Article pubs.acs.org/Langmuir
Bioinspired Orientation-Dependent Friction Longjian Xue, Jagoba Iturri, Michael Kappl, Hans-Jürgen Butt, and Aránzazu del Campo* Max-Planck-Institut für Polymerforschung (MPIP), Ackermannweg 10, 55128 Mainz, Germany S Supporting Information *
ABSTRACT: Spatular terminals on the toe pads of a gecko play an important role in directional adhesion and friction required for reversible attachment. Inspired by the toe pad design of a gecko, we study friction of polydimethylsiloxane (PDMS) micropillars terminated with asymmetric (spatular-shaped) overhangs. Friction forces in the direction of and against the spatular end were evaluated and compared to friction forces on symmetric T-shaped pillars and pillars without overhangs. The shape of friction curves and the values of friction forces on spatulaterminated pillars were orientation-dependent. Kinetic friction forces were enhanced when shearing against the spatular end, while static friction was stronger in the direction toward the spatular end. The overall friction force was higher in the direction against the spatula end. The maximum value was limited by the mechanical stability of the overhangs during shear. The aspect ratio of the pillar had a strong influence on the magnitude of the friction force, and its contribution surpassed and masked that of the spatular tip for aspect ratios of >2.
1. INTRODUCTION The ability of a gecko to strongly attach to and effortlessly detach from rough surfaces of almost any kind has inspired new designs of reversible adhesives over the past decade.1 A variety of patterned surfaces mimicking the fibrillar surface design of the toe pads of a gecko have been reported, with some of them showing even greater adhesion forces than the natural structures.2,3 The benefits of a split contact surface4 containing contact elements with overhangs for maximizing adhesion performance have been demonstrated in several structures, including symmetric T-shaped5−12 and asymmetric spatulashaped5,6,9,13,14 pillars in either vertical5−9,14 or tilted11,13 disposition. Recent theoretical calculations have shown that T-shaped terminals effectively reduce stress concentration at the edge, so that crack nucleation occurs in the middle of the contacting interface and propagates toward the outer edge, while the perimeter keeps in contact until the complete separation, thereby resulting in a net adhesion enhancement.15−17 While splitting a single contact area into smaller (microscale) and densely packed contact elements with overhangs typically increases adhesion forces, the friction properties of such surfaces remain a matter of debate. Both increased18 and reduced19−23 friction have been reported on fibrillar patterns against flat controls by different authors. Arrays of stiff (E = 1 GPa) polypropylene fibers with 300 nm diameter showed enhanced friction when compared to a smooth surface.18 The higher compliance of the fiber array (because of buckling and bending of the fibers) allowed for substantial interfacial contact, while a negligible contact area was established on the flat surface. A soft surface, such as polydimethylsiloxane (PDMS) (E of several MPa), is able to conform to the counterpart surface that patterned surfaces with micropillars without © XXXX American Chemical Society
overhangs have shown reduced friction forces compared to flat surfaces.19−22 T-shaped pillars have been reported to perform better but still did not surpass the performance of flat analogues.21,24 A strong correlation between the maximum shear force and the tip area was found.10 A few examples of surface patterns with asymmetric micropillar design showing orientation-dependent friction have been reported. This is an important issue for achieving strong but reversible adhesives.25−27 Tilted-wedge shape has been demonstrated for directional adhesion in robots.28,29 Arrays of tilted pillars showed 130−800% friction forces when shearing in the tilting direction than in the opposite direction.11,12,24,30−33 Directional friction has also been observed in vertical pillars with semicircular cross-sections.34,35 The friction differences in these two cases are based on strong differences in the surface area available for contact in opposite directions. In a different example, Janus pillars also showed directional friction properties.36,37 In this paper, we study the directional friction properties of micropillars with spatulashaped terminal elements, which are involved in most biological hairy adhesive systems.38 Direction-dependent friction in this case does not rely on contact area differences but on the asymmetric design of the contact elements. The friction behavior in the direction toward and against the spatular end was investigated and compared to friction on symmetric Tshaped pillars, pillars without overhangs, and flat controls. The role of the aspect ratio in friction was also analyzed. The role of the spatular terminals for directional friction was demonstrated. Received: July 8, 2014 Revised: August 27, 2014
