Friction and Adhesion of Gecko-Inspired PDMS Flaps on Rough

Jul 10, 2012 - Geckos have developed a unique hierarchical structure to maintain climbing ability on surfaces with different roughness, one of the ext...
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Friction and Adhesion of Gecko-Inspired PDMS Flaps on Rough Surfaces Jing Yu,† Sathya Chary,‡ Saurabh Das,† John Tamelier,‡ Kimberly L. Turner,‡ and Jacob N. Israelachvili*,† †

Department of Chemical Engineering, University of California, Santa Barbara, Santa Barbara, California 93106, United States Department of Mechanical Engineering, University of California, Santa Barbara, Santa Barbara, California 93106, United States

‡

S Supporting Information *

ABSTRACT: Geckos have developed a unique hierarchical structure to maintain climbing ability on surfaces with different roughness, one of the extremely important parameters that affect the friction and adhesion forces between two surfaces. Although much attention has been paid on fabricating various structures that mimic the hierarchical structure of a gecko foot, yet no systematic effort, in experiment or theory, has been made to quantify the effect of surface roughness on the performance of the fabricated structures that mimic the hierarchical structure of geckos. Using a modified surface forces apparatus (SFA), we measured the adhesion and friction forces between microfabricated tilted PDMS flaps and optically smooth SiO2 and rough SiO2 surfaces created by plasma etching. Anisotropic adhesion and friction forces were measured when sliding the top glass surface along (+y) and against (−y) the tilted direction of the flaps. Increasing the surface roughness first increased the adhesion and friction forces measured between the flaps and the rough surface due to topological matching of the two surfaces but then led to a rapid decrease in both of these forces. Our results demonstrate that the surface roughness significantly affects the performance of gecko mimetic adhesives and that different surface textures can either increase or decrease the adhesion and friction forces of the fabricated adhesives.

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INTRODUCTION The remarkable climbing ability of geckos has inspired numerous studies on developing gecko-inspired dry adhesives and gecko mimetic robots.1−5 Various studies have identified that geckos apply weak but universal van der Waals forces and possibly capillary forces under some circumstances to maintain their strong adhesion and adhesive friction on different surfaces.6,7 In a natural environment, almost all surfaces are rough, usually containing different scales of roughness. Surface roughness can significantly reduce the real contact area of two surfaces and therefore reduces the adhesion force between surfaces.8,9 After millions of years of evolution, geckos have developed a highly optimized hierarchical foot structure to overcome this problem (Figure 1a−d): a gecko toe has tens of arrays (lamellae) of micrometer-scale hair (setae), with each seta being about 5 μm in diameter and ∼110 μm in length on average. A single seta has 100−1000 nanoscale spatulae on the top. These spatulae, of 200 nm length and width at the tip, form the fine structure of the gecko adhesive system.6,10,11 This hierarchical structure, by conforming to both micro- and nanoscale asperities, can cover most of the scales of the surface roughness that geckos encounter in nature, achieving a large true area of contact, so that geckos can adhere to different surfaces via the weak van der Waals force.7 © 2012 American Chemical Society

Figure 1. Hierarchical structure of a gecko’s foot (a) toe, (b) seta, (c) spatula, and (d) a magnified view of a spatula pad. Scanning electron microscope (SEM) images of (e) tilted PDMS flaps and (f) enlarged view of the tilted PDMS flaps. Reproduced with permission from ref 12. Copyright 2006 National Academy of Sciences.

