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Langmuir 2006, 22, 10904-10908
Ultralow Hysteresis Superhydrophobic Surfaces by Excimer Laser Modification of SU-8 R. Martijn Wagterveld, Christian W. J. Berendsen,* Salim Bouaidat, and Jacques Jonsmann ScandinaVian Micro BiodeVices ApS, GammelgårdsVej 87C, DK-3520 Farum, Denmark ReceiVed July 13, 2006. In Final Form: NoVember 3, 2006 We present a new and simple method to produce superhydrophobic surfaces with ultralow hysteresis. The method involves surface modification of SU-8 using an excimer laser treatment. The modified surface is coated with a hydrophobic plasma-polymerized hexafluoropropene layer. The advancing and receding water contact angles were measured to be approximately 165°. The achieved water contact angle hysteresis was below the measurement limit. This low hysteresis can be ascribed to nanoscale debris generated during the excimer laser process.
Introduction Hydrophobic or water-repellent surfaces have a water contact angle of θ > 90°. The contact angle, defined as the angle between the liquid-solid and the liquid-vapor interface, is governed by an energy equilibrium and is given by Young’s equation.1 Both surface chemistry and surface topology are known to influence the contact angle and contact angle hysteresis,2 where the latter is defined as the difference between the advancing and receding water contact angles (θadv - θrec). The highest water contact angle on a flat surface is observed on fluorocarbon surfaces,3 with advancing angles up to 127°. A surface is considered to be superhydrophobic4-6 if both θadv g 150° and θrec g 150°. This requires the introduction of surface roughness. In the 1940s, both Wenzel7 and Cassie-Baxter8 formulated theories for the influence of roughness on the wetting properties of a surface. The theory of Wenzel is based on the fact that the contact area between the solid and the liquid is increased, resulting in a larger apparent water contact angle on a rough hydrophobic surface. The Cassie-Baxter theory is based on a composite surface or fakir state of the droplet. The droplet touches only the top of the extrusions and captures air underneath. The contact area is reduced, leading to a higher water contact angle and less pinning or sticking of the droplet and thus lower contact angle hysteresis. This makes the fakir state more suitable for achieving superhydrophobicity. In nature, the Nelumbo nucifera, or sacred lotus, has leaves that are superhydrophobic.9 This is due to a combination of roughness on the micro- and the nanoscale and a layer of wax crystals. These structures make the leaves self-cleaning because water droplets roll off the leaf, taking dirt with them on their paths. Cheng et al.10 demonstrated that the the nanostructures on top of lotus leaf microstructures are important to superhydrophobicity. * Corresponding author. E-mail:
[email protected]. (1) Brochard-Wyart, F. Soft Matter Phys. 1999, 335, 1. (2) Johnson, R. E.; Dettre, R. H. J. Colloid Interface Sci. 1977, 62, 205. (3) Wang, J. H.; Chen, J. J.; Timmons, R. B. Chem. Mater. 1996, 8, 2212. (4) Shirtcliffe, N. J.; Aqil, S.; Evans, C.; McHale, G.; Newton, M. I.; Perry, C. C.; Roach, P. J. Micromech. Microeng. 2004, 14, 1384. (5) O ¨ ner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777. (6) Que´re´, D.; Lafuma, A.; Bico, J. Nanotechnology 2003, 14, 1109. (7) Wenzel, R. N. J. Phys. Colloid Chem. 1949, 53, 1466. (8) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546. (9) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1. (10) Cheng, Y. T.; Rodak, D. E.; Wong, C. A.; Hayden, C. A. Nanotechnology 2006, 17, 1359. (11) Onda, T.; Shibuichi, S.; Satoh, N.; Tsujii, K. Langmuir 1996, 12, 2125.
