ARTICLE pubs.acs.org/Langmuir
Simultaneous Fabrication of Superhydrophobic and Superhydrophilic Polyimide Surfaces with Low Hysteresis Gilles Scheen,*,† Katir Ziouche,* Zahia Bougrioua, Pascale Godts, Didier Leclercq, and Tuami Lasri IEMN, CNRS, and Lille 1 University, 59652 Villeneuve d’Ascq, France ABSTRACT: Polyimide is of great interest in the field of MEMS and microtechnology. It is often used for its chemical, thermal, mechanical, and optical properties. In this paper, an original study is performed on controlled variation of polyimide film wettability. A two-step microtexturing method is developed to transform hydrophilic polyimide surfaces into a superhydrophobic surface with low magnitude of hysteresis (Δθ ≈ 0° and contact angle θ ≈ 158°). This method is based on the conception of a new kind of fakir surface with triangular cross-section micropillars, the use of a two-scale roughening, and a C4F8 coating. We demonstrate that the absence of hysteresis is related to a combination of two scales of structuring and the pillar shape. The technology that has been developed results in the simultaneous fabrication of adjacent superhydrophobic and superhydrophilic small areas, which allows an effect of self-positioning of water droplets when deposited on such a checkerboard-like surface.
1. INTRODUCTION Surface wettability is an investigation topic in constant evolution, with an increasing number of applications in various fields, for instance, the development of new generations of windows, windscreen, or boat shell using self-cleaning surfaces.1 Today, surfaces on which water droplets can roll or bounce are commonly produced.2 These kinds of surfaces are said to be superhydrophobic (SPHOB) because they do not allow water drops to stick (the contact angle that the drop makes on the surface is θ . 90°). In contrast, it is possible to create surfaces on which water droplets are spread over a wide surface, forming a very thin film. These surfaces are called superhydrophilic (SPHIL) (θ , 90°). We present in this work an original two-step method for transforming a hydrophilic polyimide surface into a superhydrophobic and superhydrophilic one simultaneously on adjacent areas. This allows the phenomena of self-positioning of the drops. The fabrication of “hierarchical” structures (nano and microstructured at the same time) allows one to reduce the adhesion to the surface significantly by deforming the contact surface of liquid solid.36 Numerous methods to prepare hierarchical structured surfaces have been reported in the literature. The most commonly employed method is the combination of fabrication of pillar arrays and the self-assembly technique7 or plasma treatment.8 This geometry is used to put the droplet in a “fakir” state lying solely on the top of the pillars. In this way, the contact surface of the droplet is essentially deformed in the plane of the top of the pillars. We have developed a simple and very efficient method to produce two-scale texturing polymer surfaces, allowing twisting of the contact surface of the drop in three dimensions. This method involves creating pillar arrays surrounded at their bases by a large roughness whose size is comparable to that of pillars. As a result, the droplet lies now both on the top of the pillars and on the top of the roughness. This configuration adds a dimension to the distortion (Figure 1). r 2011 American Chemical Society
We have compared the wetting properties and adhesion of flat, microrough, and microstructured surfaces with hierarchical structured surfaces. In this way, we have observed the effects of the structuring at different scales on the wetting. Polyimides are widely used in microelectronics and microtechnology. This family of polymers has desirable properties (chemical, thermal, mechanical, and optical) that find applications in different fields, like electronics, automotive, or aerospace industries.9 It has the characteristic of being easily spread on various kinds of surfaces via a spin-coating technique with a thickness that can be as low as a few micrometers. Nevertheless, as conventional polyimide is particularly sticky for water drops (typically θ = 75°80°), it is not used in microfluidic applications. This study aims to provide a possible pathway for the development of new families of microfluidic structures based on a simultaneous control of superhydrophobic and superhydrophilic polymide surfaces.
