Mechanically Robust Superhydrophobic Polymer Surfaces Based on

Article pubs.acs.org/Langmuir

Mechanically Robust Superhydrophobic Polymer Surfaces Based on Protective Micropillars Eero Huovinen, Laura Takkunen, Tarmo Korpela, Mika Suvanto, Tuula T. Pakkanen, and Tapani A. Pakkanen* Department of Chemistry, University of Eastern Finland, P.O. Box 111, FI-80101, Joensuu, Finland ABSTRACT: Considerable attention is currently being devoted less to the question of whether it is possible to produce superhydrophobic polymer surfaces than to just how robust they can be made. The present study demonstrates a new route for improving the mechanical durability of water-repellent structured surfaces. The key idea is the protection of fragile fine-scale surface topographies against wear by larger scale sacrificial micropillars. A variety of surface patterns was manufactured on polypropylene using a microstructuring technique and injection molding. The surfaces subjected to mechanical pressure and abrasive wear were characterized by water contact and sliding angle measurements as well as by scanning electron microscopy and roughness analysis based on optical profilometry. The superhydrophobic polypropylene surfaces with protective structures were found to maintain their wetting properties in mechanical compression up to 20 MPa and in abrasive wear tests up to 120 kPa. For durable properties, the optimal surface density of the protective pillars was found to be about 15%. The present approach to the production of water-repellent polymer surfaces provides the advantages of mass production and mechanical robustness with practical applications of structurally functionalized surfaces.

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INTRODUCTION The fabrication of superhydrophobic surfaces has been demonstrated in numerous studies.1−24 These surfaces with their unique wetting properties can be used in many practical applications, such as anti-icing,25 self-cleaning,26−28 antifogging,29 and corrosion resistance surfaces.30,31 Despite the great commercial potential and the variety of approaches available for the production of the strongly water-repellent surfaces, their use has severely been restricted due to their poor mechanical robustness.28 The reason for this has been the fragile fine-scale surface structure essential for the superhydrophobic Cassie−Baxter state to be achieved. Water droplets lie on the top of the superhydrophobic micronanostructured roughness leaving surface nonwetted and permitting easy sliding of a water drop.32 The nanoscale structures, however, have particularly poor durability in the context of mechanical wear.33 Light physical contact causes damage to the fragile nanomodified surface, resulting in losses of the Cassie−Baxter state and superhydrophobic wetting properties.33 In recent years, several methods have been demonstrated for improving the mechanical resilience of nonwetting surfaces.33−42 One of the most promising methods for protecting the fragile nanostructure against mechanical wear is to produce hierarchical roughness on a surface.33,38 When a surface roughness with two scales is exposed to wear, sacrificial microstructures provide some protection for underlying nanopatterns.33 The damaged surface can still have enough pristine surface roughness to achieve the Cassie−Baxter state.33 Two-level micromicrostructured surfaces have been shown to achieve a considerably better wear-resistance than micronanostructured surfaces.37 However, the mechanical durability of such surfaces is still far from the level that is required when © 2014 American Chemical Society

surfaces are exposed to the conditions that are normal in practical applications.33 Another method for increasing wear-resistance is to use different coating techniques.33,36,40−42 Such methods typically include the production of the coating of a hydrophobic layer on the surface of a hydrophilic material. If the hydrophobic layer is worn away, water drops will adhere to the hydrophilic areas.33 The coating process can be repeated to regenerate the hydrophobic layer.33 Most recently, methods have been presented that are concerned with the reversible recovery of surface topographies by means of heat treatment and self-healing material.43,44 However, the shape memory material is unlikely to recover from damage consisting of significant material loss on its surface. In addition, the self-healing materials are expensive and often toxic, and no healing occurs under ambient conditions. In consequence, their applicability has remained limited.44 The mechanical robustness of superhydrophobic surfaces has been evaluated using several methods. The techniques typically involve custom-made devices, but there is a lack of standardized procedures, and hence any comparison of durability characteristics remains difficult.33 Most of the fabrication techniques used to produce superhydrophobic surfaces are only effective in the generation of structures on specific materials. The mechanical durability has generally been estimated by scratching the superhydrophobic surfaces with an AFM tip,45,46 rubbing the structured surface against other surfaces,34−41,47−49 and peeling the surfaces with tape.50 The loads used in the mechanical tests are generally less Received: November 23, 2013 Revised: January 18, 2014 Published: January 20, 2014 1435

