A universally applicable method for fabricating superhydrophobic polymer surfaces

Colloids and Surfaces A: Physicochem. Eng. Aspects 407 (2012) 85–90

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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa

A universally applicable method for fabricating superhydrophobic polymer surfaces Steven M. Hurst a,1 , Bahador Farshchian a,1 , Junseo Choi a , Jinsoo Kim b,∗∗ , Sunggook Park a,∗ a b

Mechanical Engineering Department and Center for Bio-Modular Multiscale Systems, 2508 Patrick F. Taylor Hall, Louisiana State University, Baton Rouge, LA70803, USA Department of Chemical Engineering, Kyung Hee University, Yongin 446-701, Republic of Korea

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

 A simple method to produce superhydrophobic surfaces in polymers was developed.  This method was demonstrated for PMMA, PC and COC polymers.  Surface morphology and wetting were studied as a function of O2 plasma time.  The fabricated superhydrophobic surfaces were stable up to 90 days.

a r t i c l e

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Article history: Received 13 February 2012 Received in revised form 11 May 2012 Accepted 16 May 2012 Available online 25 May 2012 Keywords: Superhydrophobic surfaces Polymers Sanding Reactive ion etching Silane deposition

a b s t r a c t The modification of polymer surfaces to manipulate their wetting properties is of great technological importance. It is well known that surface chemistry and topography jointly determine the nature of wetting on a surface. In this study, we show a cheap and effective process universally applicable to fabricate superhydrophobic surfaces on various polymers. The process combines sanding and reactive ion etching treatment of the polymer surface to generate respective micro and nanoscale surface roughness, which is followed by subsequent coating of a fluorinated silane molecule to modify the surface chemistry. A 5 min reactive ion etching treatment after sanding is sufficient to achieve nanoscale roughness required for superhydrophobic surfaces. The polymer surfaces so produced retain their superhydrophobicity for more than 90 days, demonstrating the stability of the micro and nanoscale surface roughness and the hydrophobic surface coating. Similar results are obtained with different polymers such as poly(methyl methacrylate) (PMMA), polycarbonate (PC) and cyclo-olefin copolymer (COC), indicating that the process can be applied for creating superhydrophobic surfaces on general polymer substrates. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The fabrication of superhydrophobic surfaces, defined as surfaces possessing a static water contact angle greater than 150◦ and a contact angle hysteresis or sliding angle less than 10◦ , has received significant interest due to their large array of applications. Non-wetting shows industrial applications such as contamination

∗ Corresponding author. Tel.: +1 225 578 0279; fax: +1 225 578 0279. ∗∗ Corresponding author. Tel.: +82 31 201 2492; fax: +82 31 204 8114. E-mail addresses: [email protected] (J. Kim), [email protected] (S. Park). 1 S.M. Hurst and B. Farshchian contributed equally to this work. 0927-7757/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2012.05.012

prevention, anti-oxidation, and pressure drop reduction [1]. Their self cleaning nature is desirable in photovoltaics, and their ability to produce a shear-free air–water interface may drastically reduces the drag force in microfluidics [2]. These widespread applications have motivated recent efforts to improve superhydrophobic surface fabrication techniques. The wetting state of rough surfaces can be described by the well-established Wenzel [3] and Cassie–Baxter [4] models. The Wenzel model assumes that the liquid is in intimate contact everywhere with the rough surface, completely filling any surface pores or structures. The roughness factor in the Wenzel equation, in effect, enhances the natural state of the material, giving hydrophilic surfaces a lower contact angle and hydrophobic surfaces a larger

