Controlling the Wettability of Hierarchically Structured Thermoplastics

ARTICLE pubs.acs.org/Langmuir

Controlling the Wettability of Hierarchically Structured Thermoplastics Barbara Cortese* and Hywel Morgan ECS-Nano Group, School of Electronics and Computer Science, University of Southampton, University Road, Southampton SO17 1BJ, United Kingdom ABSTRACT: Surfaces play an important role in defining the properties of materials, controlling wetting, adsorption, or desorption of biomolecules, and sealing/bonding of different materials. We have combined microscale features with plasma-etched nanoscale roughness and chemical modification to tailor the wettability of the substrates. Cyclic olefin polymers and copolymers (COPs/COCs) were processed to make a range of surfaces with controlled superhydrophobic or -hydrophilic properties. The hydrophobic properties of the polymers were increased by the introduction of microstructures of varying geometry and spacing through hot embossing. The COC/COP substrates were functionalized by plasma activation in O2, CF4, and a mixture of both gases. The plasma etching introduces nanoscale roughness and also chemically modifies the surface, creating either highly hydrophilic or highly hydrophobic (contact angle >150°) surfaces depending on the gas mixture. The influence of geometry and chemistries was characterized by atomic force microscopy, contact angle measurements, and X-ray photoelectron spectroscopy. Measurements of the contact angle and contact angle hysteresis demonstrated long-term stability of the superhydrophobic/superhydrophilic characteristics (>6 months).

’ INTRODUCTION The interest in surfaces with extreme wettability is driven by their scientific and economic impact, for example, their use in biotechnology as smart structural coating materials or in microfluidics. Wettability is an important aspect of a material and is influenced by both surface chemistry and surface topography. In this context, hierarchical micro- and nanostructured surfaces are found in nature, for example, the lotus leaf, which is extremely water repellent and is self-cleaning as water drops roll off the surface.1,2 These types of surfaces can exhibit either the Cassie3 or Wenzel4 state, depending on whether the drop sits on air-filled pockets or wets the cavities of the textured surface, respectively. There are many ways of modifying surfaces to increase their hydrophobicity or hydrophilicity, for example, by inducing roughness,5,6 or with hierarchical roughness at both the micro- and nanometer scales.7,14 However, many of these approaches often involve complicated procedures and expensive materials, and the durability and robustness of the material are often problematic.15 18 In addition to enhancing the hydrophobicity of surfaces, the control of hydrophilicity is interesting as most polymers are inherently hydrophobic, making them unsuitable for many biomedical and industrial applications.19 Therefore, a simple and effective method of generating surfaces with different wetting properties and chemical functionalities would be desirable. Thermoplastic polymers are widely used in biomedical devices; some of the most commonly used are cyclic olefin polymers and copolymers (COPs/COCs) based on comonomers such as ethylene and norbornene and designated as cyclic due to the presence of the ringlike structures in the side chain on the backbone (Figure 1). They have low water resistance, have low r 2011 American Chemical Society

Figure 1. Chemical structures of (a) COCs obtained by chain copolymerization of cyclic monomers with ethene and (b) COPs obtained by ring-opening metathesis polymerization of various cyclic monomers followed by hydrogenation.

permeability to oxygen,20,21 are optical transparent,22 25 and are easy to process. However, their hydrophobic nature and passive surface chemistry can limit their use.26 One method of modifying the surface energy of a substrate is through plasma treatment, which can increase wettability through the introduction of surface polar groups.27 The effects of oxygen, argon, and fluorocarbon plasma on thermoplastic substrates can change their wettability and adhesion.28 Johansson and co-workers demonstrated modification of COCs and COPs with low-pressure radio frequency (rf) plasma in air, improving surfaces for cell culture.29 Steffen et al. investigated surface treatment with microwave plasma using NH3 and SO2 plasmas,30 and combining plasma with photografting can increase the hydrophilicity of COCs.31 A serious Received: September 23, 2011 Revised: October 31, 2011 Published: November 01, 2011 896