dx.doi.org/10.1021/la404248d | Langmuir 2014, 30, 1435−1443

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Table 1. Structural Parametersa of the Surface Patterns under Study T [μm]

surface

W [μm]

hierarchical micromicro (MM) 190 240 hierarchical micronano (MN) 190 240 multiscale structure (MS) 60 protective structures T [μm] W [μm] p05 p15 p25

190 190 190

c

480 c

In the present study, we investigate the possibilities for improving the mechanical robustness of superhydrophobic polypropylene surfaces. The main idea is to increase the durability of superhydrophobic hierarchical structures by means of protective sacrificial micropillars. The optimal surface roughness of polypropylene for achieving the Cassie−Baxter state and the optimal surface density of the protective pillars to maintain the Cassie−Baxter state will also be evaluated. The applicability of the filtered roughness values for characterizing the effect of wear will be explored. Our ultimate aim is to demonstrate a simple method based on a microstructuring technique and injection molding for the mass production of polymer surfaces that are not only water-repellent but also wear-resistant.

H t d h [μm] [μm] [μm] [μm] 70 70

20 10 20

27 9 10

20 10 b

H [μm]

R

Φp

95 95 95

1:12 1:4 2:5

0.052 0.157 0.251

a Structure parameters T, W, H, t, d, and h are presented graphically in Figure 1C,1D. For protective structures, R is the replacement ratio and Φp is the area fraction of the protective pillars. bPillars are not uniform in height; distribution is shown in Figure 1. cReplacement grid is anisotropic; see Figure 1for the placement of the pillars.

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EXPERIMENTAL SECTION

Fabrication of Structured Polymer Surfaces. Surface patterns were generated on aluminum foils using a microworking robot.6,37 The essential working parameters for different surface structures are presented in Table 1. The protective micropillars were included in the structures either by replacing some of the hierarchical pillars or by superimposing them within the structure. Schematic images of the produced surface topographies, fabrication parameters for different structures, and the positioning replacement grids are shown in Figure 1. The size of the structured polymer surface area was 5 mm × 5 mm. The patterned molds were glued onto steel plates and transferred to polypropylene (PP, HD 120 MO, density 954 kg/m3) by means of injection molding (a DSM Midi 2000 melt compounder and a microinjection molding machine).6 Characterization of Surface Structures. Structured polymer surfaces were coated with 5 nm of Au and imaged with scanning electron microscopy (SEM, Hitachi S4800) at an acceleration voltage of 3 kV. The fabricated surface structures were also characterized using an optical profilometer (Wyko NT9300) from Veeco (currently Bruker AXS) in order to obtain the Fourier filtered roughness values (filtered Rq values) at various

than 10 N. Typically, therefore, claims about the robustness and applicability of particular superhydrophobic surfaces are undermined due to the low pressures applied for contact damage. Methods of observing and characterizing wear include static contact angle and contact angle hysteresis,34−40,46 roll-off behavior,51,52 the coefficient of friction,52,53 root-mean-square (RMS) roughness,46 and the abrasion rate.54 One of the disadvantages of the resultant wear characterization is, therefore, the absence of any standardized analysis method or measurement. A filtered power spectral density (FPSD) method offers information about roughness distributions at various spatial frequency ranges.55,56 PSD curves reveal structure periodicity, whereas filtered values demonstrate how roughness has been distributed to the spatial frequencies.55,56 The method can be used to analyze structure wear in terms of the roughness changes at various spatial frequencies.