2. SOME THEORETICAL BASIS OF WETTABILITY 2.1. Surface Chemistry. Water is a polar molecule that can interact with surfaces through dipole/dipole and hydrogenbonding interactions. The possibility and the magnitude of these interactions depend on the surface chemistry (dangling bonds) of the substrate of interest. It is possible to modify the surface chemistry to increase, decrease, or reverse the wettability of a surface. Among these treatments are the activation of plasma polymer,10 the deposition of a self-assembled monolayer,11,12 or of a thin layer of polymer Received: December 23, 2010 Revised: March 17, 2011 Published: April 26, 2011 6490
dx.doi.org/10.1021/la1050805 | Langmuir 2011, 27, 6490–6495
Langmuir
ARTICLE
Figure 1. Drops are based both on top of pillars and on top of the roughness.
having the desired character of wettability.13 Currently, flat hydrophobic surfaces, where only the chemistry plays a role, have a contact angle θ not greater than 120°130° and they often have a good adherence to the water drops.10 Therefore, the texturing of the surface plays a crucial role in the surface hydrophobicity.4 2.2. Superhydrophobic Surfaces and Their Hysteresis. A drop of water deposited on a surface with low roughness will follow its profile. It is in the state of Wenzel14 for which the apparent contact angle θ* is evaluated by the following equation cos θ ¼ r cos θ0
ð1Þ
where r is the roughness coefficient and represents the ratio of real surface over the apparent surface (r g 1) and θ0 is the contact angle of a water drop on the flat surface. When the roughness becomes more important, the surface tension of the drop forces it to sit on the top of the asperities (see the example in Figure 1). This is called the “fakir” surface.11 The drop is in the CassieBaxter state.15 In this description, the apparent contact angle θ* between the drop border and the solid depends mainly on the fraction of effective solid contact at the droplet basis, jS. cos θ ¼ 1 þ jS ðcos θ0 þ 1Þ
ð2Þ
To be positioned preferentially in a CassieBaxter state, it is necessary for the drop to spend less energy compared to a Wenzel state. Zheng et al.16 have shown that the CassieBaxter state is thermodynamically stable if the pillar slenderness ratio (η = HL/ A, where H, L, and A are the pillar cross-sectional area, the perimeter, and the pillar height, respectively) is greater than the equienergy slenderness ratio (ηe): ηe ¼
1 jS 1 þ cos θ0 jS cos θ0
ð3Þ
In other words, the CassieBaxter (or Wenzel) wetting mode is stable, whereas the Wenzel (or CassieBaxter) wetting mode is metastable or even unstable if η > ηe (or η < ηe). Another important aspect of hydrophobic surfaces is the hysteresis of the contact angle. This hysteresis is at the origin of the adhesive character of the surface, which will act opposite to the droplet sliding.17 In fact, a superhydrophobic surface may have a very important contact angle θ* but also a large hysteresis. In this case, the droplet can form a quasiperfect sphere, but it cannot slide on the surface. The hysteresis, Δθ, will be opposed to its movement.18,19 Mathematically, the hysteresis is calculated by the difference between the front-side contact angle and the back-side contact angle of a drop moving on the surface. Experimentally, it corresponds to the difference between the maximum and the minimum values of the contact angle measured when feeding a drop on a surface and then shrinking it.
Figure 2. Plane view of three kinds of pillars arrays (named A, B, C) used for the fabrication of polyimide fakir surfaces: (a) disk-shaped and (b and c) triangularly shaped and nonlinearly stacked (Gray area symbolize the periodic pattern).