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Figure 2. Schematic diagram showing the fabrication of hierarchical surfaces with nano- and microstructures. Micrometer-sized pillars were embossed into COC/COP substrates. Nanostructuring and chemical modification of the surface were obtained by exposing the microstructured substrate to plasma. Superhydrophobic or -hydrophilic surfaces were obtained by chemically exposing the surface to CF4 plasma or O2 plasma, respectively.

drawback, however, is the aging that occurs during storage; the polymers are unstable, and the native hydrophobic properties return quite rapidly.32 Understanding the effect of micro- and nanoroughness combined with different surface chemistries should help in the design of biomaterials, for example, in tissue engineering or microfluidics.33,34 In this paper we describe a simple, efficient, and highly reproducible method of producing well-defined, dual scale roughened substrates on a large area to provide a surface with well-controlled and stable wettability. A surface with large (macro) scale features is combined with nanoscale features made by plasma etching together with modifications to the surface chemistry, producing a surface with controllable wettability. The micrometer-sized textured surface of COCs/COPs was fabricated by using a hot embossing approach with a soft poly(dimethylsiloxane) (PDMS) master, as shown schematically in Figure 2. After embossing, nanotexturing and surface chemical modification were created by microwave plasma etching. O2 and CF4 plasmas produced surfaces with nanoscale roughness and chemical modification that together modified surface wettability in a controlled way. A mixture of the two gases led to changes in the surfaces that depended on the mixture ratio, changing the material from hydrophobic to hydrophilic. The surfaces were characterized with contact angle (CA), CA hysteresis (CAH), and sliding angle measurements. Atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) analyses

were used to investigate changes in surface chemistry. Aging and recovery of the COC/COP surfaces (stored in ambient clean room air) were measured for up to 6 months and showed little change. This simple approach can be used to create superhydrophobic/superhydrophilic patterns on an otherwise hydrophobic surface.

’ EXPERIMENTAL APPROACH Materials. Optically transparent 1 mm thick COC/COP wafers were purchased from TOPAS Advanced Polymers GmbH, Frankfurt, Germany. Prior to plasma processing, they were cleaned in isopropyl alcohol (IPA) and deionized (DI) water. Deionized water and diiodomethane (Sigma-Aldrich) were used as test liquids. All reagents were used as received. Plasma Processing. All samples were exposed in a high-density microwave plasma generator of 2.45 GHz frequency (Tepla Giga-Etch 100-E plasma system, Pva Tepla, Asslar, Germany). In the case of the O2 and CF4plasma, the samples were treated in pure gas at a pressure of 0.8 Pa with a power of 500 W. The treatment duration was varied between 1 and 30 min. The same reactor was also used for the deposition of a thin fluorocarbon film along with plasma etching using CF4 gas at conditions of 500 W and 0.8 Pa, rendering the surface superhydrophobic. To obtain superhydrophilic substrates, samples were also exposed to a CF4/O2 (88:12) gas plasma with the following parameters: 500 W and exposure times of 1, 5, 10, 15, and 30 min. After treatment the pieces were stored in sterile containers until characterization. All experiments were performed in three replicates. 897

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Figure 3. (a) SEM image of a microstructured COC. a is the lateral dimension of the pillars, d the spacing between the pillars, and h the height. An image of a water droplet on micropillars (a = 25 μm, d = 25 μm, h = 30 μm) is shown in the inset. (b) CA and CAH as a function of the fill factor (air fraction) for a COC surface calculated as a2/(a + b)2, where the fill factor of a flat sample was obviously 1. The increase in CA values and decrease in hysteresis with increasing air fraction are probably due to an increasing amount of air trapped beneath the water droplet. COP surfaces were similar.