Onda et al.11 were the first to show extreme superhydrophobicity on artificial surfaces using fractal surfaces. This was followed by many other groups using different technologies such as lithography,4,12 carbon nanotube forests,13 physical vapor deposition,14 single-step plasma deposition processes,15,16 micromolding,17 self-assembling nanowires,18 and laser modification processes.19,20 Another approach was to mimic the lotus leaf structure.21,22 The Wenzel and Cassie-Baxter theories have also been adapted by several groups in order to create design rules for artificial superhydrophobicity.23-27 Artificial superhydrophobicity is of practical interest for creating self-cleaning surfaces.28,29 Low hysteresis will give sliding or rolling droplets, enhancing the self-cleaning effect. Another field of application is microfluidics, where small volumes of liquid are transported on a chip (e.g., in lab-on-a-chip systems30). Superhydrophobic surfaces can be helpful in reducing friction between the liquid and the contacting surface31 in dropletbased systems, contributing to precise capillary microfluidic flow (12) Bico, J.; Marzolin, C.; Que´re´, D. Europhys. Lett. 1999, 47, 220. (13) Lau, K. K. S.; Bico, J.; Teo, K. B. K.; Chhowalla, M.; Amaratunga, G. A. J.; Milne, W. I.; McKinley, G. H.; Gleason, K. K. Nano Lett. 2003, 3, 1701. (14) Tavana, H.; Amirfazli, A.; Neumann, A. W. Langmuir 2006, 22, 5556. (15) Teare, D. O. H.; Spanos, C. G.; Ridley, P.; Kinmond, E. J.; Roucoules, V.; Badyal, J. P. S.; Brewer, S. A.; Coulson, S.; Willis, C. Langmuir 2002, 14, 4566. (16) Favia, P.; Cicala, G.; Milella, A.; Palumbo, F.; Rossini, P.; d’Agostino, R. Surf. Coat. Technol. 2003, 169/170, 609. (17) Vogelaar, L.; Lammertink, R. G. H.; Wessling, M. Langmuir 2006, 22, 3125. (18) Yang, Y. H.; Li, Z. Y.; Wang, B.; Wang, C. X.; Chen, D. H.; Yang, G. W. J. Phys: Condens. Matter 2005, 17, 5441. (19) Baldacchini, T.; Carey, J. E.; Zhou, M.; Mazur, E. Langmuir 2006, 22, 4917. (20) Khorasani, M. T.; Mirzadeh, H.; Kermani, Z. Appl. Surf. Sci. 2005, 242, 339. (21) Fu¨rstner, R.; Barthlott, W.; Neinhuis, C.; Walzel, P. Langmuir 2005, 21, 956. (22) Sun, M.; Luo, C.; Xu, L.; Ji, H.; Ouyang, Q.; Yu, D.; Chen, Y. Langmuir 2005, 21, 8978. (23) He, B.; Patankar, N. A.; Lee, J. Langmuir 2003, 19, 4999. (24) Patankar, N. A. Langmuir 2003, 19, 1249. (25) Nosonovsky, M.; Bhushan, B. Microsyst. Technol. 2005, 11, 535. (26) Wolansky, G.; Marmur, A. Colloids Surf., A 1999, 156, 381. (27) Patankar, N. H. Langmuir 2004, 20, 8209. (28) Nakajima, A.; Hashimoto, K.; Watanabe, T. Monatsh. Chem. 2001, 132, 31. (29) Blossey, R. Nat. Mater. 2003, 2, 301. (30) Vilkner, T.; Janasek, D.; Manz, A. Anal. Chem. 2004, 76, 3373. (31) Torkkeli, A.; Sasarilahti, J.; Ha¨a¨ra¨, A. 14th Conference on Micro Electro Mechanical Systems, MEMS 2001, p 475. (32) Bouaidat, S.; Hansen, O.; Bruus, H.; Berendsen, C.; Bau-Madsen, N K.; Thomsen, P.; Wolff, A.; Jonsmann, J. Lab Chip 2005, 5, 827. (33) Dupuis, A.; Le´opolde`s, J.;Buchwall, D. G.; Yeomans, J. M. Appl. Phys. Lett. 2005, 87, 1.
10.1021/la0620298 CCC: $33.50 © 2006 American Chemical Society Published on Web 11/23/2006
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Langmuir, Vol. 22, No. 26, 2006 10905 Table 1. Overview of Sample Surfaces, Water Contact Angles, and Hysteresis Resultsa water contact angles, deg
hysteresis, deg
surface
microstructure present
nanoscale debris present
θadv
θrec
(θadv - θrec)
1b 2. 3. 4.
