Tailoring Normal Adhesion of Arrays of Thermoplastic, Spring-like

Article pubs.acs.org/Langmuir

Tailoring Normal Adhesion of Arrays of Thermoplastic, Spring-like Polymer Nanorods by Shaping Nanorod Tips Longjian Xue,*,†,‡ Alexander Kovalev,‡ Florian Thöle,† Gopalakrishnan Trichy Rengarajan,† Martin Steinhart,*,† and Stanislav N. Gorb*,‡ †

Institut für Chemie, Universität Osnabrück, Barbarastrasse 7, 49069 Osnabrück, Germany Institut für Zoologie, Universität Kiel, Am Botanischen Garten 1-9, 24098 Kiel, Germany

‡

S Supporting Information *

ABSTRACT: The tip shape of contact elements in hairy adhesion systems is crucial for proper contact formation and adhesion enhancement. While submicrometer terminal contact elements show much better adhesion performance than their larger counterparts, shaping their tips so as to maximize normal adhesion has remained challenging. We prepared durable nanorod arrays consisting of stiff, highly entangled thermoplastic polymers with rationally shaped tips by replication of anodic aluminum oxide (AAO). Nanorod arrays with pancake-like tips showed pronounced normal dry adhesion already for small loading forces. For small loading forces, adhesion forces significantly exceeded the loading forces. Both the absence of hysteresis in force/displacement curves and the pronounced durability of the nanorods in series of repeated attachment/detachment cycles suggest that the nanorods behave like elastic springs. Experimental load−adhesion curves were reproduced with a modified Schargott−Popov−Gorb (SPG) model, assuming that contacts between probe and individual nanorods are sequentially formed with increasing indentation depth.

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INTRODUCTION A broad variety of biological adhesion systems consists of hairlike contact elements.1−5 It has been evidenced that proper contact formation and adhesion enhancement are crucially influenced by the terminal contact shapes of the hairy contact elements.6−10 Considerable efforts have been devoted to the design of bio-inspired adhesion systems consisting of arrays of pillars with diameters ranging from a few 100 nm to several tens of micrometers that mimic the hairy structure of biological models.11−13 In the case of bio-inspired adhesion systems consisting of arrays of pillars with diameters in the 10 μm range and above, contact elements with a broad range of terminal contact shapes have been realized.14−19 Reducing the contact elements’ diameter and increasing their area density significantly enhances the performance of adhesion systems as compared to counterparts with larger feature sizes.20 Hence, arrays of fibers, rods, and tubes with diameters in the submicrometer range potentially show better adhesion than arrays of pillars with diameters of several micrometers and above. Various studies deal with the adhesion properties of arrays of condensed contact elements with diameters of a few 10 nm to a few 100 nm and high aspect ratios. Examples include arrays of aligned carbon nanotubes21−23 and negative AAO replicas.24−27 However, condensation of contact elements results in reduced adhesion.28 Thus, aspect ratios or area densities of contact elements with diameters in the submicrometer range were reduced so as to prepare arrays of © 2012 American Chemical Society

separated submicrometer contact elements having structural features close to those of biological adhesion systems. While such array geometries can easily be realized by replication of corresponding molds accessible by top-down lithography or by self-assembly, shaping the terminal contact points of the contact elements has remained challenging. Several approaches to nanorod arrays showing enhanced shear adhesion were reported. Slanted elastomeric nanorods were obtained by replication molding of nanopores with flattened pore bottoms,29 and stooped polymeric nanorods could be obtained by using polymers exhibiting controlled degradation and shrinkage under irradiation with directed electron beams.30 Following a method introduced by Nielsch et al.,31 arrays of branched contact elements were fabricated by replication of AAO with hierarchical pore morphologies.32,33 However, little efforts have been devoted to the design of submicrometer contact elements having terminal contact shapes optimized for maximum normal adhesion. Up to now, predominantly flattening of the tips of partially cured contact elements consisting of resins has been investigated.34 Here we report the preparation of durable arrays of stiff polymer nanorods (diameter ∼270 nm) with rationally shaped tips by replication of AAO. By simple mechanical shaping, flat, Received: May 18, 2012 Revised: June 29, 2012 Published: June 30, 2012 10781