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contact angle. The Cassie–Baxter model introduces a new wetting state in which the liquid may span across surface structures, leaving air pockets beneath it. The air–water interface in the Cassie–Baxter model allows for very low friction during droplet movement, leading to a low sliding angle. Although both the Cassie–Baxter and Wenzel states can cause high static contact angles, only the Cassie–Baxter state can lead to a very low sliding angle. Therefore, a true superhydrophobic surface requires the Cassie–Baxter state. Superhydrophobic surfaces have been achieved through the modification of both surface chemistry and topography. Hierarchical scale roughness, in combination with a low surface energy, has been shown to lead to superhydrophobicity [1]. Inspired by nature, some earlier efforts to create superhydrophobic surfaces attempted to mimic naturally occurring structures such as lotus (Nelumbo nucifera) or elephant-ear (Colocasia esculenta) leaves by using them as templates [5,6]. A variety of approaches for artificially fabricating superhydrophobic surfaces have also been explored. Techniques are widespread, and include solvent induced phase separation [7], template synthesis [8], growth of carbon nanotubes [9], electrochemical deposition [10], and sol–gel processes [11]. Many techniques create hierarchical scale surface roughness, and several make use of reactive ion etching (RIE) to create nanoroughness on a microfabricated surface [12–24]. Techniques for fabricating superhydrophobic surfaces have been summarized in the following reviews [25–27]. A focus on creating superhydrophobic surfaces in polymer substrates arises from their potential applications in low cost microelectromechanical systems (MEMS) devices, point-of-care bioanalytic devices, and microfluidics. Polymers are cheap, disposable, and have a wide range of properties, allowing for varying design requirements, as well as providing a number of low cost fabrication methods such as casting and hot embossing. Most techniques reported to produce superhydrophobic surfaces in polymer substrates rely on creating a master stamp with hierarchical micro and nanostructures and transferring the stamp pattern to a polymer by a molding technique [28,29]. While these techniques have their uses, the nanostructures in the hierarchical structures are usually low aspect ratio and thus the resulting water contact angles are around 140◦ [28,30,31]. In addition, the modality of polymers varies and thus molding techniques cannot be applied to all polymers. Recently, the d’Agostino and Gogolides research groups separately reported the use of plasma treatment in the fabrication of superhydrophobic surfaces in various polymer substrates such as poly(methyl methacrylate) (PMMA), polystyrene (PS), polydimethylsiloxane (PDMS) and SU-8 [15–24]. While plasma produced from fluoro gases (CF4 , C4 H8 , C3 F6 O, SF6 ) fluorinated the etched surfaces, an extensive plasma treatment enough to produce dual level surface roughness was required in their process. Nilsson et al. recently proposed a method of sanding Teflon to create a superhydrophobic surface. Mainly creating microscale roughness on the surface, sanding enables reduction of process time and cost significantly [32]. They have demonstrated the viability of this process, but used a specific method which applied only to Teflon, a hydrophobic polymer. In this work, we extend this method by inclusion of a simultaneous modification of surface chemistry and topography. By combining sanding with O2 plasma, the process time for plasma treatment to achieve dual level surface roughness can be significantly reduced. Surface chemistry was modified by the use of a fluorinated silane coating, which is more easily accessible than plasma from fluoro gases. By this modified process, we are able to expand this fabrication technique, allowing it to be applied universally to any polymer surfaces in large area. We first demonstrate the successful use of our technique on PMMA, and then show it can also be applied to COC and PC.