2.3. Texturing Properties To Reduce the Hysteresis. Some studies (for instance ref 18) have shown that one possible way to reduce the hysteresis of a surface is to distort as much as possible the so-called contact line, i.e., the line that materializes the junction among the three phases, the solid, the liquid and the air. Also, the shape of the pillars’ cross-section can significantly impact the hysteresis, as it can contribute a lot in the distortion of the corresponding contact line. The contact line will be more contorted when the fakir pillars are, for instance, lozenge-shaped or star-shaped (see, for example, ref 18). Superhydrophobic surfaces have been realized from the mimicry of some natural surfaces. The lotus leaves are the best example.1,20 In most cases, the surface roughness is developed at two scales (nanometer and micrometer). According to various studies,5,21,22 this double roughness avoids placing the drop in a Wenzel suction configuration. In most studies, nanoscale structure is fabricated at the top of the microstructure, allowing the drop to be in a double CassieBaxter state. The drop lies in fact on top of two scales of roughness. The above-mentioned effect can be generalized. One can anticipate that a surface prepared with a random roughness would lead to an even better result, i.e., lower hysteresis, than a fakir surface with a periodic distribution of pillars. Indeed the contact line would be even more disturbed if it follows a 3D route instead of the planar 2D one, as for “traditional” fakir surfaces. A random roughness will permit one to bend the contact line in all directions and thus contribute better to its unstability. In this work, we make use of a plasma etching technique to obtain an important random roughness at micrometer scale. This is realized together with the polyimide etching to fabricate pillars of different shapes. A low height and a low density of pillars favor a Wenzel state. In this case, the drop lies both on the top of the pillars and on the top of the roughness (Figure 1). The small asperities would play the role of air reservoirs and the drop would be in a “forced” CassieBaxter state for the two scales of roughness. This would result in a twisting of the contact line in three dimensions and a large reduction of the hystereris. 6491
dx.doi.org/10.1021/la1050805 |Langmuir 2011, 27, 6490–6495
Langmuir
ARTICLE
3. EXPERIMENTAL SECTION 3.1. Methodology. In this work we used a naturally hydrophilic polyimide substrate with a large hysteresis (flat polyimide: θ = 79° and Δθ = 33°). In this case, a drop of water clings to the surface. The fabrication of superhydrophobic surfaces needs microtexturing into a large roughness and series of pillars. As seen earlier, this permits air pockets to be trapped between the droplet and the substrate and a complex liquid/ solid/air interface to be engineered. For this, we have realized these pillars by a plasma etching process, which also induces the creation of a microscale roughness. The combination of the two scales of structure should amplify the character of the superhydrophobic surface. However, the plasma also induces a chemical modification of the surface, making the polyimide chemically superhydrophilic (creation of OH, COOH). As a consequence, for the creation of the SPHOB zones, it is important to correct the surface chemistry by changing the interface in contact with the liquid. This is done in a last step by depositing a 60 nm layer of C4F8 (naturally hydrophobic) which follows the profile of the two texturing scales (the pillars and microroughness).
Figure 3. SEM image of one disk-shaped polyimide micropillar.
