SEM images of a Tokay Gecko ( Gekko gecko ) (a and c) and fabricated hierarchical PS nanohairs with high AR (b and d). Inset in (b): water contact angle of the elongated hierarchical PS nanohairs. 

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... to be 135 and 22 MPa for an AR of 2 and 5 respectively. Elastic energy dissipation may also be responsible for increased adhesion, 6 e.g. , $ 2 and $ 4.2 times larger adhesion compared to AR 2 is expected for AR 5 and elongated double-level (DL) nanohairs (with AR base 12) respectively, which is qualitatively coincident with Fig. 4a (see ESI† for detailed discussion). Note that simply increasing AR to 12 (SL12) results in smaller adhesive/frictional forces than SL5, despite the lower effective modulus (3.8 MPa) and larger amount of the energy dissipation (see ESI†). This can be explained by the collapsed structure of SL12 (Fig. 3c), where the contact area is decreased by the engagement of several nanohairs into bundles that are not compliant as individual nanohairs. As shown in Fig. 4a and b, a substantial increase in adhesion/ friction forces is observed only for elongated hierarchical nanohairs with high AR. Furthermore, DL nanohairs exhibit much higher frictional forces than SL nanohairs and even flat films (under normal forces above 0.25 m N). The calculated effective moduli of these elongated base and terminal nanohairs are around 5.4 MPa and 2.6 MPa, respectively. 27 Thus, further increased compliance of hierarchical arrays of the high AR nanohairs may exhibit higher adhesive/frictional forces over SL nanohairs possibly due to the increased contact area. However, DL nanohairs produced by wet etching processes (DLW) (Fig. 1b), which have lower ARs than those of the elongated nanohairs, do not show any increase in adhesive/frictional forces, as expected. Thus, together with avoiding capillary action during the drying process in wet etching, our approach of mechanical peeling-off from the multi-branched template is highly efficient for fabricating hierarchical nanohairs with high AR induced by nanoyielding to achieve high performance in adhesion and friction. To elucidate the effects of contact area on the frictional force of the nanohairs, the coefficient of friction (CF) was calculated from the slope of a plot of the frictional force as a function of the applied normal load (Fig. 4c). The flat film shows the lowest CF, which indicates that the contact area of the tip on the flat film did not increase as much as it did for the nanohair samples by increasing the normal load. A large increase in CFs for the SL nanohairs indicates that the contact area between nanohairs and the spherical silica tip increases more readily by nanohairs bending at higher normal loads. A dramatic increase in the friction force and CF was observed for DL nanohairs (high AR). Furthermore, Fig. 4b and c notably show that the friction of DL nanohairs has two regimes (DL-I and DL-II) as a function of normal load: the frictional forces sharply increase at low normal loads (<0.25 m N, DL-I) and mildly increase at high normal loads (>0.25 m N, DL-II), which correspond to the two different CF values as shown in Fig. 4c. We speculate that this behavior is due to length distribution of the elongated terminal hairs and the hierarchical structure of the DL nanohair: only elongated terminal hairs will contact the silica ball under small normal load. In this case, the frictional force may increase rapidly with increasing normal loads due to bending of these elongated hairs. Above certain normal load ( $ 0.25 m N, in this study), the silica ball may reach the shorter terminal hairs and base hairs, which will lead to slower increase in the contact area, and consequently friction forces with the normal load. Fig. 4c shows that the CF at the regime II is much smaller than that at the regime I, and becomes similar to that of SL nanohairs. Consequently, a dramatic increase in both the frictional forces and CFs for hierarchical nanohairs with high AR, which is not the case for the flat surfaces having high frictional forces but low CF, is attributed to the maximized compliance of the elongated nanohairs without lateral collapse, which enables a ready increase in the contact area under the slight normal load. It is interesting to note that the friction force of the hierarchical structure is higher than that of the flat surface under high normal load (Fig. 4b). In addition to the increased contact area of individual fibers via lateral bending and side contact formation, 28,29 this fibrillar surface becomes much softer than the flat surface when normal load is applied, as reflected by E eff discussed previously. This implies that a large increase in the apparent contact area (area enclosed by contact periphery of silica sphere) is possible by the indentation of silica spheres on the fibrillar surface. A dramatic increase in friction, higher even than the flat surface, of the hierarchical structure may be attributed to these two combined effects, which is not applicable to adhesion in Fig. 4a where a less dramatic