Figure 1. A design schematic of fine-scale surface topographies protected with micropillars (A) micromicrostructure, (B) multiscale structure, (C) fabrication parameters for hierarchical structures, (D) fabrication parameters for multiscale structures, and positioning of the replacement grids (E) p5, (F) p15, and (G) p25. 1436


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spatial frequency ranges. The measurement areas were 4 mm2 and the measurement mode used was vertical scanning interferometry (VSI).55,56 The static and sliding angles were determined using a KSV Cam 200 contact angle meter. The reported static contact angles are the average values of six parallel measurements. Because of the slight anisotropy of the surface structures (p5 and p25), six parallel sliding angle measurements were made for each direction and the average values were reported. A detailed description of the wetting measurements has been reported elsewhere.57 Mechanical Testing. Wear measurements were carried out with a CSM+ Instruments Tribometer (TRN S/N 18-347), equipped with a reciprocating movement module. In a single-slide scratch test, the structured surfaces were worn against a rough steel counter surface (Bahco 100-06-1 Hand Oberg File) using different loads (1−5 N). The following parameters were selected for the scratch measurements: speed 1 cm/s and sliding distance 4 cm. The surface structures were positioned diagonally against the sliding direction. In order to measure the pressure resistance of the structured surfaces, the samples were exposed to compressing strength on a scale of megapascals. Each sample surface was placed between two smooth steel plates and a load was applied to the upper surface for 60 s. The structures were exposed to loads ranging from 10 to 100 kg, which corresponds to surface pressures of 4−40 MPa (sample area 25 mm2). Following the mechanical tests, the contact angles for each sample were measured and the effect of wear was studied with the aid of the profilometer.

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RESULTS AND DISCUSSION Fabricated Surface Structures. Twelve different mold structures were fabricated using the microworking robot and were replicated on a polypropylene surface by means of injection molding (Table 1). Figure 2 presents SEM images of the fabricated structures on the PP surfaces. The multiscale structure (MS) had microbumps of different heights and they also displayed a degree of submicro-roughness (Figure 2A,B). Two of the surfaces were hierarchical two-level structures where a fine-scale upper pattern was positioned on the top of the larger micropillars (Ø 190 μm). The upper structure of the hierarchical micromicrostructure (MM) consisted of densely packed barrel-shaped micropillars (Figure 2C,D). In the case of the hierarchical micronanostructure (MN), a nanoscale roughness was generated using a densely packed needle jabbing at top the similar square micropillars (Figure 2E,F). For the MM and MN surfaces, the protective features were positioned on the surface by replacing some of the hierarchically structured pillars with protective pillars. The method permits the precise positioning of the protective micropillars above the fine-scale hierarchical structures. The height and the placement of the shielded features can be controlled with micrometer precision (Figure 2G,H). The ratios of the replaced pillars were selected so that the protective micropillars would contribute approximately 0.05 (suffix p5), 0.15 (suffix p15) and 0.25 (suffix p25) to the total area fraction Φ. The replacement ratios (R) and the contributed area fractions are listed in Table 1. The surface fraction ΦB for the lower structures of the MM and MN surfaces were calculated by using eq 1, where t is the square needle tip diameter and W is the distance between the pillars. The area fractions contributed by the protective micropillars ΦP were calculated from eq 2. The same grids were used to arrange the protective pillars on the MS surface structures. ΦB =

t2 W2

ΦP = R ΦB

Figure 2. SEM images of the fabricated structures on the PP surfaces: (A) MSp0, (B) and a close-up, (C) MMp0, (D) and a close-up, (E) MNp0, (F) and a close-up, (G) MNp5, and (H) MMp15.