Surface Characterization. Contact angle measurements were made using ultrapure water at room temperature using a contact angle goniometer, drop shape analysis system, DSA30 (Kr€uss GMBH, Hamburg, Germany). Five 3 μL drops of water were placed on the substrate, and the contact angles of the left and right sides of each drop were measured and averaged. The average of the data points at different places on the surface was used to calculate the final average value. CA hysteresis was measured using the advancing and receding CAs, measured as the droplet volume was continuously increased and decreased, respectively. Surface roughness was measured with an atomic force microscope (Nano Scope IIIa, Multimode (Veeco, Santa Barbara, CA) in tapping mode. Standard tapping mode silicon cantilevers of “BS-Tap300” with a spring constant of 40 N/m and a resonance frequency of 300 kHz were used at ambient conditions. The radius of curvature of the atomic force microscope tips was nominally less than 10 nm. A scan rate of 0.3 1 Hz was employed at a resolution of 512 pixels/line. Surface roughness measurement (Rrms), defined as the standard deviation of the elevation, was determined from 25 μm  25 μm scans and is the average of at least four images scanned at different locations on the sample surface. Chemical analysis was carried out with a high-resolution XPS Theta Probe instrument (Thermo Fisher Scientific Inc., Loughborough, England) with parallel detection capability and element-specific highresolution spectra. The source was a microfocused monochromatic Al Kα X-ray set to illuminate a spot 400 μm in size, and the vacuum chamber was pumped to 10 9 Torr. Parallel ARXPS data were acquired with the analyzer pass energy set to 100 eV. Charge compensation was verified by observing the main C1s peak at a binding energy (BE) of 285.0 eV. During the measurements, an electron flood gun was used to prevent sample charging. Survey spectra were obtained with pass energy of 160 eV and a step size of 1 eV. Elemental compositions were calculated from the relative intensities of the C1s and O1s peak areas obtained from the survey spectra after a linear background subtraction.

Figure 4. AFM height image of a native flat COC surface. COP surfaces showed similar results. The scale bar is 5 μm.

To investigate the effects of microstructuring, well-defined geometrical microstructures were fabricated on COCs/COPs by embossing (Figure 2). An example of one of these surfaces is shown in Figure 3a. The dimensions (a) and spacing (d) of the square pillars were chosen to define a fill factor, defined as the percentage of the patterned area with respect to the total area, on the basis of previous work:11 the ratio of pillar area to the region without pillars, while height was kept constant throughout. For the microstructured surfaces, the contact angle increased with increasing fill factor, while the hysteresis decreased because air was trapped in the voids. Microstructured surfaces were hydrophobic, with a CA of up to 136° ((5°) for both polymers (Figure 3b). Regardless of the spacing and dimensions of the pillars, a high hysteresis was observed with the water droplet remaining motionless, as shown in Figure 3b. This behavior is typical of the Wenzel state. The pinning of the droplet is due to partial penetration of the droplet into the gaps between the pillars, confirming that microstructured features alone do not provide stable traps for air.36

’ RESULTS AND DISCUSSION Native thermoplastic COP and COC surfaces are intrinsically hydrophobic with contact angles up to 76.8° and 101.2°, respectively.35 Although COPs are slightly hydrophilic, the experimental results were quantitatively similar for both materials; therefore, only COCs will be discussed. 898

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Figure 6. SEM images of a COC surface with embossed pillars after 10 min of exposure to a CF4 plasma. Inset: image of a water drop with a contact angle of 160°.