The polyimide (PI) that was used in this study is PI2731 from Du Pont Electronics. It was spread onto silicon (100) single crystal templates. The final thickness of the PI was on average 10 μm. Preliminary wettability trials showed that inconsistent results are obtained if the PI pillar thickness was lower than 5 μm. Thus, we fixed the pillar thickness at 5.5 μm in this study. We decided to process triangularly shaped PI micropillars distributed in an imperfect array (nonstacked, see Figure 2b,c for the dimensions) in such a way that this should reduce the hysteresis, as the contact line would present a more complex contour and its instability would be enhanced. To measure the impact of the cross section of the pillars, we decided also to process disk-shaped PI micropillars with relatively the same jS ≈ 0.026 (Figure 2a). Using eq 3, we can show that the choice of the parameters given in Figure 2 favors a Wenzel state for the pillars. The pillar slenderness ratio (η) and equienergy slenderness ratio (ηe) were calculated for the three surfaces with the geometry of flat-top pillars without microroughness. The values calculated for various surfaces are summarized in Table 1. 3.2. Surface Preparation. The etching of the different abovementioned series of micropillars into the PI templates was carried out using reactive ion etching (RIE) and a silicon nitride (Si3N4) mask. Si3N4 was used instead of more classical photoresist, because its properties are clearly different from PI ones, allowing successful selective etching. The procedure was as follows: first a 0.5 μm thick Si3N4 layer is deposited by plasma-enhanced chemical vapor deposition (PECVD) on the PI surface. Classical photolithography is used to define the openings into the Si3N4 mask. CF4/CHF3 plasma is used to etch these openings. The denuded PI areas are then etched with an oxygen plasma, in an Oxford Plasmalab 80þ RIE system at a low pressure (40 mTorr) using a 35 sccm oxygen flux and 290 W power. The choice of a low-pressure and high-power etching allows both a high aspect ratio and the creation of microscale roughness.23 The definition of 5.5 μm height pillars could be realized within some 25 min. During the oxygen plasma etching into microcolumns, there is creation of oxygen radicals on the surface, such as CO, OH, and COOH,
Table 1. Values of the Contact Angle θ (calculated only for the pillars and measured), the Hysteresis Δθ, and Pillar Slenderness Ratio of Surfaces Treated Differently θ, deg surfaces C4F8 coated
untreated PI flat PI microrough fakir cylindrical A fakir triangular B fakir triangular C
Wenzel
118 124 125
pillar slenderness ratio
CassieBaxter
measured
measured Δθ, deg
η
ηe
170 170 170
79 116 158 152 157 158
33 38.1 4 6 0.6 0.2
4.9 11.6 11.6
47 49 44.3
Figure 4. SEM picture of fakir surfaces dotted with triangular pillars (a) and close up of a triangular pillar (b). 6492
dx.doi.org/10.1021/la1050805 |Langmuir 2011, 27, 6490–6495
Langmuir which induces a change of the degree of hydrophilicity.24 As stated earlier, this can contribute to make the polyimide SPHIL (illustrated later). Moreover, the oxygen plasma induces physical modifications (roughness): Figure 3 shows a typical roughening in the case of etching of cylinder-like pillars. At this stage, the designed areas to be SPHIL zones are selected and protected by a traditionnal planar photoresist (AZ1518 from Hoechst, for example) to prevent them from being perturbed by the next process step that will permit one to define areas with SPHOB behavior. 3.3. From SPHIL to SPHOB. One way to invert the wettability by an extra surface chemical treatment is to carry out a treatment in an ICP system.10,25,26 The use of ICP usually permits one to realize deep etching via the Bosch process,27 which alternates etch and passivation steps. When tuning the system in such a way that only the passivation is used, one can realize a plasma helped deposition of a 60 nm thick C4F8 compound, a particularly hydrophobic material. The C4F8 coating is seen to be continuous and uniform, espousing the entire surface whatever its topology (texture, roughness). An alternative route to render the etched PI SPHOB is to carry out a weak tetrafluomethane CF4 plasma treatment28 (not discussed here). Eventually, after the treatment, the resist protecting mask (of SPHIL areas) is removed.
Figure 5. Size distribution of the roughness after oxygen plasma etching at low pressure for 10 min (black) and 25 min (gray).
ARTICLE
In order to quantify and compare the wettability of untreated PI, superhydrophilic PI, and different hydrophobic PI coated with C4F8 (flat PI, microroughened PI, and the three kinds of hierarchical textured PI templates), we have realized wettability measurements on surfaces using distilled water droplets (a few microliters) and a digidrop GBX setup.