Table 2. Contact Angle Measurements on PP Surfaces contact angles (deg) surface structure

static

sliding

Hierarchical Micromicrostructure PP flat 103 ± MMp0 158 ± MMp5 156 ± MMp15 151 ± MMp25 149 ± Hierarchical Micronanostructure MNp0 157 ± MNp5 154 ± MNp15 150 ± MNp25 150 ± Multiscale Structure MSp0 159 ± MSp5 152 ± MSp15 150 ± MSp25 146 ±

1 2 4 2 4

60 2 6 9 20

± ± ± ± ±

15 5 8 7 8

1 5 3 6

3 5 10 15

± ± ± ±

4 9 6 10

2 5 1 3

2 8 16 30

± ± ± ±

2 5 9 9

Wetting of the Pristine Structures. The wetting properties of the PP surface structures were studied by means of static and sliding angles. The superhydrophobicity is defined as a state where a water drop has a static contact angle greater

(1) (2) 1437


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Figure 4. Profilometric (A−D) and SEM (E−F) images of multiscale structures in press tests: (A) MSp0 before mechanical compression, (B) MSp0 after mechanical compression of 40 MPa, (C) MSp5 after mechanical compression of 40 MPa, (D) MSp15 after mechanical compression of 40 MPa (E) MSp0 after mechanical compression of 10 MPa, and (F) MSp5 after mechanical compression of 40 MPa.

arrangement of protective pillars in the cases of the p5 and p25 structures. The protective pillars were positioned within the lower microstructures so that the protective features would contribute approximately 5, 15, and 25% fraction of the total area. The anisotropic arrangement was caused by replacement of every third of the micropillars in the x-direction and every fourth in the y-direction with protective features in the case of the p5 structure (Figure 1E). In the case of the p25 structure, the positioning of protective pattern can be seen in Figure 1G. Table 2 shows the static and sliding angles and their standard deviations on structured PP surfaces. The fabricated structured PP surfaces displayed superhydrophobic wetting properties with static contact angles higher than 150° and sliding angles less than 5°. This indicates that the patterns have a sufficient surface roughness and suitable chemistry for achieving the Cassie−Baxter state, where the water drops sits on the top of the surface patterns. The structures were fabricated on PP surfaces because of its high intrinsic hydrophobicity (the static contact angle of a smooth surface 103°). The structured MN and MM surfaces retained their superhydrophobic behavior even though 15% of the surface area was covered with protective pillars. The sliding angles of the structures were increased close to 10° suggesting

Figure 3. The average values contact angle measurements of multiscale structures as a function of pressure: (A) static contact angles in a mechanical compression test, (B) sliding angles in a mechanical compression test, (C) static contact angles in an abrasive wear test, and (D) sliding angles in an abrasive wear test. The black dash line represents the superhydrophobic limit.

than 150° and hysteresis, or sliding angles of less than 10°. In the present study, the sliding angles were measured instead of the contact angle hysteresis because of the slight anisotropic 1438


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Figure 6. Profilometric images (A,B) and filtered roughness values (C) in abrasive wear tests. The filtering ranges 1000−1 μm are represented as Rq1−Rq3. (A) MSp15 before abrasive wear, (B) MSp15 after abrasive wear of 200 kP, and (C) filtered Rq values as a function of pressure.

Mechanical Durability of the Structured Surfaces. The deterioration in the surface hydrophobic properties under mechanical compression and abrasive wear was studied by means of the contact angles. The wear levels of the surfaces were evaluated by profilometry images and spatially filtered roughness values. The filtering ranges used were 1000−100 μm (Rq1), 100−10 μm (Rq2), and 10−1 μm (Rq3). Multiscale (MS) Structures. The average values of six parallel static contact angle and sliding angle measurements of the MS structures as a function of pressure are presented in Figure 3. The SEM and profilometric images are shown in Figure 4. The surface patterns with no protective micropillars lost their water-repellent properties after a compressing pressure of approximately 10 MPa was applied to the surface. The pressure caused the collapse of the surface roughness, thus explaining why a water drop cannot easily roll on the top of the crumpled structure (Figure 4E). A composite surface of air and structure required for the Cassie−Baxter state does not exist in the collapsed patterns. The protected MSp5 and MSp15 structures, however, retained their superhydrophobic wetting properties even when the surface was compressed with a pressure up to 20 MPa. The protective pillars were able to resist the mechanical contact while retaining the dimensions of the fine-scale patterns unchanged (Figure 4C,D). In the case of the MSp5 surfaces, the compression was still visibly present, whereas the MSp15 sheltered surface structures displayed no visual wear. The mechanical durability of the superhydrophobic surfaces covered with protective features was a magnitude better than that of the surface with no protective structure. The surface structured with protective features also had significantly better mechanical durability against abrasive wear than the unprotected structures. The MS structure had lost its superhydrophobic behavior at a pressure of 40 kPa (Figure 3C). The protected MSp5 and MSp15 structures were robust enough to retain their nonwetting behavior, even up to a load wear pressure of approximately 80 kPa.