AFM images of the O2 plasma treated surfaces (Figure 5a e) showed nanometer long needle-like structures, but with a reduction in overall roughness compared with the native surface. When exposed to a CF4/O2 mixed plasma, surface undulations were produced (Figure 5f l). The topography of substrates exposed to the same gas mixture but for different times was similar to that of the oxygen treated surfaces, exhibiting larger round structures (Figure 5m q). Values of roughness (Rrms) are shown in Table 1. The Rrms values for the mixed gas plasma are close to those for oxygen, possibly indicating that roughening was reduced due to the formation of oxides on the substrate surface37 and/or COF2, CO, and/or CO2, which leads to polymerization and recombination of the chemical species on the surface.38 Although there is a marked decrease in surface roughness compared with that of the native polymer after only 1 min of exposure, an increase in exposure time increases the roughness. Scanning electron microscopy (SEM) images of these surfaces are shown in Figure 6. AFM height scans of the CF4 treated surface (Figure 5m q) indicate the presence of round domains of polymer, attributed to reorganization of polymer chains on the surface due to interaction with the plasma, forming conical protuberances on the surface.39 41 Figure 6 shows an SEM image of COC pillars after 10 min of exposure to a CF4 plasma. Round protuberances of reorganized polymer were formed entirely on the walls of the pillars. These etched surfaces showed structures 4 times larger than those of O2 treated surfaces. Prolonged exposure (30 min) to plasma increased the surface roughness and caused these domains to grow. This suggests that redeposition of volatile polymer molecular chains occurs during prolonged treatment.41 44 Contact angle data of a plasma treated flat surface is shown in Figure 7. A moderate increase in hydrophobicity occurs with CF4 plasma treatment, which stabilized at a value of 122° (for both COC and COP substrates). Figure 7 shows that both O2 and the mixed CF4/O2 plasma exposure create hydrophilic surfaces. The oxygen plasma produces a lower CA compared to the mixed gas plasma, because the higher oxygen content leads to an increase in the amount of polar species.45 By superimposing nanoscale roughness (plasma treatment) onto microfeatures, dramatic changes in the surface energy were observed. Etching of microstructured surfaces with CF4 plasma increases the contact angle and lowers hysteresis, presumably due to the production of a smooth surface which contributes to depinning of the droplet on the surface. The superhydrophobic nature of this surface is reported in Figure 8,

Figure 5. AFM height scans of a flat COC surface (25 μm  25 μm) for substrates treated with O2 plasma (a e), a plasma with a 1:2 mixture of CF4/O2, (f l), and a CF4 plasma (m q). Scale bars are 5 μm. For each data set, the exposure time was increased from 1 to 5, to 10, to 20, and to 30 min.

Table 1. AFM Measurements of Surface Roughness for COC Surfaces after Treatment with Different Plasmas and Exposure Timesa surface roughness Rrms (nm)

a

time of exposure to plasma treatment (min)

O2 plasma

CF4 + O2 (2:1) plasma

CF4 plasma

0

33.1

33.1

33.1

1

3.23

2.6

6.2

5

4.17

5.9

8.01

10

4.5

8.04

10.35

20

6.8

11.6

11.4

30

8.38

12.5

13.9

The untreated surface roughness (0 min) was 33.1 nm.

Nanoscale surface roughening chemical modification was achieved by exposing the surfaces to CF4 and/or O2 plasma. AFM imaging showed clear topographical changes in surface roughness before (Figure 4) and after (Figure 5) treatment, and the results are summarized in Table 1. The untreated COC substrate has an rms surface roughness of 33 nm,35 which decreases considerably after each treatment. 899

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Figure 7. Variation in the contact angle of flat COC (left) and flat COP (right) substrates after treatment with different plasmas for exposure times in minutes.

Figure 8. Image showing the spontaneous roll-off of a water drop on a high fill factor microstructured COC treated for 10 min with CF4 plasma, immediately after exposure. A high fill factor pillared surface was chosen to verify that the decrease of the hysteresis was independent of the spacing and dimensions of the pillars. A water droplet (3 μL) is dispensed to the surface (a g). The surface is lowered (h), and the droplet rolls off from the surface spontaneously. The series of images was taken over a 1.5 s period. The droplet did not wet the surface because of air pockets formed underneath. The drop quickly rolled off the surface with very little inclination and a sliding angle of ∼1°.