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2. EXPERIMENTAL SECTION 2.1. Materials and Equipment. Silicon wafers (100 orientation) were purchased from Crystec (Berlin, Germany). SU-8 photoresist types 2010 and 2015 covering a thickness range from 10 to 40 μm and the developer mr-Dev 600 were purchased from Micro Resist Technology (Berlin, Germany). 1H,1H,2H,2H-Perfluorodecyltrichlorosilane (96%) was purchased from Alfa Aesar (Karlsruhe, Germany). PDMS elastomer kits (Sylgard 184) were purchased from Dow Corning (Midland, MI). Masks for lithography were custom-made by ML&C (Jena, Germany). A mask aligner MJB 3 UV 400 (Süss MicroTec Lithography, Garching, Germany) was used for lithography. Wavelengths shorter than 350 nm were cut by a PL-360 LP filter (Omega Optical, Brattleboro, VT). A spin-coater WS-650SZ-6NPP/LITE/IND (Laurell Technologies Corporation, North Wales, PA) was used for preparation of photoresist films. Baking steps were carried out on a Präzitherm heating stage (LHG, Düsseldorf, Germany). Thin PDMS precursor films were prepared by a film applicator multicator 411 (Hemer, Germany). Before silanization, the films were treated by an oxidative Plasma Activate Statuo 10 USB (Plasma Technology GmbH, Rottenburg, Germany). Surface microstructures were characterized by scanning electron microscopy (SEM) LEO 1530VP Gemini (Carl Zeiss Jena, Oberkochen, Germany) and optical microscopy. 2.2. Fabrication of SU-8 Mold. Silicon wafers were cleaned in Piranha solution [7:3 (v/v) 98% H2SO4/30% H2O2] overnight and rinsed with deionized water. Before SU-8 lithographic processing, the wafers were rinsed with acetone and blown dry with nitrogen. Details on the processing parameters can be found in a previous paper.39,40 Pattern fields of 8 × 8 mm2 with a cubic arrangement of micropillars of 20 μm diameter and interpillar spacing and heights of 10, 30, and 40 μm were obtained. Before being used for soft lithography, the SU-8 patterned wafers were perfluorinated. For this purpose, they were treated with oxygen plasma for a few seconds, followed by silanization in an evacuated desiccator for 30 min using ∼50 μL of 1H,1H,2H,2H-perfluorodecyltrichlorosilane and baking for 1 h at 90 °C in a vacuum oven. Silanization increased the contact angle of the cured SU-8 from 73° to 115°. The obtained patterns were used as hard molds for the subsequent soft replication processes with PDMS. 2.3. Fabrication of Pillar Micropatterns by Double Soft Molding. Double soft molding was applied to obtain micropillar arrays from a PDMS replica. This process allowed us to avoid damage of the hard lithographic template and to obtain multiple samples of the same master. The PDMS precursor (ratio of 10:1 prepolymer/crosslinker) was degassed and poured onto the SU-8 patterned wafer with tetragonal-arranged round pillars and cured at 90 °C for 1 h in a vacuum oven. The obtained PDMS pattern was the negative replica of the SU-8 pattern and was used as a mold for a second replication process. For this purpose, the PDMS replica was perfluorinated using the same condition as described above for the SU-8 pattern. The PDMS precursor was cast onto the perfluorinated PDMS soft mold and cured at 90 °C for 1 h and then demolded. Micropatterned PDMS fields with arrays of round pillars packed with cubic symmetry with a diameter of 20 μm, an interpillar spacing of 20 μm, and heights of 10, 30, and 40 μm were obtained. The total thickness of the sample was 1 mm. 2.4. Fabrication of Pillars with Spatular Terminals by Inking, Printing, and Tilted Curing. The fabrication procedure is schematically outlined in Figure 1a and was adapted from ref 5. The PDMS precursor (ratio of 10:1 prepolymer/cross-linker) was degassed and left at room temperature for 8−9 h. During this time, partial crosslinking occurs and the viscosity of the system slightly increases. Aged PDMS was spread onto a glass plate using a film applicator and left for 30 min. A 4 × 8 mm2 PDMS pattern with round pillars was manually inked into the liquid PDMS film. The inked patterns were pressed against a perfluorosilanized wafer preheated to 65 °C. A piece of glass slide of 1.5 g (thickness of 1 mm) preheated to 65 °C was put onto the PDMS pattern (see Figure S1 of the Supporting Information). The assembly was tilted 60° and cured at 65 °C for 14 h in an oven. Tilting