2. Methods PMMA was purchased from United States Plastic, Lima, OH. COC was purchased from TOPAS, Florence, KY. PC was purchased from Modern Plastics, Bridgeport, CT. Plane PMMA of thickness 3 mm was cut by a band saw into dimensions of 1 cm × 3 cm. For investigations on COC and PC, samples were 1 mm and 3 mm thick, respectively. They were also cut into sizes of 1 cm × 3 cm. The PMMA samples were cleaned by an air gun. The samples were sanded manually by gently scratching with an aluminum oxide sand paper of grit size 240. A uniform, reasonable force was applied manually to each sample to impart, as consistent as possible, a surface roughening. After sanding, the samples were again cleaned by an air gun to remove any dust left over from sanding. The samples were then treated by a Technics MICRO-RIE series 800 machine. Reactive ion etching (RIE) using oxygen plasma was performed; etching was done at 150 W for times of 1, 3, 5, 10, and 20 min. In the case of 20 min RIE time, etching was performed for 10 min and then stopped for 5 min to allow the chamber to cool; then etching was resumed for the final 10 min. For each sample, immediately following its RIE treatment, it was coated with a fluorinated silane molecule in a home-built silane vapor deposition chamber [33]. The silane used for deposition was (heptadecafluoro1,1,2,2-tetrahydrodecyl)trichlorosilane (C10 H4 Cl3 F17 Si) purchased from Gelest. The samples were inserted into a vacuum chamber, which was then vacuumed to a base pressure of 0.015 Torr. Upon reaching the base pressure, the chamber was sealed and a valve was opened to allow the silane to vaporize and fill the chamber. After opening the valve to the silane, 20 min was measured, giving a silane coating time of 20 min. After this time elapsed, the pressure in the chamber was 0.034 Torr. Following silane coating, the samples required at most one hour before exhibiting a superhydrophobic nature. The samples were stored in closed containers at ambient temperature (approximately 24–28 ◦ C.) The same process was followed for investigations on COC and PC. Static water contact angles were measured by using a pipette, mounted above the samples, to drop droplets of volume 8.0 ± 0.5 ␮L. Droplets were placed on the left, center, and right sides of samples, so as to not show preference to any specific location on the sample. A minimum of three measurements were taken to calculate the average contact angle on each sample. Images of the droplets resting on the samples were analyzed using the contact angle analysis system FTA 125, First Ten Ångstrom, Inc. Portsmouth, Virginia. Sliding angle measurements were performed by dropping constant volume droplets onto a sample resting on a manual goniometer. The goniometer used was model 07GON504 from CVI Melles Griot. The stage was leveled by placing a digital level in a left-right and top-down manner, and along both diagonals of the stage. The goniometer was then tilted, taking care to increase the tilt angle slowly and smoothly. Jolting the droplets, which causes them to slide prematurely, was avoided. The same pipette used for contact angle measurements was used to drop droplets of equal volume (8.0 ± 0.5 ␮L) onto the samples. A minimum of five measurements of the sliding angle were performed for each sample. Sample surfaces were investigated by use of a JEOL JSM-840A scanning electron microscope (SEM). It is hard to observe the change in the nanoroughness values with increasing O2 RIE time with a surface profilometer because the nanoroughness produced is hidden in the microscale roughness already existing on the sanded sample surface. Moreover, we could not use atomic force microscopy (AFM) for the roughness measurement because the microscale roughness is larger than the limitation of the z-direction piezo motion (7 ␮m) of our AFM scanner. Even though it is qualitative, we believe that SEM is better suited to show the effect of RIE time on the increase in the nanoroughness.

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Fig. 1. Schematic diagram of the process used for fabricating superhydrophobic surfaces on polymers. Shown are: (a) plane sample, (b) sample after sanding. The samples were sanded by hand using aluminum oxide sand paper of grit size 240. (c) Sample after RIE treatment. RIE using oxygen plasma was performed; etching was done at 150 W for times of 1, 3, 5, 10, and 20 min, and (d) sample after silane vapor deposition. The silane used for deposition was (heptadecafluoro-1,1,2,2tetrahydrodecyl)trichlorosilane (C10 H4 Cl3 F17 Si) and it was deposited on the surface of the polymer for 20 min.