applicability of this confinement, a drop was placed in a “channel” delineated by micropillars with the entire surface treated with CF4 plasma. As shown in Figure 9, the drop remains intact, is confined to the width of the channel, and shows no adhesion to the pillars. In other words, the micropillars act as a hydrophobic barrier in which the air pockets form virtual walls defining the droplet trajectory and preventing wetting of the microstructures. This effect may be evidence that the nanoasperities that occur due to lateral redeposition of the polymer on the walls of the microstructures are responsible for the increase in low adhesion between the drop and the solid surface. These lateral micro/ nanostructures pin the liquid air interface and thus prevent

illustrating how droplets do not stay on the surface. The pillars are visible below the droplet; no liquid entrapment was observed. Advancing and receding CA values were larger than 160° with hysteresis below 5°, showing contact line depinning, depending on pillar size and spacing. Microstructuring the surface plays a key role in controlling droplet behavior. The wettability changes from the Cassie state on micrometer-sized pillars to the Wenzel state on flat CF4 plasma etched areas. When a droplet rolls from a micro- + nanopatterned surface onto a flat rough surface, the contact area between the water droplet and the structured surface decreases; therefore, the drop is pinned on the rough surface. To demonstrate a practical 900

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liquid from filling the gaps between the micropillars. This surface patterning can be used to confine drops on a surface, moving them when the surface is tilted. In contrast to the CA angles on the microstructured surfaces, a very large change was induced on substrates treated with O2 or CF4/O2 plasma. Figure 10 shows that the pillars are no longer able to trap air and the surface becomes superhydrophilic, allowing water to spread rapidly, disappearing into the gaps between the pillars. Both O2 and CF4/O2 plasma treatments produce similar hydrophilic surfaces, showing that a polar surface chemistry dominates over microstructures. It is obvious that the chemical involvement of the gas plasma played a significant role in affecting the wettability of the substrate, showing that, even though before plasma treatment the geometrical structure improved the hydrophobicity of the surface, after an oxygen containing plasma treatment the presence of a texture is ineffective. In these circumstances, the microstructures become the main factor in increasing the hydrophilicity.

For each treatment, water droplets seeped into the pillars, resulting in complete wetting of the surface as shown in Figure 10. To investigate aging effects, CAs were measured for several days after treatment (samples stored in ambient clean room air). The oxygen treated samples in Figure 11 showed a “hydrophobic recovery”, while those treated with the CF4/O2 mixture showed no discernible change for 1 month, but the CA slowly increased to approximately 45° after 6 months. The contact angle data reported in Figure 11 show that surfaces treated with O2 plasma tend to lose hydrophilic character within one month, probably from reorganization of the low molecular mass residues on the surface46 49 and loss of OH hydroxyl functional groups.16 Substrates treated with the CF4/O2 mixture have more stable contact angles. The advancing contact angle of such surfaces increases by 18° during the first months compared to 80° for the O 2 plasma treated samples.

Figure 9. Image of a water drop on a microstructured COC surface with 10 min treatment of CF4 plasma. The drop is confined in the “channel” with the micropillars acting as virtual walls.

Figure 11. Effect of age on the contact angles of the surfaces of a 10 min treated microstructured COC surface compared to a native surface.

Figure 10. Image of water drops on a microstructured COC surface after a 10 min CF4/O2 plasma treatment. The series of images was taken over less than 1 s. After touching the surface, the drop seeps through the pillars, disappearing quickly. 901

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Figure 12. Left: XPS survey spectra of COC surfaces with and without plasma treatment (10 min). Right: XPS C1s, O1s, and F1s of COC surfaces comparing the different plasma treatments with an exposure time of 10 min. Similar results were obtained for COP surfaces.

The CF4/O2 mixture causes a slight increase in the oxygen-tocarbon ratio, consistent with the expected chemical composition of plasma treatment. The survey spectra in Figure 12 show the incorporation of surface fluorocarbon groups, as expected for COC/COP surfaces treated with CF4.46 These peaks were used to determine O/C and F/C ratios for different surface modifications. The calculated atomic compositions of the substrates for different treatment times are shown in Table 2. Surfaces treated with CF4/O2 showed a gradual increase in O/C ratio because of an increase in the hydroxyl groups on the modified surfaces. The fluorine treated COC surfaces had a strong F1s peak and no oxygen peaks. The mixed CF4/O2 plasma increased surface oxygen by 53% compared with an increase of 359% for pure oxygen plasma, consistent with the contact angle data, which indicate greater hydrophilicity with plasma treatment. A further investigation involved varying the CF4/O2 ratio, which leads to large changes in wettability. Increasing the amount of CF4 relative to O2 increases the etch rate, producing a rougher surface as shown in Figure 13. The CA varies from extremely low values (on the order of 3°) with a 2:1 mixture to a very hydrophobic surface, close to 118° with a 2.5:1 mixture; see the images in the insets in Figure 13. AFM examination of these surfaces showed an increase of roughness and undulations with increasing CF4 concentration (Figure 13). The high hydrophobicity of the surface is caused by the higher fluorine content. The different contact angles and the different etch rates indicate that a lower CF4/O2 ratio creates more hydroxyl groups (OH) during plasma treatment. Under these experimental conditions, the change from superhydrophilic