Figure 1. (a) Scheme of fabrication of micropillars with spatular tips via inking−printing−tilted curing technology. (b) Optical and (c) scanning electron microscopy (SEM) images of the spatular tips on micropillars with an aspect ratio of 1.5. The scale bar equals 20 μm. Blue and red lines in panel b represent the diameter of the pillar and the width of overhang at its widest part, respectively. slightly shifted the pillars from their initial printed position and created an asymmetric terminal profile. 2.5. Fabrication of Pillars with T-Shaped Terminals. The inked patterns were pressed against a perfluorosilanized wafer and cured in an oven at 65 °C for 14 h. Special care was required to maintain the wafer in the horizontal position for obtaining symmetric T-shaped tips. 2.6. Flat PDMS Films. The PDMS precursor (ratio of 10:1 prepolymer/cross-linker) was degassed and poured onto a glass petri dish and allowed to relax for 0.5 h, followed by curing at 90 °C for 1 h and then at 65 °C for 14 h. The total thickness of the sample was 1 mm. The side exposed to air was used for friction measurements. 2.7. Friction Testing. Friction measurements were performed using a custom-built device (see Figure S2 of the Supporting Information). The samples were brought in contact with a spherical ruby probe of 5 mm diameter. The lateral (friction) and normal force between the sample and sphere were measured. Before taking data, pretests were run on each sample to set a predefined and constant normal force during the friction test. This was achieved by changing the offset of the vertical piezo translator while monitoring the corresponding output of the vertical force sensor. In cases where a small sample tilt was observed during sliding, we used a linear movement of the vertical piezo stage (typically around 10 nm vertical movement per 1 μm lateral sliding distance) to compensate for the tilt and keep the vertical force constant during sliding. The sample was moved at 100 μm/s over a distance of 500 μm forward and backward (trace and retrace), which corresponds to the maximum travel range of the lateral piezo stage. In contrast to a classical pin-on-disk tribometer, the direction of motion changes periodically within each measurement cycle. This can lead to reorientation effects at the reversal points, especially for high aspect ratio pillars, but should not influence steadystate sliding in between. Preliminary experiments performed at different shearing rates between 20 and 200 μm/s at 1 mN preload did not show any influence of the shearing rate on the friction force. During the experiment, the contact area between the spherical probe and PDMS sample was recorded by a camera attached to an optical microscope.
3. RESULTS AND DISCUSSION 3.1. Fabrication of Micropatterned Surfaces with and without Overhangs and Characterization of Their Adhesion Behavior. Homogeneous cubic arrays of cylindrical micropillars without overhangs and with symmetric (T-shaped) and asymmetric (spatula-shaped) terminals were prepared by adapting a previously reported method (panels b and c of B
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Figure 1 and see Figure S3 of the Supporting Information).5,6 The micropillars had a diameter of 20 μm, heights of 10, 30, and 40 μm, and an interpillar spacing of 20 μm. Pattern dimensions were selected on the basis of maximized friction performance and experimental limitations during the fabrication process.5,6 The spatular tips had a mean width at the widest part of 4.3 μm, more than 1/5 of the pillar diameter. Tshaped pillars had an annular overhang with a mean width of 2.1 μm (see Figure S4a of the Supporting Information). The area density covered by the micropillars was ca. 20%. Micropatterned surfaces showed higher adhesion than flat controls (Figure 2 and Figure S5 of the Supporting