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foot-like, and pancake-like tip shapes roughly corresponding to three main types of biological contact shapes were accessible, as schematically outlined in Figure 1. The procedures described

Figure 1. Illustration of the shaping of the PS nanorod tips.

below allow forming highly entangled thermoplastic polymers with proper stiffness into such customized contact elements. In particular, they allow processing high-molecular-weight polymers that yield mechanically highly stable contact elements. Normal dry adhesion increased from hemispherical to flat to foot-like to pancake-like tips having significantly larger diameters than the stems of the contact elements. We show that nanorod arrays with pancake-like tips exhibit extraordinarily high normal dry adhesion (up to ∼234.0 kPa, ∼2.3 times of gecko foot) already for small loading forces Fl, and for small loading forces adhesion forces significantly exceeded the Fl. The pronounced durability of the nanorod arrays in series of repeated attachment/detachment cycles suggests that contact elements, consisting of thermoplastic, highly entangled polymers behave like elastic springs. Experimental force/ displacement curves showed no hysteresis and could be reproduced with a modified Schargott−Popov−Gorb (SPG) model35 taking into account that contacts between probe and individual nanorods are sequentially formed with increasing indentation depth.

Figure 2. Scanning electron microscopy (SEM) images of arrays of PS nanorods released from AAO. The hemispherical nanorod tips are inverse replicas of the AAO pore bottoms. (a) Top view and (b) bird’s eye view.

AAO pore bottoms and in widening of the AAO pore segments in the vicinity of the pore bottoms, while the AAO membrane as such remained intact. As a result, the PS nanorods protruded ∼100 nm from the remaining AAO layer (Figure S1b). Pressing glassy PS nanorod tips protruding from the remaining AAO layer against a plane surface at a pressure of ∼250 kPa at room temperature (cold pressing) resulted in tip flattening. Thus, the average tip diameter increased to 334.7 ± 29.9 nm and was, therefore, ∼24% larger than that of untreated PS nanorods (Figure 3a,b). The tips of cold-pressed nanorods had a reverse trapezoid contour seen along the direction perpendicular to the nanorod axes. Pressing the tips against silicon wafers heated to 170 °C for 1−2 s at a pressure of ∼50 kPa (hot pressing) led to viscous flow of the liquid PS and beginning spreading on the wafer surface consisting of silica. Thus, the PS was pressed into the gap between the PS nanorods and the AAO pore walls formed during the opening of the AAO pore bottoms (Figure 3c) and flat pancake-like tips with flat top (mean diameter 405.8 ± 27.4 nm, 1.5 times larger than the initial PS nanorod diameter) were obtained (Figure 3d). Shearing the tips of the PS nanorods protruding from the AAO with a flat surface tilted at a small angle with respect to the AAO underside at room temperature (shear pressing) led to elongation of the nanorod tips along the shear direction (Figure 3e,f). The top surfaces of the shorn and flattened nanorod tips had long and short axes with lengths of 362.3 ± 25.4 and 301.7 ± 22.4 nm, respectively (eccentricity 0.54). Thus, foot-like tips having their long axes preferentially oriented along the shear direction were obtained, which resembled terminal contact shapes of the setae of geckos, spiders, and insects42 as well as artificial adhesive systems showing pronounced shear adhesion. It should be noted that the pancake-like tips of PS nanorods obtained by hot pressing partially impinged on each other as their diameter was close to the lattice constant of the PS nanorod arrays (Figure 4). However, it is reasonable to assume that the contact area between neighboring pancake-like tips, which should roughly