Figure 5. Filtered roughness values of multiscale structures as a function of pressure in mechanical compression tests. The filtering ranges 1000−1 μm are represented as Rq1−Rq3. (A) MSp0, (B) MSp5, (C) MSp15, and (D) MSp25.

that protective features produces a Wenzel state, where a water drop wets the protective micropillars.58 However, most of the contacted area of a water drop to the surface still exist in the Cassie−Baxter state that determines the wetting properties of the surface and permits easy sliding of water drops. In the case of the MSp15 structure, the sliding angles are notably greater than the sliding angles of the MMp15 and MNp15 structures. The possible explanation for this phenomenon is the pinning of the water drop on the side walls of the protective micropillars due to the different surface topography. When the density of the protective micropillars on the surface was 25%, the wetting properties dropped below the superhydrophobic limit. The water drops stick to several protective pillars on the p25 surfaces, forming local Wenzel states and explaining the increased sticking of water. The protective pillars were initially fabricated 15−30 μm taller than the hierarchical structures in order to provide better mechanical durability. These surfaces did not, however, exhibit superhydrophobic properties. The high protective pillars produced the Wenzel state, explaining why water droplets stuck to the surfaces. In consequence, the protective structures were fabricated ca. 5 μm higher than the hierarchical patterns. 1439


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Figure 7. The average values contact angle measurements of hierarchical structures as a function of pressure. (A) Static contact angles for the MM in a mechanical compression test, (B) sliding angles for the MM in a mechanical compression test, (C) static contact angles for the MM in an abrasive wear test, (D) sliding angles for the MM in an abrasive wear test, (E) static contact angles for the MN in a mechanical compression test, (F) sliding angles for the MN in a mechanical compression test, (G) static contact angles for the MN in an abrasive wear test, and (H) sliding angles for the MN in an abrasive wear test. The black dash line represents the superhydrophobic limit.

filtering. The wear analysis is presented in Figure 6. When the images of the pristine and worn surfaces (Figure 6A,B) are compared, it is evident that only the protective structures had worn. Similar behavior is observed in the filtered Rq values; the Rq1 roughness declines as a result of the increasing pressure, whereas Rq2 and Rq3 ranges appear to retain their original roughness. Hierarchical (MM, MN) Structures. The average values of static contact angles and sliding angles for the hierarchical twoscale structures MM and MN after the compression and abrasive tests are shown in Figure 7. These surfaces display behavior similar to that of the MS surfaces regarding their superhydrophobic properties. The unprotected surfaces deteriorated quickly under compressive and abrasive wear but the addition of protective features greatly improved their mechanical resilience. The MNp15 structure maintained its superhydrophobic behavior in mechanical compression up to 20 MPa and in abrasive wear tests up to 120 kPa.