stiffness of the sample and force sensor. When the maximum value of static friction is overcome (transition), sliding between the probe and sample starts (kinetic friction). For symmetric surface patterns, the trace and retrace friction curves are symmetric around the zero lateral force offset V0 (Figure 3a). The kinetic friction force signal of trace and retrace was obtained by averaging Vt for trace and Vr for retrace and subtracting the offset V0, leading to the values of ΔVt and ΔVr. The force sensor signal in volts was converted to force in Newtons by multiplying with the known calibration factor of the sensor, which was determined before by applying defined loads with known weights. The value of V0 is essentially given as the voltage reading of the force sensor at zero applied force. However, the exact value V0 depended slightly upon the applied load because of a small residual torsion of the lateral force sensor in the presence of an applied load. In the case of a smooth surface or a symmetric pattern, one can obtain V0 directly from the friction curves as the mean value of Vt and Vr. For asymmetric patterns, one obtains friction curves that are no longer symmetric around the zero force offset (Figure 3b), and thus, determination of V0 is no longer straightforward. In this case, we determined the correct value of V0 for a certain load from an analogous symmetric system; i.e., V0 from symmetric pillars with an aspect ratio of 0.5 was used to define the zero lateral deflection of pillars with spatular terminals of the same aspect ratio. 3.3. Friction of Patterned Surfaces with Symmetric Features. Friction measurements performed on patterned surfaces with T-shaped micropillars and on pillars with no overhangs as controls (panels a and b of Figure 4) showed similar features. After the maximum static friction force was reached, smooth sliding of the probe across the sample at a constant friction force was observed. No stick−slip behavior was detected.19−21 The friction force increased linearly with the applied load (Figure 4d) within the force range tested (1−10 mN). Measured friction forces were slightly smaller for Tshaped pillars than for pillars without overhangs. Friction coefficients on the microstructured patterns, i.e., friction force divided by the load, were 0.9 for pillars without overhangs and 0.7 for T-shaped pillars. Thus, T-shaped structures do not lead to increased friction, in contrast to the adhesion, which is higher with overhangs (Figure 2). Microscopic inspection of the T-shaped patterns after the measurements revealed partial damage of the overhang after iterative measurements at large normal loads. These measurements were disregarded in Figure 4d. It is important to note that friction studies on T-shaped pillars with thicker overhangs did not lead to mechanical damage during experiments.21,41 In contrast to the microstructured surfaces, flat PDMS surfaces showed a periodic zigzag profile in the kinetic part of the friction curve (Figure 4c), characteristic for stick−slip behavior. Stick−slip on flat elastomeric surfaces has been associated with the formation and attachment to Schallamach waves.42 These seem to be suppressed in the patterned samples (panels a and b of Figure 3), in accordance with earlier observations.19,22 Flat PDMS showed larger friction forces than our patterned surfaces (Figure 4d). This has been associated with the reduced effective contact area and the reduced shear resistance of micropatterned surfaces.19−22 It also indicates a negative effect of contact splitting for friction on flat surfaces (under dry conditions). In contrast, adhesion is increased by contact splitting in our patterns (Figure 2).4,43 For a soft surface that is as compliant as PDMS, friction may no longer be
Figure 2. Adhesion forces for a 5 mm diameter ruby sphere on flat and patterned surfaces measured with a loading force of 9 mN.
Information). Arrays of T-shaped micropillars demonstrated the best adhesion performance (37.9 ± 3.4 mN), ca. 22 times higher than flat PDMS and 5.4 times higher than arrays of pillars without overhangs. Micropillar patterns with spatular tips showed intermediate adhesion values (12.1 ± 1.5 mN), slightly better than the pillars without overhangs (7.1 ± 0.2 mN). These results agree with previous literature reports.5,39 3.2. Friction Force Curves. Figure 3 shows raw data of two representative friction force measurements on patterned
Figure 3. Raw data of lateral force sensor signal versus displacement curves obtained by testing friction with a ruby sphere of 5 mm diameter. The arrows indicate the direction of movement to left and right (trace and retrace) of the ruby sphere relative to the sample surface. V0 denotes the zero lateral force offset of the sensor. Vt and Vr are the kinetic friction force signal in trace and retrace direction, and ΔVt and ΔVr are the average values of kinetic friction. (a) Data taken on a structured sample with normal pillars with symmetric friction and (b) data taken on a sample with pillars with spatula tips with asymmetric friction.