Received: May 1, 2012 Revised: July 6, 2012 Published: July 10, 2012 11527

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investigated using the surface forces apparatus, showing that the adhesion between two surfaces decreased exponentially with increasing surface roughness.37 The SFA experiments clearly demonstrate the importance of surface roughness on the adhesion and sliding friction; however, root-mean-square (rms) roughness studied is very small (up to 10 nm) and significantly lower than the surface roughness of many practical applications.8 Previously, some work on characterizing synthetic bio-inspired adhesives on rough substrate surfaces has been reported. However, this work was seriously limited in that the rough substrate surface consisted of a regular pattern.1,38 When tests were reported against real world surfaces such as smooth granite and wood, the variation in roughness values between surfaces was large and abrupt and not fully characterized.39 Thus, despite various attempts at modeling rough surface adhesion and friction on the theoretical side,40 not many systematic experimental investigations have been reported on the effect of nano and sub-micrometer scale roughness on the adhesion and adhesive friction of gecko mimetic adhesives.41 Most of the surfaces that the geckos and gecko mimetic robots encounter have different topological surface profiles with various length scales of roughness. In order to better design gecko-inspired structures for real applications, understanding the performance of the fabricated structure on surfaces with different roughness is of extreme importance. In this study, we fabricated gecko-inspired tilted PDMS flaps (Figure 1e,f) which showed anisotropic friction and frictional adhesion properties when sliding in different directions. We also investigated the effect of nano to sub-micrometer scale surface roughness on the friction and adhesion properties of these tilted flaps.

Although a lot of attention has been paid to the adhesion force of the gecko adhesive system and gecko mimetic adhesives, the lateral force or the friction force, F∥, that balances the gravity force of the body of the gecko is even more important for the gecko’s extraordinary climbing ability on walls but yet has not gained as much attention.12 A general equation that describes the adhesive friction force between two surfaces is F = μF⊥ + ScA real

(1)

where μ is the friction coefficient, F⊥ is the normal load, Sc is the shear strength, and Areal is the real contact area.13 In eq 1, the first term is the simple Amontons’ law for a nonadhering surface, representing the load contribution to the total friction force. The second term gives the contribution of adhesion, which is proportional to the “real” molecular contact area.14 This “real” molecular contact area rapidly decreases with increasing surface roughness, and therefore the adhesion contribution is very small, almost negligible in many practical situations.15 In many biological adhesive systems, however, this adhesion contribution is essential and dominates the friction force.16,17 Another key feature for the success of geckos is that they can switch from strong adhesion and friction forces, in the attaching state, to weak adhesion and friction forces, the detaching state, in 10 ms.6 Geckos achieve this accomplishment by employing articulated peeling motion of their feet and the anisotropic adhesion and friction properties of the setae on their feet.18,19 When a gecko wants to detach a foot from a surface, instead of pulling off the whole foot at once, it rolls the toes backward, peeling off the foot gradually, which requires a much smaller force. Studies have also demonstrated that a single seta is naturally tilted, exhibiting anisotropic adhesion and friction properties: sliding in the tilted direction gives much stronger adhesion and friction forces, whereas sliding in the direction against the tilt shows no adhesion and small friction force.19,20 This anisotropy is a key factor in developing gecko mimetic adhesives and related robotic applications.4,21−25 On developing gecko mimetic adhesives, much effort has been spent to fabricate different structured surfaces that mimic the hierarchical structure of gecko feet and their adhesion and friction properties on model smooth surfaces;1−3 however, less attention has been paid to the effect of surface roughness, one of the most crucial parameters in the study of adhesion and friction. Tests on geckos and beetles have shown that surface roughness significantly affects the adhesion of those animals on model substrates.26−30 Both the geckos and beetles showed stronger adhesion on smooth surfaces. On rough substrates, both tests show that the minimum adhesion could be achieved at certain roughness, while increasing or decreasing the roughness can lead to an increase of adhesion. The effect of surface roughness on the adhesion of elastic plates has been mainly studied theoretically.9 Various theoretical works using both continuum theory and molecular simulation have shown that increasing the roughness can significantly reduce the adhesion between two surfaces.16,31−34 Simulation studies have also shown that the surface topology is one of the most important parameters affecting sliding friction.35 Anisotropic friction was observed when sliding a topologically anisotropic surface against a flat surface.36 The effects of roughness on the adhesion and sliding friction between two polymeric surfaces of 1−10 nm roughness and between a rough surface and a smooth mica surface have been