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RESULTS AND DISCUSSION PS Nanorod Arrays by Infiltration of AAO. Highmolecular-weight polystyrene (PS) (mass-average molecular weight Mw = 524 kg/mol; polydispersity index = 1.04) with a Young modulus E ∼ 3 GPa similar to that of gecko seta consisting of keratin (E ∼ 2−4 GPa)36,37 was filled into selfordered AAO with a pore diameter of 270.1 ± 21.0 nm (standard deviation), a pore depth of 1.5 μm, and a lattice constant of 500 nm38 adapting a procedure reported previously.39 Complete etching of the AAO (see Methods section) revealed that arrays of PS nanorods formed whose tips were inverse replicas of the hemispherical AAO pore bottoms, evidencing complete filling of the AAO pores. The average weight Me of PS chain segments between two entanglements in softened PS was reported to amount to 13 000−18 000 g/ mol.40,41 Owing to the high molecular weight of the PS we used, each PS chain in the PS nanorods formed ∼28 entanglements. Hence, the PS nanorods were highly stable against mechanical impact and did not condense even after the wet-chemical etching of the AAO (Figure 2). Tip Shaping. Etching only the barrier oxide forming the hemispherical AAO pore bottoms (Figure S1a), as described in detail in the Methods section, resulted in the opening of the 10782

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Figure 5. Typical force−displacement curve obtained by testing adhesion of a PS nanorod array with a spherical sapphire probe (diameter 3 mm) firmly mounted to the free end of a metal doubleleaf spring. The insets illustrate the position of the sapphire sphere relative to the surface of the PS nanorod array (for the sake of clarity, the suspension of the sapphire sphere is represented by a cantilever). The sapphire sphere approaches the PS nanorod array. After contact formation, the sapphire sphere is further displaced toward the PS nanorod array until a preset loading force Fl is reached. Then, the sapphire sphere is retracted. During retraction, the PS nanorods remained attached to the sapphire sphere beyond the position where contact formation had initially occurred because of their adhesion to the sapphire sphere. Detachment finally occurs at a certain pull-off force corresponding to the adhesion force Fad. Note that Fad has a negative sign and Fl a positive sign.

Figure 3. SEM images of the tips of PS nanorods after shaping. Panels a, c, and e show undersides of partially etched AAO layers with opened pore bottoms containing PS nanorods. Panels b, d, and f show released PS nanorod arrays after complete etching of the AAO. (a, b) Flat tips obtained by cold pressing; (c, d) pancake-like tips obtained by hot pressing; and (e, f) foot-like tips obtained by shear pressing. The PS nanorod arrays seen in panels b, d, and f were tilted by 30°. The scale bars correspond to 200 nm.

mounted on a metal spring with a velocity of 2 μm/s to the PS nanorod array under investigation. At a certain displacement, the sapphire sphere forms contact with the PS nanorod array. Further displacement of the sapphire sphere deforms the PS nanorods, and the sapphire sphere indents the PS nanorod array. The sapphire sphere is displaced into the PS nanorod array until a predefined target value of the loading force Fl is reached. Then, the sphere is retracted with the same velocity as it was approached to the PS nanorod array in the direction opposite to the approach direction. The pull-off force Fad that represents the normal adhesion force of the PS nanorod array is the force that has to be applied to separate the sapphire sphere from the PS nanorod array. Fad has the opposite sign of Fl. No significant adhesion force was apparent for smooth PS films (Figure 6a) and arrays of PS nanorods having hemispherical tips corresponding to Figure 2 (Figure 6b), even if large loading forces of ∼1 mN were applied. Arrays of PS nanorods with modified terminal tip geometries showed significantly enhanced adhesion even under a moderate loading force of about 500 μN (Figure 7a). Remarkably, no hysteresis was apparent in the load-retraction cycles, suggesting the PS nanorod arrays deformed rather elastically than inelastically.43 For flat, foot-like, and pancake-like tips Fad amounted to 92.5, 183.0, and 677.9 μN, respectively (mean values of at least six measurements at different positions on two samples; see Figure 7b). To calculate adhesion strength, the diameter of the apparent contact area D = 2r between the sapphire sphere with radius R and the PS nanorod array under investigation (Figure 8a,b) needs to be known. The apparent contact area corresponds to the projection of the curved contact area between the sapphire sphere and the probed PS nanorod array into the plane normal to the long axes of upstanding nanorods. The indentation depth