surfaces using a spherical ruby probe. The lateral deflection signal of the force sensor represents the lateral displacement of the sample in two opposite directions (trace and retrace). The curves show two distinct regions that correspond to static and kinetic friction. In the region of static friction, the lateral deflection of the force sensor rapidly increases, while the sample is laterally displaced. Sliding does not yet occur. The slope of the curve in this region reflects the combined shear C
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Figure 4. Representative friction curves for three different surfaces at normal loads of 1, 4, and 7 mN. (a) Pillars without overhangs, (b) T-shaped pillars, and (c) flat PDMS. The inset indicates the corresponding structure. The aspect ratio of pillars for both panels a and b was 0.5. (d) Load dependence of friction forces for the three different surfaces.
dominated by surface roughness and Amonton’s law of friction may no longer hold. In such a system, friction is expected to become proportional to the shear strength of the contact times the contact area. For the fully elastic contact between a sphere and a planar surface, the relation between the true contact area and applied load will be nonlinear and given by the Hertz or Johnson−Kendall−Roberts (JKR) model, where normal load FL and adhesion (Fad) will both contribute.44 In such a case, the friction force F can be described as follows: F = α(FL + Fad)2/3
where α is proportional to the shear strength. For the case of the flat PDMS surface, this model fits the experimental values of friction force as a function of the applied load and gives an adhesion force of Fad = 0.3 mN, which is lower than the measured adhesion force for that surface of around 2 mN (Figure 4d). A linear fit of the data, assuming Amonton’s law, does not fit the data points as well but gives a value of the adhesion force of 2.6 mN, which is close to the measured adhesion force value (see Figure S6 of the Supporting Information). This indicates that, for the flat PDMS system, we are in a transition regime between Amonton’s friction and pure elastomer friction.45 All of the micropillar-patterned surfaces showed lower friction force and a linear relation between the friction force and applied load, while the measured adhesion was higher than that of flat PDMS. T-shaped pillars showed almost the same friction force values as the pillars without overhangs but a 20 times adhesion force compared to normal pillars. Therefore, for the micropillar-patterned surfaces, adhesion seems not to contribute significantly to the friction force compared to the applied load. 3.4. Friction of Asymmetric Pillar Patterns. In the following, the subindices “T” or “A” in the friction force symbol F will indicate the sliding direction toward or against the spatular end (Figure 5a). Arrays of pillars with spatula-like terminals showed asymmetric friction curves when sliding the probe toward and against the spatula tip. When sliding against
Figure 5. (a) Schematic of the experimental procedure for probing directional friction. The probe is first slided against (leading to a force FA) and then moved back toward the spatular end (leading to a force FT). The arrows indicate the sliding direction of the probe. (b) Representative friction curves for pillars with spatula tips and aspect ratio of 0.5. (c) Friction forces FA and FT extracted from panel b compared to friction forces on pillars without overhangs.
the spatular end, the kinetic part of the friction curve did not exhibit a constant force value, as for the symmetric pillars, but increased until a plateau value was reached (Figure 5b). The corresponding minimum and maximum values of the friction D
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force FA are represented as the bottom and upper border of boxes in Figure 5c for the different loading forces. The minimum values of FA (i.e., those obtained at the transition between static and dynamic friction) were similar to the friction forces of pillar patterns without overhangs. This result indicates that the spatular shape at the contact terminal does not play a major role for static friction. However, the presence of spatulae seems to enhance dynamic friction forces when sliding against the spatular end, as evidenced by the increase in the friction force during sliding. The length of the box, i.e., the difference between the minimum and maximum values of FA, increased with the loading force (from 1.4 ± 0.3 to 4.1 ± 0.9 mN when the loading force was increased from 1 to 10 mN). It indicates a larger chance for spatulae sticking to the probe under a larger loading force contributing to a larger kinetic friction. Microscopic observation of the contact area during friction provides additional information about the role of spatulae during sliding. Figure 6 shows a snapshot of the contact plane of the ruby sphere with the array of spatula-terminated pillars. The pillars in contact with the probe appeared dark, and the