Table 2. Elemental Composition (%) of Cyclic Olefin Polymer and Copolymer Surfaces before and after Treatment with Different Plasmas elemental composition (%)

element ratio

plasma treatment polymer

C1

O1

F1

O1s/C1s F1s/C1s

untreated

COC

94.41

5.59

92.63 34.82

7.37

CF4

COP COC COP

32.10

COC

34.01

8.53

57.64

0.25

1.69

COP

36.19

6.28

57.53

0.17

1.58

COC

74.32

25.68

0.345

COP

75.53

24.47

0.324

CF4O2 O2

0.06 0.08 65.18

1.87

67.9

2.11

Furthermore, a faster hydrophobic recovery of the surface was observed for samples with longer exposure times (more than 10 min), possibly due to faster movement of polar groups inward to the COC bulk. High-resolution XPS data for untreated and plasma treated COCs are shown in Figure 12. COPs have similar chemical properties; therefore, only the data for COCs are shown. The untreated polymer contains some oxygen species; see Table 2. XPS data indicate that both O2 plasma and the gas mixture gave a surface with a higher oxygen content, with a reduced carbon peak diminishing with no evidence of hydroxyl groups. 902

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Figure 13. Left: vertical etch rate of a COC substrate as a function of the CF4/O2 flow rate. Center and right: AFM height scans (10 μm  10 μm) and images of water droplets in the insets. Surfaces were treated with a 2:1 mixture (center) and 2.5:1 mixture (right). Scale bars are 5 μm.

to superhydrophobic regimes occurs with a change in the plasma treatment time and can be easily controlled.