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MATERIALS AND METHODS

Plasma Etching. In order to test the roughness adaptation of the angled microflap adhesive system, rough substrate surfaces that could be easily integrated into the SFA test apparatus were required. Initial attempts to roughen SFA pucks included HF etching, sandblasting, and etching with a commercial glass etching kit (Armour Etch Cream, Armour Products, Hawthorne, NJ). While no roughening was observed after etching with both buffered HF and 48% HF, the roughness asperities generated by both the sand blasting and Armour etching techniques were ∼10−20 μm in height, nearly an order of magnitude greater than the level of roughness desired for this study. Therefore, smooth SFA glass pucks (Product No. NT63-476, Edmund Optics Inc., Barrington, NJ) were etched in a Materials Research Corporation MRC 51 reactive ion etching system (Praxair Surface Technologies, Indianapolis, IN) using a CF4/SF6/O2/Ar plasma to generate a nonuniform surface roughness pattern with maximum asperity heights lower than 3 μm. No cracks or line textures were observed on the pucks after etching. Maskless dry etching of glass substrates using fluorine containing plasmas has been shown to cause surface roughening in certain parameter ranges. This phenomenon is attributed to the production of less volatile metal halogen compounds on the glass surface during etching. The uncoated SFA glass disks used were manufactured with Schott N-BK7 517/642 material, which is a borosilicate glass (Schott North America, Inc., Elmsford, NY) with a refractive index of 1.517. In addition to SiO2, the BK7 borosilicate glass material has been reported to contain oxides of sodium, aluminum, and barium.42,43 It has been postulated that the creation of less volatile species such as NaF, AlF3, and BaF2 from these materials in a fluorine chemistry RIE plasma may lead to a roughening of the glass substrate by micromasking.44 Unetched and uncoated SFA disks were found by AFM (Tapping Mode) analysis to have a polished surface with an rms roughness value of ∼11 (±8) nm. Through trial and error, a dry etch recipe using a mixture of O2, SF6, CF4, and Ar as process gases in an RIE plasma was 11528

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developed to etch these optically smooth SFA glass disks and generate nonuniformly rough test substrate surfaces. Surfaces of increasing rms values of roughness were generated by exposing smooth SFA disks to the RIE plasma for increasing lengths of time (Table 1).

Table 1. Etching Times for the Creation of Nonuniformly Rough Surfaces on Smooth Silica Disksa etching time (min) 0 35 48 65 120

rms height roughness and std dev (nm) 11 73 133 192 308

± ± ± ± ±

8 5 20 9 56

av distance between asperities and std dev (μm) N/A 8.6 ± 8.1 ± 4.9 ± 4.8 ±

4.5 3.2 3.3 2.7

a Process conditions: O2 flow rate = 30 standard cubic centimeter per minute (sccm), SF6 flow rate = 30 sccm, CF4 flow rate = 3 sccm, Ar flow rate = 30 sccm, chamber pressure = 10 mTorr, bias voltage = 500 V.

Measuring the Surface Topology by Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). Examination of the surface topography after etching using an optical microscope, SEM (XL-30, FEI Co., Hillsboro, OR), and AFM (MFP3D SA, Asylum Research) revealed circular mesa-like structures. The rms roughness of each etched surface was measured using tapping mode AFM scanning over various areas of the surface and a scan area of 50 μm by 50 μm. AFM images showed that the size and density of the asperities increase with increasing etching time, while the gap distance between two neighboring asperities becomes smaller with a longer etching time. Similar features were also confirmed by the SEM measurements (Figure 2). Measuring Normal Load and Friction Forces. The adhesion and friction properties of the tilted PDMS flaps structure on different rough substrates were tested using a 3D displacement and force sensing probe attachment conjugated surface forces apparatus 2000 (SFA 2000) in a previously reported sphere against a flat surface configuration.4,45 A CCD camera mounted on a microscope is incorporated in the system, which allows us to view the local contact during the sliding and adhesion testing. The bottom surface is a flat glass disk with the fabricated PDMS flaps glued on top. The top glass surface, either smooth or with different roughness, is a spherical glass disk with a radius of 2 cm. The top surface can slide laterally over a distance of 200−500 μm with different sliding speeds. During microfabrication, testing setup, and measurements, all work was performed in a clean, sealed environment (laminar flow hood or sealed SFA). The surfaces were stored in a sealed container before testing, protecting them from dirt or dust from the atmosphere (a few small dust particles can be seen in the SEM images, shown in Figure 1S in the Supporting Information). The adhesion tests were reproducible in our measurements as long as the structured surface was not damaged, or only damaged in a few places, which only happened after prolonged high-load and sliding (see Figure 1S). This reproducibility shows that the effects observed could not be attributed to removal of a surface layer or “dirt” particles.