3. Results and discussion A schematic depicting the fabrication process for creating a superhydrophobic polymer surface is shown in Fig. 1. First, a plain polymer surface is manually sanded to roughen the surface mainly producing microscale surface roughness. Next, the surface is treated by O2 RIE. The RIE process introduces nanoscale roughness into previously formed microscale roughness, realizing a dual-scale or hierarchical roughness and, simultaneously, prepares the surface for the subsequent fluorinated silane coating by introducing reactive surface groups. Finally, a fluorinated silane is adsorbed by vapor deposition, yielding the desired superhydrophobic surface. The microscale surface roughness produced by manual sanding was reproducible, mostly depending on the grit number of the sand paper used. Nilsson et al. reported that while surface roughness of sanded Teflon samples increases with decreasing grit number (or increasing particle sizes on the sand paper), the minimal contact angle hysteresis between advancing and receding water contact angles (150◦ /146◦ ) was obtained with an intermediate grit number of 240 [32]. The root mean square (RMS) surface roughness reported for the sanded Teflon with this grit number was 13.7 ␮m. Therefore, in the present work we used a fixed grit number of 240 for the sanding process for microscale surface roughness. The O2 RIE process following sanding induces both chemical and physical modification of the PMMA surface through strong interactions with various particles existing in the plasma which include electrons, ions, radicals and neutral molecules. In order to investigate the effect of O2 RIE on the generation of nanoscale surface roughness, we used our standard RIE recipe (DC power = 150 W and gas pressure of 150 mTorr) and varied the RIE time. SEM images characterizing sanded (top three images: a–c) and unsanded (bottom three images: d–f) PMMA samples for different O2 RIE time

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are shown in Fig. 2. The images were taken one day after fabrication. Images, from left to right, correspond to samples which were O2 RIE treated for 1, 5, and 20 min. In both (sanded and unsanded) cases, as the RIE time increases, a greater degree of roughness is produced. In the case of unsanded samples, RIE treatment produces porous structures. At longer RIE times, these pores can merge, both enlarging the pores and reducing their overall number. For sanded samples, on the other hand, the RIE treatment does not form pores, but instead creates a nanoscale roughness on top of sanded microstructures, resulting in hierarchical micro and nanostructures, a prerequisite for achieving a superhydrophobic surface. Static water contact angles and sliding angles measured on these samples (after the fluorinated silane coating) as a function of increasing RIE time are shown in Fig. 3. As the RIE time increases, the contact angle on a sanded surface increases to a value of 151.0 ± 2.4◦ . The measured static contact angle values were taken by a software program which fits the perimeter of the droplets images and finds their contact angle. However, as can be seen in the inset of Fig. 3(a), the actual contact angle is consistently larger by approximately 5◦ . The sliding angle value decreases with increasing RIE time, reaching a final value of 1.8 ± 0.7◦ at 20 min of RIE time. The results indicate that 5 min RIE time after sanding is sufficient to generate hierarchical micro and nanoroughness required to achieve superhydrophobic surfaces. The sanding process inevitably produces large scratches as seen in Fig. 2a–c, which leads to wetting inhomogeneity in the microscale roughness. However, as the size of large scratches is still far smaller than the size of the water droplets used for contact angle measurements, the wetting inhomogeneity from the large scratches is included in the measurement error. Fig. 3(a) also includes the water contact angle as a function of RIE time for unsanded samples (also after the fluorinated silane coating), which contain only nanoscale roughness produced by RIE. The water contact angle for unsanded samples was significantly lower than that for sanded samples even after 20 min RIE time. This indicates that without sanding, the Cassie–Baxter wetting state for superhydrophobicity cannot be achieved solely by the generation of nanoscale roughness. The results also corroborate with the fact that the existence of hierarchical structures greatly improves hydrophobicity [1,34,35]. The stability of the PMMA superhydrophobic surfaces fabricated by the proposed method was studied via SEM and static water contact angle measurement with aging time after fabrication. Fig. 4 shows SEM images of PMMA samples which had aged 90 days after fabrication. RIE time was 20 min, and both sanded and unsanded samples are shown. For unsanded samples, the number of pores at day 90 has significantly decreased from that at day 1. Some smaller pores seem to close and merge with nearby pores, which can be attributed to a polymer relaxation process (Fig. 4(b)). In the case of sanded samples, however, there is no significant decrease in visible roughness, which indicates that the hierarchical micro/nanoscale surface roughness required for superhydrophobicity is retained despite the presence of polymer relaxation. Fig. 5 shows static water contact angle as a function of aging time after fabrication. The sample used for this study is PMMA treated with 20 min O2 RIE and the fluorinated silane coating. It can be seen that for both sanded and unsanded samples, the contact angle does not change significantly with aging time. We also investigated sliding angles after 90 days of aging, which showed a minimal increase by ∼5◦ . Even though we only show the contact angle and sliding angle values up through 90 days of aging, the fabricated surface still showed superhydrophobicity even after 90 days. The results indicate that the superhydrophobic surfaces produced by the proposed method are stable both chemically and mechanically under no external load. In addition, the surface roughness modification and silane coating remained unchanged when the sample was washed with