Figure 4. Top-view SEM image of an array of PS nanorods with pancake-like tips obtained by hot pressing. The scale bar corresponds to 1 μm.

correspond to that of deformable spheres with the same diameter, is quite small. Therefore, it is unlikely that significant adhesion between the contact elements occurs. On the contrary, pancake-like tips might even help prevent condensation of the contact elements under strong load. Similar considerations also apply to arrays of PS nanorods with footlike tips obtained by shear pressing. Adhesion Measurements. Adhesion measurements (Figure 5) were performed by approaching a sapphire sphere 10783

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Figure 6. Force−displacement curves of (a) a smooth PS film and (b) an array of PS nanorods (diameter 270 nm, lattice constant 500 nm) with hemispherical tips corresponding to those seen in Figure 2. No pull off-forces were detected on both samples. The curves were shifted horizontally 2 μm apart from each other.

Figure 7. Adhesion measurements on PS nanorod arrays (nanorod diameter 270 nm; lattice constant 500 nm). (a) Typical force− displacement curves (shifted horizontally by 2 μm apart from each other) of PS nanorod arrays with flat tips obtained by cold pressing, foot-like tips obtained by shear pressing, and pancake-like tips obtained by hot-pressing. (b) Adhesion force Fad (mean values of at least six measurements at different positions on two samples) of PS nanorod arrays with flat tips obtained by cold pressing, foot-like tips obtained by shear pressing, and pancake-like tips obtained by hotpressing for a loading force Fl ≈ 500 μN. The error bars indicate standard deviations.

Δl was calculated by subtracting the deformation Cspring of the spring on which the sapphire sphere is mounted from the loading distance Dl (Figure 8c), using a calibration factor α = 1.137: Δl = Dl −

αFl Cspring

shape is central to the adhesive function of biological and biologically inspired adhesive systems. Modeling Loading Force/Adhesion Curves with a Modified SPG Model. For all tip shapes, Fad increased with increasing Fl (Figure 9). For the pancake-like tips, Fad exceeded Fl up to Fl values of ∼600 μN and reached a plateau for Fl > 1200 μN, while no such a plateau was apparent for PS nanorod arrays with flat and foot-like tips. The observed dependence of Fad on Fl is not in agreement with the classical Johnson− Kendall−Roberts (JKR) theory,44 which predicts constant Fad independent of Fl. However, in line with the SPG model,35 some insect attachment pads45 and artificial surfaces14,43 show a similar relation between Fl and Fad. The SPG model considers each PS nanorod as an independent spring with a spring constant E/Al0, where E is the Young modulus of a PS nanorod, l0 its length, and A its contact area. The classical SPG model assumes that once a nanorod is in contact it remains in contact until pull-off. As obvious from Figures 3 and 4, the tips of the PS nanorods are not arranged in a coplanar way. At first the sapphire sphere forms contact with the locally highest nanorods (Figure S2a). As the sphere moves further toward the nanorod

The diameter of apparent contact area D was calculated as follows, assuming that Δl ≪ R: D = 2r = 2 R2 − (R − Δl)2 ≈ 2 2R Δl

The diameters of the apparent contact areas D on PS nanorod arrays with flat, foot-like, and pancake-like tips at Fl ≈ 500 μN amounted to ≈54, ≈58, and ≈70 μm. Dividing the corresponding Fad values (cf. Figure 7b) by the apparent contact area yielded adhesion strengths for flat, foot-like, and pancake-like tips of 40.4, 69.3, and 176.2 kPa. Under the assumption that all nanorods within the contact area have effective contact with the sapphire sphere, each PS nanorod with flat, foot-like, and pancake-like tip contributes 3.8, 5.7, and 9.9 Pa, respectively, to Fad (hexagonal nanorod arrays with a lattice period of 500 nm contain 4.62 nanorods/μm2). Fad of PS nanorod arrays with foot-like tips was 1.5 times stronger that of arrays with flat tips, although in both cases the contact area of a single nanorod was similar. This outcome suggests that the tip 10784