Figure 2. AFM and SEM images of the rough surfaces with different rms roughness: (a, c, e, g, i) AFM images of the etched SFA silica surfaces with 11 ± 8, 73 ± 5, 133 ± 20, 192 ± 9, and 308 ± 56 nm rms roughness; (b, d, f, h, j) SEM images of the same surfaces with 11 ± 8, 73 ± 5, 133 ± 20, 192 ± 9, and 308 ± 56 nm rms roughness.

on the top surface of the flaps were observed to have a peak-tovalley height of less than 250 nm. The asperities on the sidewalls are of lower height than the asperities on the top surface (Supporting Information). Because of this nano scale roughness of the flaps, under small loads ( 3 mN, highlighted regime). The adhesion force first increased with increasing roughness, with stronger adhesion forces measured on the surfaces with 73 ± 5 and 133 ± 20 nm rms roughness under similar loads. Further increasing the rms roughness to 192 ± 9 and 308 ± 56 nm rapidly decreased the adhesion forces, with the lowest adhesion forces measured with the surface of 308 ± 56 nm rms roughness. Clearly, surface roughness had a pronounced effect on the adhesion force between the tilted PDMS flaps and the glass surface measured after sliding. Our results, however, show that higher roughness does not always lead to lower adhesion forces. Strong anisotropy was also noticed in the adhesion measurement after sliding in either +y or −y direction (Figure 4a,b). For each individual roughness, sliding in the +y direction gave rise to 50% to 100% increase of adhesion force in comparison to sliding in the −y direction under the same or similar loads. While the weakest adhesion forces were measured with the roughest surface (rms roughness = 308 ± 56 nm) we tested, it showed the strongest anisotropic adhesion after sliding. Almost no adhesion force (less than 0.1 mN) was detected after sliding in the −y direction under various loads up to 9.3 mN, whereas an adhesion force of 0.4 mN was detected after sliding the top surface in the +y direction under a preload of ∼9.8 mN. Friction Force Measurement. Anisotropic friction force was also measured when sliding the tilted flaps against surfaces with various roughness (Figure 5a,b). For each rough surface tested, sliding in the +y direction always gave higher friction forces than sliding in the −y direction. This asymmetry perfectly correlates with the adhesion measurement: the higher adhesion force while sliding in the +y direction results in higher adhesive friction force. Increasing the rms roughness led to first increasing and then decreasing of the friction force. When sliding the tilted flaps against the glass surface with a rms roughness of 73 ± 5 nm, in both +y and −y directions, the friction force, initially lower than that of the smooth surface (rms = 11 nm) at lower load (F⊥ < 2 mN), rapidly increased to similar magnitudes in the middle load regime (2 < F⊥ < 5 mN, highlighted). Further increasing the load, however, again led to lower friction force in both directions in comparison to the friction force measured when sliding the flap against the smooth glass surface. The friction force was more enhanced on the surface with a rms roughness

Figure 3. Static adhesion (without shearing) test of tilted PDMS flaps on the surfaces with different rms roughness. Strong adhesion was only measured on the surface with the smooth glass surface (rms roughness = 11 ± 8 nm). No adhesion force was detected on other surfaces with higher roughness. Higher adhesion forces were measured even at lower loads after sliding against the top glass surface. The colored regimes represent the trend of the adhesion forces and the experimental errors.