4. RESULTS 4.1. Wettability Measurement. So we have chemically and physically modified a PI surface in order to transform it into one with superhydrophobic and superhydrophilic adjacent surfaces. In Figure 4 are shown SEM pictures of one PI fakir surface realized using one of the three above-mentioned patterns (Figure 2b). The posts are clearly triangle-shaped, and it is also clear that a severe roughness has superimposed on the processed posts. The etching is not anisotropic. The pillars were slightly underetched and a small cap is created. We can assume that this phenomenon facilitates the CassieBaxter state and increases the surface hydrophobicity. The roughness has been measured by means of AFM (atomic force microscopy). Figure 5 gives the size distribution for the roughness after the plasma etching for two different etching times. The average size of the roughness depends on the etching time. After 25 min of etching, the roughness size is very important and reaches 467.2 nm (rms value) with a peak to valley height of more than 2.5 μm. As stated before, a totally untreated PI surface is slightly hydrophilic (θ = 79°) and particularly adhesive for the drops (Δθ = 33°). An illustration of this behavior is given in Figure 6a. In contrast, Figure 6b,c shows the result of a water drop deposited on two examples of treated surfaces, respectively: i/PI fakir textured with triangular posts (Figure 4b) but with no C4F8 deposit, which behaves with a SPHIL character (Figure 6b, drop completely collapsed), and ii/PI fakir treated with the same kind of triangle posts and covered with a C4F8 deposit, which permits it to display SPHOB character (Figure 6c, spherical drop). The contact angle for the superhydrophobic PI surface in Figure 6c is 158°, which is clearly larger than the one of the initial flat PI surface (79°). This angle is reasonably high, but the most
Figure 6. Pictures of a distilled water drop on a polyimide surface: (a) untreated, (b) textured and activated by oxygen plasma (SPHIL) and (c) after deposition of C4F8 and texturing (SPHOB). 6493
dx.doi.org/10.1021/la1050805 |Langmuir 2011, 27, 6490–6495
Langmuir
Figure 7. A water drop is deposited on a surface polyimide covered with C4F8 (a) without microtexturation and (b) with microtexturation. The drop is maintained by the pipet while the surface moves from right to left.
striking result is that this fakir surface has almost no hysteresis, which means that this kind of surface is particularly nonadhesive. The experimental values obtained for the contact angle (θ) and the hysteresis (Δθ) for the different situations are summarized in Table 1. They will be interpreted in next section. The measured contact angle θ is lower than the prediction made by the CassieBaxter theory applied to the surfaces without roughness. This can be explained by the fact that the drop does not lie only on the pillars but also on the microroughness. Indeed, eq 3 and η values calculated in Table 1 show that the drop tends to be in a Wenzel state. But the microroughness between the pillars prevents the drop from impaling itself on the pillars. 4.2. Effects of the Roughness. When polyimide surfaces undergo the hydrophilic treatment (structuring and activation by O2 plasma etching), they present total wetting properties: the droplets spread into a thin film of liquid. Figure 6b displays this result (the photo is taken 20 ms after depositing the drop). After 80 ms, the drop is not visible by the camera. The contact angle has no meaning because the wetting is total. According to Wenzel’s theory (eq 1), we can assume that the superhydrophilic character of the PI surface generated by the O2 activation during plasma etching is amplified (SPHIL) by the presence of a roughness (roughness coefficient r g 1 and cos θf0). To evaluate the impact of surface texturing in the case of hydrophobic wetting, we make the following experiment: a droplet is maintained on a particular PI surface then this later is put into motion and we look at the evolution of the droplet shape. Figure 7 shows two cases: in Figure 7a the droplet is on a flat PI surface only covered with C4F8 and in Figure 7b the droplet is on a PI surface that has received a complete SPHOB treatment with fakir triangular C pattern and plasma-induced roughness. The displacement of the droplet on the first flat, C4F8-coated surface displays a large hysteresis (Δθ = 38.1°, Table 1). The
ARTICLE