(8) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Perry, C. C. Langmuir 2005, 21, 937. (9) Huang, L.; Lau, S. P.; Yang, H. Y.; Leong, E. S. P.; Yu, S. F. J. Phys. Chem. 2005, 109, 7746. (10) Martines, E.; Seunarine, K.; Morgan, H.; Gadegaard, N.; Wilkinson, C. D. W.; Riehle, M. O. Nano Lett. 2005, 5, 2097. (11) Cortese, B.; D’Amone, S.; Manca, M.; Viola, I.; Cingolani, R.; Gigli, G. Langmuir 2008, 24, 2712. (12) Marmur, A. Langmuir 2003, 19, 8343. (13) Gao, L.; McCarthy, T. J. Langmuir 2006, 22, 2966. (14) Lafuma, A.; Quere, D. Nat. Mater. 2003, 2, 457. (15) Nosonovsky, M; Bhushan, B. Microelectron. Eng. 2007, 84, 382. (16) Sunanda, R.; Yue, C. Y.; Lam, Y. C. Vacuum 2011, 85, 1102. (17) Ma, M. L.; Hill, R. M. Curr. Opin. Colloid Interface Sci. 2006, 11, 193. (18) Yao, X.; Chen, Q. W.; Xu, L.; Li, Q. K.; Song, Y. L.; Gao, X. F.; Quere, D.; Jiang, L. Adv. Funct. Mater. 2010, 20, 656. (19) Hatke, W. Kunststoffe 1997, 87, 58. (20) Rohr, T.; Ogletree, D. F.; Svec, F.; Frechet, J. M. J. Adv. Funct. Mater. 2003, 13, 264. (21) Khanarian, G. Opt. Eng. 2001, 40, 1024. (22) Lim, H. S.; Lee, S. G.; Lee, D. H.; Lee, D. Y.; Lee, S.; Cho, K. Adv. Mater. 2008, 20, 4438. (23) Belder, D.; Ludwig, M. Electrophoresis 2003, 24, 3595. (24) Shah, J. J.; Geist, J.; Locascio, L. E.; Gaitan, M.; Rao, M. V.; Vreeland, W. N. Electrophoresis 2006, 27, 3788. (25) Song, S.; Singh, A. K.; Kirby, B. J. Anal. Chem. 2004, 76, 4589. (26) Garbassi, F., Morra, M. Occhiello, E. Polymer Surfaces: From Physics to Technology; John Wiley & Sons: Chichester, U.K., 1998; p 317. (27) Sunanda, R.; Yue, C. Y.; Lam, Y. C.; Wang, Z. Y.; Hu, H. Sens. Actuators, B 2010, 150, 537. (28) Ahn, C. H.; Choi, J. W.; Beaucage, G.; Nevin, J. H.; Lee, J. B.; Puntambekar, A; Lee, J. Y. Proc. IEEE 2004, 92, 154. € (29) Johansson, B. L.; Larsson, A.; Ocklind, A.; Ohulund, A. J. Appl. Polym. Sci. 2002, 86, 2618. (30) Steffen, H.; Schr€oder, K.; Busse, B.; Ohl, A.; Weltmann, K. D. Plasma Processes Polym. 2007, 4, S392. (31) Stachowiak, T. B.; Mair, D. A.; Holden, T. G.; Lee, L. J.; Svec, F.; Frechet, J. M. J. Sep. Sci. 2007, 30, 1088. (32) Tsao, C. W.; Hromada, L. L.; Liu, J. Lab Chip 2007, 7, 499. (33) Nikolova, D.; Dayss, E.; Leps, G.; Wutzler, A. Surf. Interface Anal. 2004, 36, 689. (34) Ahn, C.; Kim, S.; Chao, H.; Murugesan, S; Beaucage, G. Mater. Res. Soc. Symp. Proc. 2002, 729, 131. (35) Cortese, B.; Mowlem, M. C.; Morgan, H. Sens. Actuators, B 2011, 160, 1473. (36) Bhushan, B.; Jung, Y. C.; Koch, K. Philos. Trans. R. Soc. London 2009, A367, 1631.

’ CONCLUSIONS We have demonstrated a method for controlling the wettability of thermoplastic surfaces by combining different plasma etching treatments with lithographically defined macroscale roughness. Depending on whether oxygen, fluorocarbon, or a mixture of both is used, the contact angle of the surface can be tailored over a wide range from superhydrophobic to superhydrophilic behavior. Combining microscale patterns with nanoscale roughness enhanced the hydrophobic properties of the polymer, but this effect was not observed when using an oxygen plasma. A mixture of CF4/O2 increased the hydrophilic nature of the surfaces, producing stable contact angles with little recovery of surface properties. Varying the CF4/O2 ratio can be used to tailor the hydrophobic or hydrophilic nature of the surface. These results demonstrate that combining macro and nanoscale features provides a simple and reproducible method to modify functional thermoplastic surfaces with controllable wettability. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT Financial support from FP-7 IP LABONFOIL and the Engineering and Physical Sciences Research Council is gratefully acknowledged. Also Dr. Nicholas Green, Iain Ogilvie, and Vincent Sieben are acknowledged for technical support. ’ REFERENCES (1) Sun, T. L.; Feng, L.; Gao, X. F.; Jiang, L. Acc. Chem. Res. 2005, 38, 644. (2) Koch, K.; Bhushan, B.; Barthlott, W. Soft Matter 2008, 4, 1943. (3) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546. (4) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988. (5) Herminghaus, S. Europhys. Lett. 2000, 52, 165. (6) Bico, J.; Tordeux, C.; Quere, D. Europhys. Lett. 2001, 55, 214. (7) Onda, T.; Shibuichi, S.; Satoh, N.; Tsujii, K. Langmuir 1996, 12, 2125. 903

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