pillars out of contact appeared bright. The pillars in the image were in different frictional situations that can be associated with different stages in the friction curve. The pillars far from the contact line were under static friction: the pillar tops, including the overhangs, were in contact, and the pillars were stretched in the shear direction (yellow contour line and white dash lines). At a critical displacement, detachment started at the point of highest stress and kinetic friction started. The detachment of the stretched pillars started at the opposite side of the spatular edge. This is visualized in Figure 6a (white contour line highlighting brightness at that part of the pillar). The flexible spatular edge remained attaching to the ruby probe during sliding, and therefore, it contributed to an enhancement of kinetic friction force. It is important to note that these effects were not seen in T-shaped pillars with symmetric overhangs and are only characteristic of the asymmetric, spatular contact geometry, which seems to play a relevant contribution to the friction force during sliding. With further displacement, the spatula was further stretched and, occasionally, it folded up, reducing the friction force values in the next cycle (see Figure S7 of the Supporting Information). In the retrace curve, the kinetic friction force toward the spatular end, FT, remained constant during sliding (Figure 5b), as observed in symmetric pillars. The obtained FT values were lower than in the previous tracing experiment (FA). This result demonstrates that the spatular terminal of micropillars allows for directionality in the friction properties of the surface. Sliding against the spatular tip leads to higher friction forces than toward the spatular tip (Figure 5c). When sliding toward the spatular tip, the maximum stress is concentrated at the point where the spatula protrudes from the pillar stem.41,46 Detachment is expected to start at this point, and the crack is expected to propagate outward to the periphery. The microscopic image (Figure 6b) shows the detached spatular edges as brighter areas (white contour line) during sliding, while the rest of the pillar area remains in contact with the ruby probe. Therefore, the spatular tip contributed to an increased static friction of patterned surfaces when they are probed in the direction toward the spatular end. This was also reflected in the probe displacement required to reach the static−dynamic transition. In the direction toward the spatular end, the probe moves over longer displacements to reach the static−dynamic transition than when sliding against the spatular end (Figure 6c). Finally, the kinetic friction in the direction toward the spatular end proceeded in a similar way as on pillars without overhangs, because the spatula is detached from the probe during sliding and does not contribute to the kinetic friction. Note FT was slightly lower than that on pillars without overhangs, presumably because the detached spatula opened a crack for the detachment/sliding. 3.5. Influence of the Pillar Aspect Ratio on the Asymmetric Friction. The experiments on previous sections were performed on pillars with an aspect ratio of 0.5 (ratio of height/diameter). For higher aspect ratios (1.5 and 2), the friction behavior changed. First, the profile of friction curves presented two transitions in tracing and retracing directions. These transitions were observed both on pillars without (Figure 7 and see Figure S8 of the Supporting Information) and with overhangs (see Figure S9 of the Supporting Information). Optical microscopy analysis of the contact zone during sliding revealed that the pillars with an aspect ratio of 1.5 or 2 strongly stretched and tilted over into the sliding direction. When the sliding in one direction started, the pillars in contact with the
Figure 6. Snapshots from optical video microscopy of an array of spatular-tip-terminated micropillars with an aspect ratio of 0.5 during (a) sliding against spatular end and (b) sliding toward spatular end. The cartoons at the top right corner indicate the orientation of the spatular tip and the moving direction of the probe (red arrow). The cartoons below the snapshots illustrate the deformation of pillars. The pillars in contact with the probe appear dark, and the pillars out of contact appear bright. The green circles mark pillars without contact. The yellow lines outline pillars in full contact. The white lines outline the detached area of a pillar partially in contact. The white dash lines indicate relative positions of the pillars in and without contact with the ruby probe. The scale bar in both images equals 50 μm. (c) Displacements at which the transition from static to kinetic friction appears while sliding toward and against the spatular end. All pillars had an aspect ratio of 0.5. E
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Figure 7. (a) Representative friction curves for pillars with an aspect ratio of 1.5 at loading forces of 1, 4, and 7 mN. (b) Video microscopy snapshot of the array of pillars with an aspect ratio of 1.5 (20 μm diameter, 20 μm spacing, and 30 μm height) during sliding with a loading force of 7 mN. The scale bar equals 100 μm. (c) Scheme showing the deformation of pillars with an aspect ratio of 1.5 during sliding. The red arrow indicated the direction of displacement of the probe. The numbers correspond to the different sections of the curve in panel a.
