC could be predicted by the Eq. (2):
cos hC ¼ f ð1 þ cos hI Þ 1
ð2Þ
where f was the fraction of the solid/liquid interface under the water droplet. In Cassie’s model which was different from Wenzel’s was that the spaces among the rough structure was full of air instead of been filled by water, as shown in Fig. 1b. The regular rough surface packed with cylinder array is defined by cylinder size diameter d, height H, spacing a, as shown in Fig. 1c and d, the r and f in Eqs. (1) and (2) were given as:
r¼
ða þ dÞ2 þ pdH ða þ dÞ2
¼1þ
pdH ða þ dÞ2
ð3Þ
pd2 4ða þ dÞ2
131
ð4Þ
According to Eq. (4), the value of f should be determined by the diameter of cylinder d and the space size of cylinder a, but the height of cylinder H is irrelevant to the f. So, it is possible that the hydrophobicity could be obtained and tuned by the controlling the diameter d and spacing a. 3. Micro-fabrication Surface micromachining technology was widely applied in the manufacturing of MEMS devices [22–26]. In this paper, we fabricate microarray by metal-based surface micromachining technology. The fabrication process was illustrated in the Fig. 2. Copper/ chrome was sputtered on the quartz substrate, followed by the photo-resist spin-coat (Fig. 2a and b). After photolithography process, the photo-resist pattern was formed. Next, electroforming was conducted by using nickel sulfamate solution (Fig. 2c and d) [26]. The photo-resist was removed with acetone (Fig. 2e). After these steps, the substrate packed with cylinders with equal patch size. The prepared microarray was dipped into nickel electrolyte to electrodeposit nickel for 1–2 min to keep the homogeneity of the surface of microarray, so the manufacturing process was accomplished (Fig. 2f). The nickel electrolyte mentioned above consisted of Ni [NH2SO3]2 (600 g/L), H3BO3 (25 g/L), NiCl26H2O (10 g/L). The temperature of the electrolyte was 52 °C, and the pH value was 4.0. The deposition current density was 2 A/dm2. Topology of the sample was imaged by scanning electron microscope (SEM). The wetting ability of microarray was studied by static contact angle (CA) measurement. De-ionized water droplet of 6–8 lL was dropped onto the surface of microarray using a micro syringe. Photos of water drops were recorded with a CCD camera.
Fig. 1. Sketches of two states of water on sough surface and schematic depiction of the surface covered with designed regular array of cylinders with diameter d, height H, spacing a. (a) Wenzel’s model (b) Cassie’s model (c) top view (d) sectional view.
132
G.-y. Li et al. / Microelectronic Engineering 95 (2012) 130–134
Fig. 2. Fabrication process of a microarray.
4. Results and discussion Fig. 3 shows the SEM images of prepared microarrays and their apparent CAs. With the help of transilluminator, the transmitted light could be observed between the droplet and the surface, which showed the air was trapped under the droplet, so the Cassie’s law was suitable in this experiment. Eq. (2) suggested that the air was the key factor that contributed to the hydrophobicity. Available air that trapped among the cylinders formed a curtain at the cylinder–water interface, which prevented the water infiltrated into the gaps among the cylinders. That is why the hydrophobicity of Ni film with rough surface was better than the flat ones. So it could be concluded that the hydrophobicity of the microarray should be attributed to the cylinders that trapped the air. It was necessary to investigate the relationship between the parameters of cylinders and hydrophobic behaviors. The obtained surface topography was simplified and depicted in Fig. 1. The designed parameters, predicted CAs and experimental results are listed in Table 1.
Fig. 4 shows the curves of the predicted apparent CAs according to Wenzel and Cassie formulas versus geometric parameter a/d. The experiment result was also included. The height of the arrays in Fig. 4 was 10 lm. According to the experiment result line in Fig. 4, the CAs of the prepared arrays failed to increase continuously with the increase of the a/d which was directly proportional to the fractional solid–gas interface area, but the calculated parameters increased with the increase of the a/d based on Cassie’s model. It was obviously that the prepared array surface would be similar to smooth surface when the a/d decreased to infinitesimal or increased to infinite. As the increase of a/d which could be interpreted as the density of the array, the experiment result was accord with the calculated value based on Wenzel’s model firstly, then were correspondent to Cassie’s, and finally regressed to Wenzel’s model. However, it was impossible to figure out the exact point at which the transition between the Cassie’s state and Wenzel’s occurred. Researchers have proposed several criterions for the design of regular rough surface [15,27–30]. The transition from Cassie’s state to Wenzel’ state could be attributed to the critical height of the de-
Fig. 3. SEM images of prepared cylinder arrays and the corresponding CAs. (a) d = 10 lm, a = 10 lm, H = 10 lm, CA = 135°. (b) d = 10 lm, a = 20 lm, H = 10 lm, CA = 155°. (c) d = 30 lm, a = 60 lm, H = 10 lm, CA = 152°. (d) d = 10 lm, a = 30 lm, H = 20 lm, CA = 155°. (e) d = 10 lm, a = 50 lm, H = 20 lm, CA = 155°. (f) d = 30 lm, a = 45 lm, H = 20 lm, CA = 152°. (g) Sectional view of cylinder (h) magnified sectional view of cylinder.