no yes yes no
no no yes yes
118.6 ( 1.75° 164.1 ( 1.50° 165.0 ( 1.40° 164.2 ( 1.50°
80.5 ( 1.05° 136.3 ( 1.68° 164.3 ( 1.89° 164.8 ( 1.83°
38.1 ( 2.04° 27.8 ( 2.25° 0.7 ( 2.35° -0.6 ( 2.37°
a Microstructures and nanoscale debris were created by laser ablation of SU-8 (surfaces 2-4). Sample 1 and 2 were sonicated in isopropanol after excimer treatment to remove the nanoscale debris. Finally, all surfaces were coated with a hydrophobic polymer. Advancing and receding water contact angles were measured by increasing the droplet volume from 2 to 12 µL and subsequently reducing the volume to 2 µL at a rate of 60 µL/min. The contact angle average and standard deviation were calculated from the data points shown in the hysteresis graphs in Figure 3. b Reference sample: not subjected to excimer laser treatment.
Figure 1. Scanning electron microscope images of the four sample surfaces. All samples were sputter coated with a 20 nm gold layer before imaging. (a) Surface 1: flat SU-8, not subjected to excimer laser ablation but cleaned before applying the hydrophobic top coating. (b) Surface 2: microstructured by excimer laser ablation and subsequently cleaned by sonication to remove the debris generated during the excimer laser treatment. (c) Surface 3: microstructured surface with nanoscale debris. (d) Surface 4: flat SU-8 surface covered with redeposited nanoscale debris, generated in the excimer laser treatment.
control32 by creating virtual channel walls, and assisting in the positioning of liquid samples33 by surface patterning. This letter presents a new and simple process for creating superhydrophobic surfaces with ultralow water contact angle hysteresis. High-energy excimer laser pulses are used to create microstructures in SU-8 by photochemical laser ablation.34,35 As a side effect of the ablation process, nanoscale roughness is introduced as a result of the generation of debris.34 After the excimer treatment, a hydrophobic coating is applied by plasma polymerization of hexafluoropropene. The water contact angles (34) Ghantasala, M. K.; Hayes, J. P.; Harvey, E. C.; Sood, D. K. J. Micromech. Microeng. 2001, 11, 133. (35) Dyer, P. E. Appl. Phys. A 2003, 77, 167.
and hysteresis of different surfaces are compared in order to investigate the influence of the microstructures and the nanoscale roughness on the superhydrophobicity. Materials and Methods The superhydrophobic surfaces presented in this letter were produced using a three-step process. First, a layer of SU-8 was prepared on silicon. This flat SU-8 surface was then modified by an excimer laser treatment, creating a rough surface and nanoscale debris. The final step was the deposition of a hydrophobic plasmapolymerized hexafluoropropene coating. Preparation of SU-8. A layer of SU-8 50 (MicroChem) with a thickness of 50 µm was applied to a silicon wafer by spin coating,
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Figure 2. Scanning electron microscope close-up image of the nanoscale debris generated by the excimer laser ablation of SU-8. The sample was sputter coated with a 20 nm gold layer before SEM imaging. This image clearly shows the nanometer-sized details and the porous texture of the debris.
followed by a soft bake. Further steps involved crosslinking of the layer by UV exposure and a postexposure bake. Excimer Laser Treatment. The excimer laser setup used in this research is a ProMaster (Optec, Belgium) equipped with an SP 500i short pulse KrF excimer laser (ATL Lasertechnik, Germany) with a typical wavelength of 248 nm and a pulse duration of 5 to 6 ns. The treatment was carried out at a fluence of 5.5 J/cm2, with 30 shots per location and a repetition rate of 500 Hz. In this study, squares of 1 × 1 cm2 were modified. The process time for creating these squares is approximately 90 s, but this can be improved using a different laser setup. For microstructuring of the SU-8 layer, a 250 × 250 µm2 aperture was used with a horizontal grating structure having a pitch and line width of 10 µm. The structure was created by a scan in the horizontal direction, followed by a scan in the vertical direction using a vertical grating with the same dimensions. This resulted in 10 × 10 µm2 pillars with 10 µm spacing, approximately 10 µm in height. This length scale is comparable to that of the microstructures on the lotus leaf. Nanoscale debris was generated on top of the microstructures during the process. To create surfaces with only nanoscale debris but with no microstructure, a 250 × 250 µm2 square aperture (no grating) was used at the same laser settings. As a result, nanoscale debris is created and partially redeposited on the flat surface outside the excimer-treated area. This debris can be observed as a haze on the surface. Prior to coating, the debris is hydrophilic and can be removed with water. Sample Cleaning. To evaluate the influence of the nanoscale debris on the wetting properties, some samples were sonicated in a bath of isopropanol for 5 min after excimer treatment, prior to coating. Rinsing in isopropanol is a standard step in SU-8 processing and does not affect the surface texture. Hydrophobic Plasma Coating. For stabilization of the nanoscale debris and in order to generally make the surfaces hydrophobic, a fluorocarbon coating is applied by plasma polymerization using a simple (50 Hz AC-voltage) plasma generation technique, described in previous work.36 A flow of 100 sccm hexafluoropropene (>98.5%, ABCR, Germany) and 20 sccm argon is fed into the chamber and the deposition is performed at a pressure of 1.3 Pa with a plasma power density of 2 W/L for 45 s. This gives an approximate total thickness of 30 nm, determined using a QCM (Quartz Crystal Microbalance) during deposition. Water Contact Angle and Hysteresis Measurement. To determine the wetting properties of the sample surfaces, the advancing (36) Bouaidat, S.; Berendsen, C.; Thomsen, P.; Guldager, Petersen, S.; Wolff, A.; Jonsmann, J. Lab Chip 2004, 4, 632.