droplet remains attached. Conversely, the textured surface has a low hysteresis (Δθ ≈ 0°) and the drop slips without adhering. So our kind of texturation of PI surfaces permits one to decrease considerably the hysteresis and thus its adhesive character. Further analysis of the wettability and hysteresis measurements presented in Table 1 permits one to evaluate the benefit of combining the two scales of roughness: the results obtained on singly microrough surface (plasma induced roughness) and on a two-scale structured surface show that the contact angle is relatively the same (θA ≈ 158°). However, the hysteresis is higher on the singly microrough surface (4° for the first compared with 1° for the triangle-shaped Fakir surfaces B and C). As reported earlier (first section) this can be related to the fact that the triple line is confined in a more distorted situation than on a single-scale structured surface. One can also see from the experiment (Table 1) that the shape of the pillars’ cross section also impacts clearly the wettability. Though the fakir surfaces with cylindrical posts (A) and with triangular posts (B and C) have almost the same fraction of effective solid contact at the droplet basis [jS ≈ 0.026 in CassieBaxter theory (eq 2)], the latter have a markedly improved “unwettability”: the contact angle is slightly larger and the hysteresis is close to zero (instead of ≈6° for the former). The triangle-fakir textured and C4F8-coated PI surfaces present interesting nonadherence properties to the water drop. 4.3. Rebounding and Self-Positioning. We have looked at the bouncing of a drop on different surfaces using a fast camera (500 frames/s). Figure 8 shows the recorded pictures taken for the SPHOB polyimide surface textured following pattern C. The acceleration makes the droplet flatten after it impacts the fakir surface. Then it gets back to its spherical shape thanks to the surface tension and is eventually expelled upward as the impact energy has not been lost. If the surface hysteresis was significant, the drop would not have succeeded in rebounding from the surface. In the case of cylindershaped pillars (pattern A), an amortized bouncing was observed compared to the triangle case, consistent with the slightly higher hysteresis. Another experiment was carried out: we observed the selfinitiated gliding of a drop on a SPHOB surface. Once a drop of a few microliters is deposited on this kind of surface, it quickly starts to put itself into motion (Figure 9). The surface “drives out” the drop. This “moving droplet” ultimately stops when it reaches a border of the sample (if all its surface is uniformly treated) or this moving droplet shrinks when it reachs a superhydrophilic area (when there are side by side areas with different nature, e.g., alternating SPHOB and SPHIL). In the case of a checkerboard patterned PI surface with adjacent SPHOB and SPHIL square areas, a water droplet (smaller than the dimension of an area) placed on it will systematically move to the nearest SPHIL area and collapse in it (only on its limited perimeter). This is what we call self-positioning capability.
Figure 8. Observation of bounds on a SPHOB fakir triangular surface with pattern C. 6494
dx.doi.org/10.1021/la1050805 |Langmuir 2011, 27, 6490–6495
Langmuir
ARTICLE
’ ACKNOWLEDGMENT The authors want to thank Stephane Dorbolo (ULg, Belgique) for the fast camera imaging. ’ REFERENCES
Figure 9. Spontaneous displacement of a droplet on a superhydrophobic surface.
5. CONCLUSION We have transformed a naturally hydrophilic surface of polyimide into adjacent superhydrophilic (SPHIL) and superhydrophobic (SPHOB) areas with near-zero hysteresis by using a single manufacturing process. The fabrication of unwettable areas on polyimide surface was carried out first by a two-scale texturation achieved simultaneously by O2-based RIE etching: definition of a pattern of micropillars and development of a superimposed micrometric roughness allowing the deformation of the contact line in three dimensions. Second, because the O2 plasma creates O-based radicals on the textured surface and so transforms the surface into SPHIL, an extra technological step is applied on the areas to be made SPHOB: a few tens of nanometers of C4F8 are deposited to reverse locally the wettability character (from SPHIL to SPHOB). We have demonstrated that the combination of the twostructuration scales decreases the hysteris to near zero. For texturing we proposed a new design of pillars with triangular cross-section because it allows a better distortion of the contact-line (water contour in contact with the solid and the air); it contributes to increase the hydrophobicity of the polyimide surface by reducing its hysteresis. Triangularly shaped micropillars were compared to cylinder-shaped ones. Although a similar large contact angle is obtained on both types of textured SPHOB surfaces (152° versus 158°), the fakir surface based on triangular posts has a much reduced hysteresis, almost zero, thus preventing a water drop to get hooked. Our approach provides a simple process for making simultaneously adjacent superhydrophobic and superhydrophilic small areas on various substrates. On the first kind of area, a water drop spreads completely and there is a total wetting. On the second kind, the drop is repelled by the surface and slides. A drop placed on the border between the two kinds of surface moves spontaneously toward the superhydrophilic zone without external intervention. It is a self-positioning behavior that may be very interesting for different applications.