friction force of the flat surface at normal loads smaller than 6 mN. Note that previous reported works on friction properties of patterned surfaces with soft materials have never been able to surpass flat controls. These results provide evidence for the relevance of the proper surface design for maximizing friction performance on micropatterned surfaces. Orientation-dependent friction measurements were performed with spatula-terminated pillars with an aspect ratio of 1.5 and 2 (Figure 9 and see Figure S9 of the Supporting Information). In general, the differences in friction force in the direction toward and against the spatular end were less pronounced in pillars with a higher aspect ratio. The bending of the longer pillars during shearing allows for sliding to occur
probe were already pretilted in the opposite direction from the previous friction cycle (note that the probe remains in contact with the sample between friction cycles). When the new cycle began, the deformed pillars first recovered their upright position (first static−dynamic transition) and then they were deformed in the opposite direction (i.e., the actual sliding direction) before the second static−kinetic transition was reached. Sliding occurred on not only the pillar tips but also the stems of the deformed pillars once pillars had flipped over and detachment between pillar tops and sphere had occurred. The friction force and the friction coefficient increased with increasing the aspect ratio (Figure 8). This can be attributed to
Figure 8. Friction force versus normal force measured on patterns of micropillars without overhangs with different aspect ratios and on flat PDMS as the reference. The lines are the linear fits. The slopes of the linear fits are indicated.
the larger contact area formed with the ruby sphere. The indentation depths for pillars with an aspect ratio of 1.5 and 2 at the largest loading force applied (10 mN) were about 30 and 35 μm, close to the pillar height, suggesting that the larger contact areas were mainly contributed by the pillars instead of the supporting layer. The larger contact area can be expected because of two contributions: on one hand, the highly deformed pillars offer additional contact at their stems, and on the other hand, the effective modulus of the patterned surface will decrease with increasing the aspect ratio. In fact, the friction force of the pillar with an aspect ratio of 2 surpassed the
Figure 9. (a) Representative friction curves of spatular-tip-terminated micropillars with an aspect ratio of 1.5 under loading forces of 1, 4, and 7 mN. (b) Friction forces extracted from the friction experiment shown in panel a. F
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on the tilted stems instead of on the pillar tips, reducing the influence of spatular tips on the friction performance.
4. CONCLUSION The geometry of the tip of gecko-like micropillar patterns has been demonstrated to have a crucial impact on their adhesion performance.1 Terminals with T-shaped overhangs have generated the highest adhesion forces.5,6 Asymmetric designs of the contact area, such as the spatular terminal widely found in the biological models, are expected to mediate directiondependent adhesion/friction and to play an important role in effortless detachment.38 Our studies demonstrate that the spatula-like overhangs allow for direction-dependent friction. Sliding against the spatular tip leads to higher friction forces than sliding toward the spatular tip on patterns with low aspect ratios. The contribution of the spatula terminal to friction is different in the opposite directions, as evidenced by microscopic observation.
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ASSOCIATED CONTENT
S Supporting Information *
Protocol to fabricate spatular terminals (Figure S1), particle interaction apparatus (PIA) image and description scheme and calibration curve (Figure S2), optical and SEM images of micropillar with spatular tips (Figure S3), optical image of micropillar with T-shaped tips before and after friction measurement (Figure S4), dependence of adhesion of different structured surfaces and flat control on normal loading (Figure S5), comparison of data fitting to the dependence of friction force upon normal load for flat PDMS and normal pillar with an aspect ratio of 0.5 using Amonton’s law and Hertz model (Figure S6), schematic drawing of shearing first against the spatular end and then toward the spatular end (Figure S7), typical friction curves of a normal pillar with an aspect ratio of 2 (Figure S8), and typical friction curves on spatula-tipterminated micropillars with an aspect ratio of 2 under loading forces of 1, 4, and 7 mN (Figure S9). 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]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the Deutsche Forschung Gemeinschaft for financial support within the program SPP1420 “Biomimetic Materials Research: Functionality by Hierarchical Structuring of Materials” (Projects CA880/1, BU 1556/26) and Dirk Drotlef (MPIP, Mainz, Germany) for preliminary experiments and discussions.
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