Figure 3. Advancing (θadv) and receding (θrec) water contact angles as a function of droplet volume on four samples surfaces. The measurements were performed using DI water at room temperature. The droplet volume was increased and subsequently reduced at a rate of 60 µL/min. The marked points were used to determine the water contact angles, hysteresis, and standard deviation values listed in Table 1. (a) Hysteresis curves of the two surfaces that were cleaned by sonication before the application of the hydrophobic coating. (1) Flat surface (lower in graph). This curve shows the hysteresis of the hydrophobic coating without micro- or nanoroughness. Symbols b and O denote θadv and θrec, respectively. (2) Microstructured and cleaned surface (upper in graph). The introduction of the microstructure increases both θadv and θrec, but because θrec < 150°, superhydrophobicity is not achieved. Symbols 2 and 4 denote θadv and θrec, respectively. (b) Hysteresis curves of the two surfaces with nanoscale debris. (3) Microstructured surface with nanoscale debris (lower in graph, left axis). Symbols 2 and 4 denote θadv and θrec, respectively. (4) Flat surface with debris (upper in graph, right axis). The same low water contact angle hysteresis is observed. Symbols b and O denote θadv and θrec, respectively. Surfaces 3 and 4 exhibit superhydrophobicity because θadv > 150° and θrec > 150°.
and receding water contact angles were measured using a FTA125 contact angle analyzer (First Ten Ångstroms) and a syringe pump (KdScientific). Measurements were carried out at room temperature, and the DI water used for the measurements was dispensed and retracted at a rate of 60 µL/min. The advancing angle is determined by increasing the volume of a water droplet from approximately 2 to 12 µL. The receding contact angle was measured by reducing the droplet volume back to 2 µL. Scanning Electron Microscopy (SEM). To investigate the surface topology, a 20 nm gold layer was deposited using a K765X sputter coater (Emitech, U.K.), and SEM images were produced using a JSM 5500LV scanning electron microscope (JEOL, Japan).
Results and Discussion Table 1 lists the characteristics of four different sample surfaces together with the water contact angles (θadv and θrec) and corresponding hysteresis values. Surface 1 is a SU-8 reference sample that was not modified by an excimer laser but only
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Langmuir, Vol. 22, No. 26, 2006 10907
Advancing and receding contact angle measurements were performed on all four substrates. The results of these experiments are displayed in Figure 3. The advancing and receding water contact angles and hysteresis data listed in Table 1 were derived from this data. The contact angles listed in Table 1 (surfaces 1 and 2) and the hysteresis curves in Figure 3a show that the introduced microstructuring has a positive effect on the hydrophobicity. Both advancing and receding contact angles are increased considerably because of the microstructures, but superhydrobicity is not achieved (θrec < 150°). The surface containing both microstructures and nanoroughness has a water contact angle of approximately 165° and shows superhydrophobicity (Figure 3b). The water contact angle hysteresis of this surface is lower than the standard deviation of the contact angle measurement (Table 1, surface 3). The literature suggests that this two-scale roughness, which is also found in the lotus leaf, is an important requirement for obtaining very high water contact angles and low hysteresis.17,21,22,27 Our results, however, show that a flat surface with nanoroughness (Figure 1d) exhibits equally high water contact angles and low hysteresis (Table 1, Figure 4). We can therefore state that the nanoroughness is the most important factor for ultralow hysteresis surfaces. This is confirmed in other publications.14-16,19 Other materials showing similar debris generation during laser ablation might also be used to produce superhydrophobic surfaces.