increase is observed. The hierarchical nanohairs fabricated here resembled gecko foot hairs, which feature multi-branched hairs (Fig. 5). Addi- tionally, the hierarchical structure of the PS nanohairs rendered the surface superhydrophobic (with a water contact angle of 170 , see the inset in Fig. 5b) with a low water contact angle hysteresis (<2 ) compared to that of other structures (SL2: $ 115 , SL5: $ 135 , and SL12: $ 137 ). Although the hierarchical structures showed higher adhesion and friction force than the SL as well as remarkable superhydrophobic characteristics, they did not satisfactorily mimic the gecko foot hairs. Previous experi- ments, supported by theoretical studies, have demonstrated that a very thin backing substrate can significantly enhance adhesion because it permits flexibility, enables equal load sharing, and prevents edge stress concentration. 30 The backing PS layer of the hierarchical nanohairs fabricated here was thick (1 mm), which decreased the macroscale adhesion and frictional properties. Further studies are necessary to develop mimics of gecko foot hairs using flexible ductile polymers via the nanoyielding tech- nique. Our results suggest that the hierarchy combined with high AR improved the adhesion and friction relative to the other structures as measured using nano/microscale adhesion/friction tests. Recently, we fabricated DL nanohairs with high AR and directional orientation (45 with respect to the normal) by changing the angle of release from the template (Fig. 6). These nanohairs are predicted to provide unidirectional frictional force and wetting behavior 12,31 and will be described in a forthcoming report. We have developed a facile approach for fabricating hierarchical nanohairs with high AR. The formation of these structures was controlled using an AAO template with multi-level porous structures and the nanoyielding processes. These hierarchical nanohairs exhibited higher adhesion and frictional forces relative to the SL nanohairs and hierarchical nanohairs with a low AR. In particular, hierarchical nanohairs with a high AR exhibited both higher frictional force and CF than flat surfaces, which are attributed to the maximized compliance of elongated hairs without lateral collapse. This approach provides a simple and versatile route to obtain artificial dry adhesives with remarkable adhesive and frictional properties as well as superhydrophobicity. AAO templates with multi-level pores were prepared by elec- tropolishing pure aluminium sheets (99.999%, Goodfellow), followed by two-step anodization and barrier layer thinning. The specimens were electropolished in a mixture of perchloric acid and ethanol (HClO 4 : C 2 H 5 OH 1⁄4 1 : 4 in volumetric ratio) at 20 V and 7 C for 5 min to remove surface irregularities. Anodization was then conducted using 0.1 M phosphoric acid (85%, Aldrich) at 195 V and 0 C for 16 h. After the first anodization, the porous alumina layer was etched away using an alumina etchant (1.8 wt% chromic acid and 6 wt% phosphoric acid at 65 C for 2 h). The second 5 min anodization was performed under identical conditions as those used for fabrication of the primary nanopores. After the second anodization, the barrier was modified by continuously reducing the voltage from 195 to 70 V in phosphoric acid at 0 C. The primary nanopores of the AAO templates were then widened by immersing in an etching solution (in 0.1 M phosphoric acid solution at 30 C for 3 h 30 min). The secondary nanopores were then generated by an additional anodization process (in 0.3 M oxalic acid solution at 70 V, 15 C and 150 s), followed by widening in a 0.1 M phosphoric acid solution at 30 C for 1 h. A thick polystyrene (PS, M w 1⁄4 1.9 Â 10 5 ) sheet on top of a silanized AAO template was heated (to about 185 C) above the glass transition temperature of PS ( T g 1⁄4 100 C) in a vacuum oven. After 3 h, the assembly was cooled to room temperature, and the template was dissolved away by wet etching solution (in a mixed solution of phosphoric acid, nitric acid, and acetic acid) at room temperature. The multi-level AAO porous template cleaned by UVO for 10 min was silanized with octadecyltrichlorosilane (ODTS) and anhydrous toluene in a flask for 3 h under argon. The nano- templates were removed from the solution, rinsed several times with toluene and ethanol, and then baked in an oven for 1 h at 120 C. Silanization with the ODTS increased the water contact angle of the AAO template from 5 to 142 . After filling the multi-level AAO template with PS by annealing, the template was removed at a constant speed of 5 mm s À 1 at room temperature. Adhesion and friction measurements were conducted using a commercial atomic force microscope (AFM, Digital Instru- ment Multimode). A silica ball of radius 24 m m was mounted on the cantilever (Contact Mode type, spring constant 14 N m À 1 ) and was used as a tip (ESI, Fig. S2†). Adhesion measurements were conducted in force–displacement mode, and the frictional force was measured in lateral force microscopy (LFM) mode. Each test was repeated 20 times and the ...