(1) Furstner, R.; Barthlott, W.; Neinhuis, C.; Walzel, P. Langmuir 2005, 21, 956–961. (2) Merlen, A.; Brunet, P. J. Bionic Eng. 2009, 6 (4), 330–334. (3) Li, W.; Amirfazli, A. Soft Matter 2008, 4, 462–466. (4) Li, X.-M.; Reinhoudt, D.; Crego-Calama, M. Chem. Soc. Rev. 2007, 36, 1350–1368. (5) Minko, S.; Muller, M.; Motornov, M.; Nitschke, M.; Grundke, K.; Stamm, M. J. Am. Chem. Soc. 2003, 125 (13), 3896–3900. (6) Sajadinia, S. H.; Sharif, F. J. Colloid Interface Sci. 2010, 344, 575–583. (7) Bhushan, B.; Koch, K.; Jung, Y. C. Ultramicroscopy 2009, 109, 1029–1034. (8) Cortese, B.; D’Amone, S.; Manca, M.; Viola, I.; Cingolani, R.; Gigli, G. Langmuir 2008, 24, 2712–2718. (9) Wilson, D.: Stenzenberger, H. D.; Hergenrother, P. M. (Eds.) Polyimides; Chapman and Hall: New York, 1990. (10) Coulson, S. R.; Woodward, I.; Badyal, J. P. S.; Brewer, S. A.; Willis, C. J. Phys. Chem. B 2000, 104, 8836–8840. (11) Bico, J.; Marzolin, C.; Quere, D. Europhys. Lett. 1999, 47 (2), 220–226. (12) Gnanappa, A. K.; O’murchu, C.; Slattery, O.; Peters, F.; O’hara, T.; Aszalos-Kiss, B.; Tofail, S. A. M. Appl. Surf. Sci. 2011, 257, 4331–4338. (13) Van Der Wal, P.; Steiner, U. Soft Matter 2007, 3, 426–429. (14) Wenzel, R. J. Phys. Chem. 1949, 53 (9), 1466–1467. (15) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546–551. (16) Zheng, Q. S.; Yu, Y.; Zhao, Z. H. Langmuir 2005, 21, 12207–12212. (17) Kim, J.; Kim, C. Micro-Electro-Mech. Syst. 2002, 479–482. (18) Oner, D.; McCarthy, T. Langmuir 2000, 16, 7777–7782. (19) Yang, J.; Chen, J.; Huang, K.; Yeh, J. J. Microelectromech. Syst. 2006, 15 (3), 697–707. (20) Roach, P.; Shirtcliffe, N.; Newton, M. Soft Matter 2008, 4 (2), 224–240. (21) Callies, M.; Quere, D. Soft Matter 2005, 1, 55–61. (22) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Chabrol, G.; Perry, C. C. Adv. Mater. 2004, 16 (21), 1929–1932. (23) Agarwal, N.; Ponoth, S.; Plawsky, J.; Persans, P. J. Vac. Sci. Technol., A 2002, 20 (5), 1587–1591. (24) Egitto, F.; Emmi, F.; Horwath, R.; Vukanovic, V. J. Vac. Sci. Technol. B 1985, 3 (3), 893–904. (25) Fresnais, J.; Chapel, J.; Poncinepaillard, F. Surf. Coat. Technol. 2006, 200, 5296–5305. (26) Kim, S.; Kim, J.; Kang, B.; Uhm, H. Langmuir 2005, 21, 12213–12217. (27) Guidini, V.; Moshkalyov, S.; Tatsch, P. Electrochem. Soc. Proc. 2004, 3, 381–386. (28) Egitto, F. Pure Appl. Chem. 1990, 62 (9), 1699–1708.
’ AUTHOR INFORMATION Corresponding Authors
*E-mail:
[email protected];
[email protected]. Present Addresses †
Research Center in Micro and Nanoscopic Materials and Electronic Devices, Place du Levant, 3, Maxwell Building, B-1348 Louvain-la-Neuve, Belgium. 6495
dx.doi.org/10.1021/la1050805 |Langmuir 2011, 27, 6490–6495