Figure 4. Photographs of a piece of a silicon wafer with three superhydrophobic patches before (a) and during (b) immersion in DI water at room temperature. The patches are excimer laser-treated squares of 1 × 1 cm2 area. The nanoscale debris is partially redeposited outside of the treated squares (to the right, seen as haze), increasing the superhydrophobic area. When the wafer is immersed in water, air is trapped on the superhydrophobic surfaces, shown as lightreflecting patches.
sonicated in isopropanol. The other surfaces were excimer laser treated according to the procedures described above. Surface 2 was sonicated in isopropanol to remove the nanoscale debris, whereas surfaces 3 and 4 were not cleaned before coating. Finally, all samples were coated with a hydrophobic plasma-polymerized hexafluoropropene layer. Figure 1 shows SEM images of the four surfaces. Figure 1a shows surface 1, the SU-8 reference substrate that was not excimer treated. Parts b and c of Figure 1show surfaces 2 and 3, respectively, with square protrusions that are approximately 10 µm in width and height. Nanoscale debris is visible on top of the microscale protrusions on surface 3. Comparing Figure 1b and c clearly shows that the cleaning step has efficiently removed the nanoscale debris from the microstructures of surface 2. Figure 1d shows surface 4, a flat surface covered with nanoscale debris. The image was recorded just outside the area modified by excimer laser ablation with a square aperture. The nanoscale debris is redeposited in this area. A close-up of the debris can be seen in the SEM image of Figure 2. The image clearly shows the nanometer-sized details and the porous texture of the debris generated during the excimer laser ablation process.
To investigate the robustness of the surfaces, the samples were immersed in water and in ethanol for 10 min. Figure 4a shows a piece of a silicon wafer with three superhydrophobic patches of 1 × 1 cm2 area. Figure 4b shows that air is trapped on the superhydrophobic patches upon immersion in water, indicating that the general concept of the Cassie-Baxter model is valid for this surface. The redeposited debris is visible as a haze on the surface area outside the excimer-treated square (Figure 4a) and as part of the air-trapping superhydrophobic patch (Figure 4b). Immersion in ethanol wets the squares because of the lower surface tension of ethanol. After immersion in ethanol or in water, the advancing and receding water contact angles were measured again, resulting in the same contact angle and hysteresis values. Apparently, the crosslinked plasma polymer coating has sufficient strength to be able to stabilize the debris, even during immersion in wetting liquids. The superhydrophobic surfaces were also subjected to mechanical forces. After sonication in water for 1 min, the haze had disappeared, indicating the removal of the nanoscale debris. This was confirmed by the water contact angles that were comparable to those of hydrophobic surfaces without the debris. A powerful boost of helium gas had the same effect on the superhydrophobic surfaces, demonstrating the low mechanical robustness of the nanostructures. This can possibly be optimized by changing the properties of the hydrophobic coating.
Conclusions Superhydrophobic surfaces with ultralow water contact angle hysteresis have been created using excimer laser surface modification of SU-8, followed by a plasma deposition step. The plasma-polymerized hexafluoropropene layer renders the surface hydrophobic and stabilizes the nanoscale debris generated by the excimer laser process. The surfaces show water contact angles of approximately 165° and hysteresis values below the measurement limit. The nanoroughness of the debris is shown to be the
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most important factor for the ultralow hysteresis of these surfaces. The surfaces are stable in water and ethanol but cannot resist mechanical stress. The presented method is a fast and simple way of producing low hysteresis superhydrophobic surfaces with potential applications as self-cleaning surfaces or in microfluidics. It can be expected that this method for creating ultralow hysteresis superhydrophobic surfaces by excimer laser modification can
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also be applied to other materials showing similar debris generation during laser ablation. Acknowledgment. We thank Michael B. Christiansen for preparing the SU-8 substrates. The work described in this letter is supported by the European Commission through the NMP2CT-2005-515846 project NAPOLYDE. LA0620298