Shearing deformable surfaces, structures on surfaces, or trapped liquids have been shown to cause alignment between the surfaces, affecting both the adhesion and friction forces.13,46 For the smooth glass surface (rms roughness = 11 nm), a strong adhesion force of 0.95 mN was measured after sliding the top glass surface under only 1 mN load, whereas no adhesion force was detected under similar load without sliding. Although the final saturating values of the adhesion forces on the smooth glass surface for both static adhesion and the adhesion measured after sliding are almost the same (∼1.2 mN), sliding reduced the required load to reach the maximum adhesion force. Sliding also significantly enhanced the adhesion of the tilted flaps structure to the surfaces with various roughness: adhesion forces were measured for all the rough surfaces during the separation of two surfaces after first sliding the top glass surface in the direction along the tilt, the +y direction or in the direction against the tilt, the −y direction, and then stopping sliding but keeping the shear stress between two surfaces unreleased. After sliding in the +y direction, except for the roughest surface (rms roughness = 308 ± 56 nm), the adhesion forces between the PDMS flaps and the rough surface first rapidly increased with load at low loads (F⊥ < 3 mN) and then

Figure 4. Adhesion measured during the separation of silica surfaces, the top surface, with various roughness and the tilted PDMS flaps after sliding the silica surface in +y direction (a) and −y direction (b). The adhesion forces measured after sliding the top surface in the +y direction are significantly higher than the values measured after sliding in the −y direction. The solid black lines with arrows serve to illustrate the trends observed with surfaces of increasing roughness. 11530

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Figure 5. Anisotropic friction forces of the tilted PDMS flaps sliding against silica surfaces with various roughness in +y direction (a) and −y direction (b). The solid black lines with arrows serve to illustrate the trends observed with surfaces of increasing roughness.

of 133 ± 20 nm: highest friction force was measured in both +y and −y directions for all the loads tested. The effect of roughness is more pronounced at higher rms roughness. Not only smaller friction forces were measured while sliding the flaps against the surfaces with 192 ± 9 and 308 ± 56 nm rms roughness under similar applied loads, but also the friction forces showed fairly linear dependence on the applied load in both the low load regime and the high load regime with no friction force measured at zero load. These two surfaces also showed the lowest adhesion forces under different loads. The high friction forces measured with surfaces of relatively low rms roughness are probably due to the adhesion between two surfaces during sliding. This is clearly demonstrated by the strong adhesion force measured after sliding two surfaces. When the flaps were sliding against the surfaces with 11 ± 8, 73 ± 5, and 133 ± 20 nm rms roughness, finite friction forces were measured with zero even negative loads when sliding in both the +y and −y directions, another indication of strong “adhesive controlled” friction. Our results, at least at first glance, contradict the common knowledge in tribology. It is usually assumed that increasing surface roughness leads to smaller contact area between two surfaces during sliding and therefore smaller adhesion and friction forces. However, increasing the surface roughness also changes the surface topology, which could significantly change friction behavior of the flaps on different rough surfaces. With certain “matching” geometries, this interlocking of flaps can even lead to higher adhesion and friction forces, which will be further discussed in the Discussion section.