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Fig. 2. SEM images of sanded (a,b,c) and unsanded (d,e,f) PMMA samples for RIE times of: 1 (a,d); 5 (b,e); and 20 min (c,f). Images are taken one day post-fabrication. Insets show higher magnification of their respective sample.

Fig. 3. (a) Static water contact angle as a function of RIE time for sanded and unsanded samples. (b) Sliding angle as a function of RIE time on sanded samples. All measurements were taken on PMMA samples one day post-fabrication. For 1 min RIE time, the 90◦ sliding angle indicates the droplet sticking to the sample surface (no sliding at any tilt angle.) For unsanded samples, sticking occurred at all RIE times (no droplet sliding.).

water. We have not studied the degradation behavior of the fabricated superhydrophobic surfaces under external mechanical and chemical load. But the degradation behavior is very important and needs to be thoroughly investigated for practical applications of the fabricated superhydrophobic surfaces. It is also worth noting that, immediately following the silane coating, the samples do not exhibit a hydrophobic nature. It requires at most 1 h before the samples become superhydrophobic. We believe that re-orientation of the hydrophobic fluoro-alkyl branches of silane molecules which are covalently bonded to the

RIE treated surface occurs. It is also possible that 3-D cross-linking between neighboring silane molecules in the form of Si O Si bonding can occur, creating a polymer network [36]. This would also improve the stability and extent of surface coverage of the silane groups. Some combinations of these effects can explain the observed time required between silane coating and surface superhydrophobicity. After examining the feasibility of the proposed process for creating superhydrophobic surfaces and their stability with PMMA, the next question is whether the developed process is applicable

Fig. 4. SEM images of sanded (a) and unsanded (b) PMMA samples, both of which were RIE treated for 20 min. Images were taken 90 days post-fabrication.

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to other polymer materials. Since the micro and nanoscale roughnesses are readily controllable by selecting an appropriate sand paper and RIE conditions, the remaining issue is how much the chemical modification by a combination of RIE and the fluorinated silane coating contributes to the increase in surface hydrophobicity with respect to what the physical modification via sanding and RIE does. This is because different amounts of reactive groups where silane molecules can bind will be formed for different polymer materials and this will result in different nominal coverage of the silane coating. In an effort to understand the effect of physical modification decoupled from that of chemical modification, static water contact angles were measured as a function of aging time for the PMMA samples sanded and treated with O2 RIE for 3 and 20 min (without silane coating). After allowing sufficient time for the hydrophobic recovery to occur, the surface chemistry returns to a near unmodified state. This allows for the opportunity to examine the effect of surface topography with little change in the surface chemistry. This is presented in Fig. 6. Immediately after the RIE treatment, the water contact angle of the sample becomes even lower than that of the pristine PMMA due to the increased surface roughness in combination of the formation of high surface energy components such as free radicals and hydroxyl polar groups. Then the samples experience a hydrophobic recovery with an increase in the contact angle while the hierarchical surface roughness is retained as shown in Fig. 4. The mechanisms of the hydrophobic recovery include the reaction and diffusion to the bulk of surface free radicals, and diffusion or reorientation of polar surface groups into the bulk [37–41]. This phenomenon can