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p=

Δl if Δl < Δl1 Δl1

p = 1 if Δl > Δl1

and

During the indentation of a structured surface with a sphere, Fl is related to Δl as follows:

Fl =

πER Δl 2 l0

During pull-off, the PS nanorods are stretched until their maximal extension Δlm is reached; the pulling force reaches a Table 1. Values of Fl1 and Δlm/Δl1 Used for Data Fitting tip geometry

Fl1 (mN)

Δlm/Δl1

flat foot-like pancake-like

14.07 7.74 0.53

0.226 0.444 1.378

maximum at Fad. Using the values given in Table 1, Fad can then be calculated as follows: Figure 8. Schematic diagrams illustrating the definition of the apparent contact area between sapphire sphere and PS nanorod array. (a) Side view and (b) top view of the sphere-on-flat geometry used to measure force−displacement curves. R is the radius of sapphire sphere, Δl is the indentation depth, r is the radius of the apparent contact area. (c) Graphical representation of the loading distance Dl determined from force−displacement curves (Fl = loading force, Fad = pull-off force).

⎛ Δl 2 Fad = Fl ⎜⎜ m − 3 l Δ ⎝ 1

Fl ⎞ ⎟⎟ , if Fl ≤ Fl1 Fl1 ⎠

and

⎛1 2Δlm Δl ⎞ Fad = ⎜ − m ⎟Fl1 + Fl1Fl − Fl , if Fl ≤ Fl1 Δl1 ⎠ Δl1 ⎝3

Fad =

Δlm 3

Δlm 2

3Δl1

Δl12

F , if Fl > 3 l1

Fl1

and

Δlm ≤ Δl1

⎛ Δl 2 Δl Δl 2 1⎞ Fad = ⎜ m2 − m + ⎟Fl1 , if Fl > m2 Fl1 3⎠ Δl1 Δl1 ⎝ Δl1

and

Δlm > Δl1

where Fl1 is the loading force at Δl1. Note that the relations between Fl and Fad are different for loading forces below and above Fl1. A general feature of the proposed model is that Fad increases with Fl until Fad reaches a plateau. The dependence of Fad on Fl is the more pronounced the larger the contact area per nanorod tip is (Figure 9). Moreover, the Fl1 values summarized in Table 1 suggest that, in the case of the pancake-like tips, not only the larger contact area per nanorod but also enhanced ability to form contact plays a role. We speculate that this is the reason why only nanorod arrays with pancake-like tips reach the plateau at large Fl values, while for PS nanorod arrays with flat and foot-like tips the plateau apparently lies outside the experimentally accessible Fl range. Durability Tests. Arrays of collapsed PS nanorods did not show apparent adhesion (Figure S3), indicating that durability is essential for the functionality of the PS nanorod arrays. The durability of a PS nanorod array with pancake-like nanorod tips was tested by carrying out a series of attachment−detachment cycles at the same position. At first, we successively increased Fl from 265 to 1474 μN in seven consecutive attachment/ detachment cycles (Figure 10a). Then, 10 attachment/ detachment cycles were carried out with Fl ≈ 1000 μN followed by 10 more attachment/detachment cycles with Fl ≈ 300 μN. During all consecutive attachment/detachment cycles performed with the same loading force, Fad remained essentially constant (∼1500 μN for Fl ≈ 1000 μN and ∼700 μN for Fl ≈ 300 μN) (Figure 10b). The durability test revealed that the PS