others on one surface could totally change the adhesion and friction between two surfaces without significantly changing the rms roughness. To accurately define the surface roughness, we need to characterize a series of parameters including different length scales of the surface topology and the distribution of the size and shape of asperities.15 We also encountered this difficulty in our study. Although we have also used the rms roughness to present the roughness of the surfaces studied, it indeed did not capture the nature of our rough surfaces. The rough surfaces used in this study have randomly distributed asperities, whereas the height and the size of the asperities, due to the limit of our etching technique, are quite uniform rather than randomly distributed. Increasing etching time just increases the height and the size of the pillar shape asperities and decreases the gap distance between two asperities. In general, the rms height roughness is not a sufficient characterization of structured surfaces, which must also include the width roughness between asperities. Table 1 shows both the height and width roughness values of the surfaces studied. As discussed further below, both roughness parameters exhibit similar commensurability trends, for both the adhesion and friction forces. For example, with the 73 ± 5 and 133 ± 20 nm rms (height roughness) surfaces, the average gap distances between neighboring asperities (width roughness) were 8.6 ± 4.5 and 8.1 ± 3.2 μm, respectively. Therefore, the gap distances between neighboring asperities were close to 10 μm, which is very close to the width of the tilted PDMS flaps. In the static adhesion test, because of the asperities on the top glass surface, the flaps mainly made point contacts with some pillar-like asperities, leading to a small real contact area and no adhesion was measured during the separation. Sliding the top rough surface along the +y or −y directions, however, gave the flaps the chance to slide past or over different asperities. The low modulus PDMS flaps could also deform during this process and therefore could penetrate into the gaps between randomly distributed asperities (Figure 6b). This penetration gave rise to interlocking like structures between the flaps and asperities, which led to much stronger adhesion and friction forces during and after sliding. Interestingly, this interlocking mechanism is widely used in many biological systems, such as insects, plant spreading, and bird feathers, and has also inspired the engineering design of structured adhesives.48−50 When the rms roughness of the surface was further increased to 192 ± 9 nm, the size of the asperities grew bigger, and most of the sizes of the gaps between different neighboring asperities were smaller than 10 μm (the average distances between neighboring asperities was 4.9 ± 3.3 μm).

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DISCUSSION Effect of Roughness. One particular difficulty in studying rough surface friction and adhesion is that quantification of roughness is not trivial. The root-mean-square roughness (rms roughness) is the most widely used parameter in describing the surface roughness and the only parameter used in many studies. However, rms roughness is just an averaged value of the whole surface topological profile, and it alone gives no information about the shape of the surface profile: different surface topological profiles can give the same rms roughness but totally different adhesive and tribological behaviors.47 Two surfaces with cone-shaped peaks and valleys could have exactly the same rms roughness, whereas the adhesive and tribological properties of these two surfaces against a smooth surface are totally different. When compressing two rough surfaces, they only make contacts at a few highest asperities at the beginning, and the number of contacts increases with increasing loads. Therefore, adding a few asperities significantly higher than 11531

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smooth surface. However, the elastic deformation of the flaps can only cover a small range of surface roughness. Beyond this range, increasing the roughness rapidly reduces the contact area of the surfaces and thereby the adhesion and friction forces. Effect of Anisotropic Structure. In the gecko adhesion system, the setae are naturally tilted, with a tilted angle of 45°. This anisotropic structure enhances the adhesive friction during the attachment of the foot (in the gripping motion) and reduces the adhesive friction during the detachment (in the peeling motion).18 Mimicking this anisotropy is crucial for developing gecko mimetic robots: the robot should be able to achieve strong adhesion and friction when it attaches to a surface, and should also be able to easily peel its feet off in the movement. By adding a tilting angle to our flap, we successfully mimic the anisotropy of the gecko setae. The flaps, tilted by about 20°, have much smaller stiffness in the +y direction compared to the −y direction, and bend over in the +y direction easily.4 Made of PDMS with an elastic modulus of 0.75 MPa, the flaps can also have large deformation during compressing and sliding, resulting in a larger contact area with the opposing surface. When sliding backward, in the −y direction, the geometry of the flaps does not allow them to bend over naturally, leading to a smaller total contact area. The adhesion force between two surfaces is proportional to the total contact area, and therefore the sliding in the +y direction gives much higher adhesion forces and adhesive friction forces under different loads. The strong anisotropy of the adhesion and friction forces measured on all the surfaces demonstrated that this anisotropy held fairly well on not only on smooth surface but also on rough surfaces with various roughness.