The filtered roughness values for the mechanical compression tests are shown in Figure 5. Without protective structures (Figure 5A), the filtered Rq values decrease in the Rq2 and Rq3 ranges, thus reflecting the compression of the fine-scale structures. Filtering range Rq1 includes no notable roughness changes because the MSp0 surface contained no structures in this range. As the size and periodicity of the protective structures falls within the Rq1 range, they appear in the Rq1 roughness range (Figure 5B,D). The filtered Rq values indicate that the Rq2 and Rq3 roughness decline less due to the increasing proportion of protective features, which is also visible in the Rq1 range, although on a smaller scale. The results observed in the filtered Rq values are comparable to the information obtained from the profilometric images and static contact angles (Figure 3). When the fine-scale structure (Rq2, Rq3) remains relatively intact, the static contact angle remains high. The MSp15 structure used in the abrasive scratch testing was analyzed using profilometric imaging and roughness values 1440


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(120 kPa), it was selected for the measurement. The MNp15 structure retained its superhydrophobic wetting properties after a compression with a pressure of 20 MPa for 48 h. The static contact angle (150°) and sliding angle (10°) of the MNp15 structure remained unchanged when 30 cycles of a compressing pressure of 20 MPa was applied to the surface for 60 s. The structure also had superhydrophobic behavior with a static contact angle of 150° and a sliding angle of 9° after 10 abrasive wear cycles at a pressure of 120 kPa.

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CONCLUSIONS The present work demonstrates a simple mass-production method for fabricating mechanically robust nonwetting structured polymer surfaces. Fine-scale surface roughness was protected against mechanical wear by larger microscale features. The structured polypropylene surfaces had high static contact angles and low roll-off angles, although a certain proportion of the area of fine-scale patterns was replaced by larger micropillars. The method permits a precise optimization of protective features on a surface without any significant decline in its superhydrophobic behavior. The wetting experiments showed that water-repellent surfaces with shielded structural features have superior durability against mechanical compression and abrasive wear as compared with unprotected surfaces. Filtered roughness values at various structure periodicities were used to analyze the effect of protective features providing an alternative characterization tool for wear. The texturing technique can be also used for wear protection of nanostructures. A small fraction of sacrificial surface is sufficient to protect fragile nanostructures while maintaining the functional surface properties such as nonwetting and self-cleaning. The new method has been demonstrated to produce mechanically robust superhydrophobic structured polymer surfaces for practical applications.

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Figure 8. Filtered roughness values of hierarchical micromicro and micronanostructures as a function of pressure in mechanical compression tests. The filtering ranges 1000−1 μm are represented as Rq1−Rq3. (A) MMp0, (B) MMp25, (C) MNp0, and (D) MNp25.

AUTHOR INFORMATION

Corresponding Author

*E-mail: tapani.pakkanen@uef.fi. Tel: +358405028982. Fax: +358132513390. Notes

The authors declare no competing financial interest.

The filtered roughness values for the hierarchical structures undergoing compression are presented in Figure 8. In Figure 8A,C, the Rq1 and Rq2 roughness values do not change, indicating that the base squares and microscale structures offer some resistance against the compressing pressure. In contrast, the Rq3 roughness decreases, suggesting a flattening of the finer structures. When protective features were added, the roughness levels decreased to some extent in all of the filtering ranges (Figure 8B,D). The contact angles are strongly dependent on the surface area fraction. As the protective features had been manufactured taller than the two-scale hierarchical pillars, the majority of the surface damage caused by compression and abrasive wear was inflicted upon those protective features. The features fall within the Rq1 range according to their periodicity and size, thus permitting monitoring of the surface damage using the filtering technique (Figure 8). The breakpoint at which the high contact angle with protected surfaces is lost is clearly visible as sudden fall in the Rq1 roughness value with the protected surfaces (around 15−20 MPa for p25 surfaces). To evaluate the robustness of superhydrophobic polypropylene surfaces, three fatigue resistance tests were performed. Because the MNp15 structure gave the highest durability in mechanical compression (20 MPa) and abrasive wear tests

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ACKNOWLEDGMENTS Financial support provided by the Finnish Funding Agency for Technology and Innovation TEKES, the European Union/ European Regional Development Fund (ERDF), and the Inorganic Materials Chemistry Graduate Program (EMTKO) is gratefully acknowledged. We would also like to thank Toni Pennanen for his help with the mechanical tests.

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REFERENCES

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