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G.-y. Li et al. / Microelectronic Engineering 95 (2012) 130–134 Table 1 Geometric parameters of designed cylinders and the CAs of rough surface. Sample
Designed parameters (lm)
Calculated parameters
Calculated CAs (°) C
Experimental CAs (°)
H
d
a
r
f
W
10 10 10 10 10 10 10 10 10 10
10 10 10 10 30 30 30 30 30 30
20 30 50 100 45 50 60 90 100 150
1.35 1.2 1.09 1.03 1.17 1.15 1.12 1.07 1.06 1.03
0.087 0.05 0.022 0.0065 0.126 0.11 0.087 0.05 0.042 0.022
79.2 80.4 81.3 81.8 80.6 80.8 81.0 81.5 81.6 81.8
154.2 160.7 167.2 173.0 148.9 150.9 154.2 160.8 162.3 168.0
153 154 153.6 87.0 152.0 153.0 152.0 155.0 97.0 89.0
20 20 20 20 20 20 20 20 20 20
10 10 10 10 30 30 30 30 30 30
20 30 50 100 45 50 60 90 100 150
1.7 1.39 1.17 1.05 1.34 1.29 1.23 1.13 1.11 1.06
0.087 0.05 0.022 0.0065 0.126 0.11 0.087 0.05 0.042 0.022
76.3 78.8 80.6 81.8 79.3 79.6 80.1 80.9 81.1 81.5
154.2 160.7 167.2 173.0 148.9 150.9 154.2 160.8 162.3 168.0
153.6 155.0 155.0 85.0 152.6 154.6 154.2 153.3 155.7 102.5
h
h
h*
Group 1
Group 2
*
The superscript C represents the CA was calculated based on the Cassie’s theory; the superscript w represents the CA was calculated based on the Wenzel’s theory.
signed array which was larger than the height of prepared array. It was necessary that the height of array should be larger than the critical height to stop the water protrusion between the cylinders from contacting the substrate. Otherwise the water droplet would collapse. A concise model was established to discuss the critical height of the cylinder in this experiment (Fig. 5). As we know, the large CA could be observed at Cassie’s model, which the air was trapped in the gaps among the cylinders, and the water droplet protruded between the cylinders, depicted in Fig. 5. The protrusion depth h depended on the spacing a and the angle a between the vertical plane of cylinder and the water protrusion curve. An assumption was made that the water protrusion could be described as a segment of a circle with a radius r, and based on geometry knowledge the h could be calculated as:
h¼
Fig. 4. Curves of predicted and measured CAs versus a/d, the height of the array was 10 lm.
a tanða=2Þ 2
ð5Þ
In Extrand’s experiment [29], the h0 was considered as the true contact angle, and the material of his experiment was hydrophobic with an intrinsic CA (h0 > p/2). However, in this research, the sub-
Fig. 5. Sectional view of water drop suspended on regular array. a was angle between the vertical plane of cylinder and the droplet curve, a was the spacing between adjacent cylinders, hA was the apparent angle on the array surface, h was height of the water protrusion which was also the critical height of array.
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strate was hydrophilic nickel with h0 < p/2. The angle between the vertical plane of cylinder and the water curve could not be deduced by h0 directly. Another method to obtained h0 was presented in Wang’s experiment [31], in which the h0 was calculated based on Cassie’s model, and most of the h0 at the edge of the silicon pillars was more than 150°, according to Eq. (2), the critical height would be far greater than the height of arrays. For example, the critical height of the array of 10 lm 10 lm 50 lm (height/diameter/ spacing) in our experiment would be 39.9 lm, which was far larger than the height of the array, but the water droplet suspended on the array successfully and the CA was 153.6°. So, this method was not proper to obtain h0 on hydrophilic material. The critical height could be determined by the diameter and the spacing of cylinder by comparison of the result listed in Table 1. Further research should be carried out to propose a mathematic model to calculate the critical height of the array fabricated by hydrophilic material, which would be a meaningful factor to the design of regular hydrophobic array. 5. Conclusion In this paper, the microarrays consisted of nickel cylinders were fabricated by surface micromachining technology. It proved that hydrophobicity could be obtained by constructing regular rough structure on hydrophilic surface. Without any coating of low-surface-energy material on the fabricated microarray, the maximum contact angle was up to 155°. The super-hydrophobicity was tuned by changing the parameters of cylinders, such as the diameter d, space size a and height H. Based on Cassie’s model, the larger of the fractional the liquid–gas contact area is, the more hydrophobic surface. However, while the distance between adjacent cylinders was too large to suspend the water droplet, the water protrusion would touch the substrate, then the Cassie’s model transmitted to Wenzel’s, and the water drop collapsed. The compromise be-
tween the pursuing superhydrophobicity and the sustainability of Cassie’s state was necessary. The experiment result in this research provided useful guidance to the design of regular hydrophobic array. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
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