Figure 6. (a) A schematic shows the flaps on smooth surface (rms = 11 ± nm). (b) The geometric interlocking mechanism that gives high adhesion and friction forces when sliding the PDMS flaps on the etched rough surface with 73 ± 5 or 133 ± 20 nm rms roughness. (c) Increasing the roughness increases the size of the asperities while decreasing the gap distance between neighboring asperities, preventing the interlocking mechanism.

During sliding, the flaps, now not being able to penetrate into the gaps between asperities, mainly stayed on top of the asperities (Figure 6c). This effect was even more pronounced with the surface of 308 ± 56 nm rms roughness, leading to the smallest friction forces and almost no adhesion forces measured under different loads. Previous SFA experiments on random rough surfaces have shown that the adhesion of two surfaces decayed exponentially with increasing rms roughness. This conclusion, although also confirmed by computer simulation, cannot be applied to our current study on the tilted PDMS flaps. This again showed that fully characterizing the profiles of two surfaces is extremely important in order to study the adhesion and friction between rough surfaces. Unlike ideal surfaces, many real surfaces contain different length scales of roughness, which may have a correlative effect on the adhesion and friction between surfaces. Different geometries of the opposing surfaces can either enhance or diminish the adhesion and friction between two surfaces. Computer simulation and experimental studies have also shown that with an anisotropic surface the adhesion and friction forces could be significantly different while sliding along different axes of the surface.36 Another important parameter affecting the adhesion and friction forces between the flaps and the rough surface is the elastic moduli of the two materials that the surfaces are made of, especially at the contact area.47,51 In our test, the elastic modulus of SiO2 (∼70 GPa) is much higher than the elastic modulus of PDMS (∼0.75 MPa); therefore, the rough surface can be considered as not deformable, whereas the flaps are highly deformable. With low surface roughness, high elastic deformation may lead to higher contact area, similar to the function of setae on the gecko feet. This could also be one reason for the higher adhesion and friction forces we measured on the surfaces with lower rms roughness than on the control

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CONCLUSION Natural surfaces are usually rough, containing various scales of roughness. Surface roughness greatly affects the adhesion and friction properties of almost all tribological and adhesive systems, including gecko mimetic adhesives, yet not much attention has been paid thus far. Our work presents the first experimental effort to systematically study the effect of nano scale surface roughness on gecko mimetic structured adhesives. Our results demonstrate that suitable surface roughness could effectively enhance the adhesive friction between tilted PDMS flaps and a rough/patterned substrate in comparison to smooth surface, most likely due to a good geometric match between the flaps and the surface structure. Thus, above some threshold loads, both very low and very high, maximized forces could be achieved when the mean space between asperities equals the width of flaps. Although surface roughness strongly affected the magnitude of the adhesion and friction forces, anisotropic friction and adhesion properties of the tilted flaps were still preserved while sliding along different directions on surfaces with various roughness values. Although the results of our work cannot be directly applied to all gecko-inspired adhesive structures, they clearly illustrated the importance of load, surface roughness, structure, anisotropy, and direction of sliding. In order to develop better structured adhesives for gecko robots, additional systematic studies on the effect of surface structure are required.

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ASSOCIATED CONTENT

S Supporting Information *

Figures 1S and 2S. This material is available free of charge via the Internet at http://pubs.acs.org. 11532

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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 fabrication of micro flaps and rough surfaces was sponsored by the Institute for Collaborative Biotechnologies Grant W911NF-09-D-000. The content of the information does not necessarily reflect the position or the policy of the Government, and no official endorsement should be inferred from the U.S. Army Research Office. The development of the 3D SFA and measurements of rough surface adhesion and friction were supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under Award DE-FG02-87ER-45331. The fabrication of PDMS flaps was done in the UCSB nanofabrication facility, part of the NSF-funded National Nanotechnology Infrastructure Network (NNIN). The AFM measurements were done in the MRL Central Facilities. The MRL Central Facilities are supported by the MRSEC Program of the NSF under Award DMR 1121053; a member of the NSF-funded Materials Research Facilities Network. Jing Yu acknowledges the CPS Technology Fellowship of the Material Research Laboratory, University of California, Santa Barbara.

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