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be easily understood by considering the Wenzel’s wetting model where the contact angle of a hydrophilic (or hydrophobic) surface decreases (or increases) with addition of surface roughness. After a period of 7 days, the 20 min etched sample approached a contact angle of 130◦ , an increase of 55◦ from its unmodified contact angle of approximately 75◦ . The samples were measured again after 90 days, and show no substantial change occurring between days 11 and 90. By including silane coating, the contact angle reaches its highest value at 151◦ ; however, this shows that modifying the surface chemistry results only in a contact angle increase of about 20◦ . Even though our discussion is qualitative, the results indicate that surface topography contributes more to the formation of superhydrophobic surfaces than the surface chemistry does and that the process developed in the present work can be extended to other polymeric materials. As can be seen from Fig. 6, our method is a general fabrication process that can be successfully applied to other polymers, such as COC and PC. While the quality of the silane coating will vary based on RIE conditions and the polymer’s chemistry, the hierarchical roughness resulting from sanding and RIE is universally applicable to all polymers. As the surface topography plays the major role in determining superhydrophobicity, the proposed fabrication process should be universally applicable. In Fig. 7, graphs for COC and PC show water contact angle measurements as a function of RIE time on each sample. The static water contact angles on the pristine COC and PC surfaces are 78.8 ± 2.2◦ and 82.3 ± 3.0◦ respectively. Data for sanded and unsanded samples immediately following fabrication are shown. Once again, the contact angles improve with increasing RIE time, and the sanded samples exhibit significantly higher contact angles. Contact angle measurements performed on these samples after 90 days of aging showed no measurable loss in hydrophobicity. Sliding angle measurements performed on samples RIE treated for 20 min showed angles well below 10◦ . Thus, superhydrophobic

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surfaces were successfully obtained on both COC and PC polymer surfaces using our fabrication method, showing its potential as a universal method to create superhydrophobic polymer surfaces. It is difficult to use the sanding process to directly fabricate superhydrophobic surfaces for microfluidics. However, after producing a hydrophobic surface on a plane substrate, it can be easily incorporated inside the walls of microfluidic channels using various 3-D fabrication techniques that have been developed recently [42–44]. Therefore, the fabrication method developed in this paper still has great potential to be used for microfluidic applications. One may argue that such hierarchical structures could be fabricated by direct replication, such as nanoimprint lithography, from pre-patterned stamp. However, with nanoimprint lithography it is difficult to replicate the nanoscale roughness produced by RIE with high replication fidelity. Moreover, the RIE process used in this study produces nanoscale surface roughness in all directions of microstructures. However, nanoimprint lithography allows replication of 2-D structures only, namely protruded or recessed structures vertical to the substrate surface, and thus any nanoroughness that is not vertical to the substrate surface cannot be replicated well by the nanoimprint process. Also, not all the polymers can be used for the replication process. In all, the fabrication process that we developed here is more flexible, and more generally applicable to various polymer materials than other replication techniques. 4. Conclusions We developed a simple process by a combination of sanding, O2 RIE and fluorinated silane coating to produce superhydrophobic surfaces in polymer substrates. The sanding and O2 RIE treatment create a hierarchical scale roughness on any polymer surface while the combination of activation by O2 RIE and subsequent silane coating of the surface allows for an effective surface chemistry modification. This process combines these beneficial effects to easily fabricate a superhydrophobic surface on COC, PC and PMMA polymer surfaces, which clearly shows that the proposed fabrication technique effectively creates a superhydrophobic surface and can be applied universally to various polymers. Acknowledgements This research was supported by National Science Foundation CAREER Award (CMMI-0643455) and by the Korea Research Foundation and The Korean Federation of Science and Technology Societies Grant funded by Korea Government (MOEHRD, Basic Research Promotion Fund). The authors gratefully acknowledge fruitful discussions with Profs. Michael C. Murphy and Steven A. Soper.

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