Figure 9. Dependence of Fad on Fl for PS nanorod arrays with flat tips obtained by cold pressing, foot-like tips obtained by shear pressing, and pancake-like tips obtained by hot-pressing. Each data point is the mean value of at least six measurements at different positions on two samples. The error bars indicate the standard deviations. The solid lines are fits obtained with the modified SPG model described in the text.

array, it builds contact also with shorter nanorods or nanorods having tilted tips. The number of nanorods in contact with the sapphire sphere reaches its maximum at an indentation depth Δl1 (Figure S2b). Our experimental data could be reproduced well with a modified SPG model in which all contact elements were modeled as elastic springs uniform in length with length l0. Additionally, the probability p of formation of contact to the sapphire sphere persisting until pull-off is linearly proportional to a “nanorod compression” Δl (the indentation depth) until p equals one (at Δl1, all PS nanorods are in contact with the sapphire sphere): 10785

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significantly better normal adhesion than foot-like tips commonly used if high shear adhesion is desired. Both the absence of hysteresis in the force/displacement curves and the pronounced durability of the nanorod arrays in series of repeated attachment/detachment cycles suggest that the nanorods behave like elastic springs. Experimental load− adhesion curves were reproduced with a modified Schargott− Popov−Gorb (SPG) model, assuming that contacts between probe and individual nanorods are sequentially formed with increasing indentation depth.

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METHODS

Preparation of PS Nanorod Arrays. PS (Mw = 524 kg mol−1; PDI = 1.04) and epoxy resin (ATACS5104/4103) purchased from Sigma-Aldrich were used as received. Self-ordered nanoporous AAO templates attached to underlying Al substrates were fabricated by two step mild anodization with phosphoric acid as electrolyte.38 The AAO templates had a pore diameter of ∼270 nm, a lattice period of ∼500 nm, and a pore depth of ∼1.5 μm. To infiltrate PS into the AAO nanopores, AAO was placed on 150 μm thick PS films and heated to 120 °C for 48 h and to 200 °C for 1 h under vacuum while a pressure of about 160 bar was applied. Then, the PS-infiltrated AAO templates were cooled to room temperature at a rate of 1 K/min. As a result PS nanorods formed, which were faithful inverse replicas of the AAO pores (cf. Figure 2). The PS nanorods were connected with a 150 μm thick bulk PS film on the AAO surface. The PS-infiltrated AAO templates were embedded into epoxy resin in such a way that the underside of the Al substrate remained uncovered while the interface between the PS film and the AAO was sealed. After curing the epoxy resin at room temperature for 24 h the Al substrates were selectively etched with a solution of 100 mL of 37% HCl and 3.4 g of CuCl2·xH2O in 100 mL of water at 0 °C. Thus, the hemispherical pore bottoms of the AAO layer consisting of barrier oxide were uncovered. During the etching of the Al, the epoxy resin protected the AAO layer. The remaining epoxy resin-supported AAO membranes infiltrated with PS were immersed into 10% phosphoric acid at 60 °C for 11 min to open the AAO pore bottoms by etching the barrier oxide so as to release the tips of the PS nanorods. During this step, the protection by the epoxy resin ensured that the etching solution exclusively attacked the barrier oxide at the undersides of the AAO layers, while the etching solution was prevented from attacking the surfaces of the AAO layers next to the PS surface films. Penetration of the etching solution between the AAO layers and the PS surface films would have resulted in complete detachment of the AAO layers from the PS nanorod arrays. Tip Shaping. For the preparation of PS nanorod arrays with flat tips by cold pressing, the smooth base of a PS cylinder was pressed against the AAO-supported PS nanorod tips at room temperature and at a pressure of ∼250 kPa. For the preparation PS nanorod arrays with pancake-like tips by hot pressing, pieces of silicon wafers were placed on the tips of PS nanorods protruding from supporting AAO membranes. The backsides of the Si wafers were then heated to 170 °C for 1−2 s with a waxing-up instrument waxlectric light I (Renfert) while a pressure of ∼50 kPa was applied. After cooling to room temperature, the silicon wafers were detached by immersing the samples in deionized water. For the preparation of PS nanorod arrays with foot-like tips by shear pressing, a PS cylinder tilted at a small angle was moved across the arrays of PS nanorods protruding from supporting AAO membranes at room temperature. The mechanical stabilization by the epoxy resin support was central to successful largearea tip shaping (typical sample area: 5 × 5 mm2); conducting the tip shaping procedures described above without epoxy resin support led to cracking of the AAO membranes. The AAO was finally completely removed by etching in 10% aqueous HCl solution at 60 °C. Electron Microscopy. SEM investigations were carried out on a Zeiss Supra 55 microscope operated at an accelerating voltage of 1.5 kV. The samples were sputter-coated with a ∼5 nm thick iridium layer. To obtain mean values and standard deviations of the diameters of the

Figure 10. Durability test of a PS nanorod array with pancake-like tips. (a) At first, seven consecutive attachment/detachment cycles with successively increased Fl were performed. Fad of these seven initial attachment/detachment cycles is plotted as a function of Fl. The applied Fl values were 265, 508, 779, 978, 1145, 1307, and 1474 μN. (b) Subsequently, 10 attachment/detachment cycles with Fl ≈ 1000 μN and 10 further attachment/detachment cycles with Fl ≈ 300 μN were carried out. All measurements were performed at the same position on the tested array.

nanorod array withstood large loading forces without structural damage and decrease in Fad. PS nanorod tips with flat and footlike tips showed similar durability (Figure S4).

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CONCLUSIONS Stiff hairy adhesion systems were obtained by molding highly entangled, thermoplastic polymers against nanoporous anodic aluminum oxide templates and subsequent mechanical shaping of the tips of the polymer nanorods formed in the AAO pores. The results presented above provide design principles for customizing the adhesion properties of nanorod arrays. Hemispherical tips are associated with low adhesion even at high loading force (Fad not detectable with sapphire spheres as probes even for Fl ≈ 1 mN). Thus, arrays of nanorods with hemispherical tips might be used as nanostructured surfaces with antiadhesive properties. While, up to now, predominantly shear adhesion of hairy adhesive systems with nanoscale feature sizes has been optimized, we have presented adhesive systems combining nanoscale feature sizes and terminal contact shapes optimized so as to maximize normal adhesion. Pancake-like tips yielded high normal adhesion (Fad ≈ 700 μN) even for small loading forces (Fl ≈ 300 μN), and up to ∼600 μN Fad even exceeded Fl. Thus, pancake-like tips, which have for a given area density of array elements a larger contact surface, show 10786

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shaped PS nanorod tips, representative grayscale SEM images were converted into binary images and analyzed with the program ImageJ 1.37v. Adhesion Measurements. The adhesion performance of the PS nanorod arrays and of smooth PS films was tested by recording force− displacement curves with a home-built force tester Basalt-02. A sapphire sphere with a diameter of 3 mm glued to the free end of a metal double-leaf spring was used as probe. The sapphire sphere can be moved up and down by a piezo drive. The use of spherical probes overcomes alignment problems inherent to flat-on-flat geometries. The samples to be investigated were placed on a hexapod nanopositioning stage (Physik Instrumente, Karlsruhe, Germany). The measurements were carried out at room temperature at humidities of 30−40%. The approach and retraction speeds were 2 μm s−1. Since the spring constant (298 N/m) was known, the spring deflection during the loading and unloading, which was monitored with a laser interferometer, could be converted into force. For data collection, a custom-made Labview software package was used. The sapphire sphere was cleaned with acetone prior to measurements on new samples.

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S4. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (L.X.); martin.steinhart@ uni-osnabrueck.de (M.S.); [email protected] (S.N.G.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the German Research Foundation (Priority Programme 1420 “Biomimetic Materials Research: Functionality by Hierarchical Structuring of Materials”) for financial support and C. Hess and H. Tobergte for the preparation of AAO membranes. L.X. thanks the Alexander-von-Humboldt Foundation for a fellowship.

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