1. Introduction

Owing to unique surface morphologies,natural biomaterials exhibit many excellent biofunctionalities,such as self-cleaning ability of lotus leaves ^{[1, 2]},anisotropic wettability of rice leaves ^{[1, 3]},antireflection property of cicada wings ^[4],etc. The intimate relation between them attracts considerable interest to fabricate hierarchical micro- and nanostructured materials. To date,the most commonly used techniques for micro/nanostructuring can be roughly classified into two categories: t_OP-down and bottom-up methods ^{[5, 6]}. t_OP-down methods are based on creating an organized structure either by etching or by manipulating components into specific locations,such as photolithography and replica molding ^{[7, 8]}. Bottom-up methods refer to organization of smaller components into lager or more complex assemblies, such as self-assembly ^{[9, 10]}. Considering internal advantages and limitations (e.g.,cost,output,operation conditions) for each method,smart combination of them is one desirable strategy to make best use of them for the controlled fabrication.

Hereinto,replica molding (i.e.,template replication) with high fidelity and high precision makes itself a facile technique to fabricate various micro/nano structures ^{[7, 8]}. Selection of suitable duplicated materials is of importance in the replica molding technique. Polydimethylsiloxane (PDMS) is the most widely used elastomer because of its outstanding properties: excellent elasticity,high thermo-stability,chemical-stability,and low surface energy ^{[11, 12]}.

Surface wrinkling,with typical examples of dried fruits and aging human skin,is a universal and ubiquitous phenomenon in nature ^{[13, 14]}. As one of the mechanical instabilities,surface wrinkling is derived from the strain mismatch between a stiff skin and the underlying compliant substrate. When the induced compressive stress exceeds the material-defined critical wrinkling stress,wrinkles with typically sinusoidal profiles occur for minimizing the system free energy ^{[15, 16]}. Evidently,surface wrinkling provides a simple yet versatile efficient bottom-up way to create micro/nanoscale textured surfaces over large areas ^{[17, 18, 19, 20, 21, 22]}.

In recent years,increasing attention has been paid to fabrication of hierarchical micro/nano surfaces based on surface wrinkling ^{[17, 23, 24, 25, 26, 27, 28]}. On the one hand,the resultant microstructures are fully based on the controlled surface wrinkling. For example, Efimenko et al. fabricated self-similar multi-generations of wrinkles by release of the pre-stretched UVO-exposed PDMS sheet with a large pre-strain ^[17]. The similar nested hierarchical wrinkles have also been generated through multi-step plasma treatment/prestrain release ^[23]. Additionally,Crosby et al. used a sequential strain wrinkling process to create a series of biaxial wrinkle morphologies with two distinct wavelengths and orientations ^[24]. In our group,a variety of well-organized wrinkle patterns have been available via the confinement-induced wrinkling from the wrinkled template twice ^[25]. On the other hand,hierarchically composite structures can be realized by means of other patterning techniques combined with surface wrinkling. Namely patterned substrates (or films),which could be prepared from the t_OP-down methods,were introduced into the film/ substrate bilayer system for surface wrinkling. The primary patterns were based on the t_OP-down methods,while the secondary ones were from the surface wrinkling. Thus the twolevel patterns can be independently manipulated. For instance, Chiche et al. utilized the surface wrinkling and template replication to create checkerboard-like patterns ^[27]. Here the used PDMS stamp was replicated from the wrinkled surface. Recently,Carter et al. have combined nanoimprint lithography with chemically induced wrinkling process to fabricate hierarchically wrinkled superhydrophobic surfaces ^[28]. Of course,in some cases,the primary patterns have an effect on the secondary wrinkling patterns. For instance,in the film/substrate system composed of a planar PDMS substrate and the patterned polystyrene (PS) film, Stafford et al. found that the induced wrinkle wavelength strongly depended on the geometry and the orientation of the patterned PS film ^[26]. Especially,the wrinkle wavelength increased obviously with the decrease of the angle between the film pattern direction and the compression direction,which was originated from the anisotropic property in the effective bending rigidity of the oriented wrinkled film. This intimate relation of wrinkling behaviors with the film pattern could be explained by their provided composite mechanics model. However,few reports have been concerned with the case of patterned compliant substrates. Indeed,the effect of the primary patterns from the patterned substrate on the induced stress field in the film/substrate system should be different from that of the patterned film applied.

Here,we report a simple low-cost method to create a series of hierarchical structures on polydimethylsiloxane (PDMS) elastomer, which is based on the combined bottom-up/t_OP-down strategy. Firstly,PDMS stamps with line gratings are formed by duplicating the commercialized compact disk (CD) and digital versatile disk (DVD). Secondly,the replicated PDMS stamps are pre-strained for oxygen plasma (OP) exposure,followed with the slow release of the pre-strain. Consequently,the secondary microscale patterns are produced via surface wrinkling. We systematically investigate the effect of experimental factors (e.g., plasma exposure time,the prestrain,and the angle between the two-level pattern directions) on the well-organized composite patterns. Since the thin oxidized layer and the bulk PDMS substrate are essentially from the same material,we can avoid potential delamination at a large strain,which is one serious problem in the previous reports ^{[26, 28]}. More importantly,the replicated PDMS stamp is utilized to provide the first-level patterns,which is different from the previously reported patterned film ^{[26, 28]}. Thus the influence of the substrate geometry on the induced wrinkling wavelength has been considered. Furthermore,the applied structured templates from the well commercialized and cheap CD and DVD endow our combined strategy with low cost and high output in the controlled surface patterning. Evidently,our work presents a new perspective to fabricate hierarchically functional structured surfaces for advanced and extended applications. 2. Experimental

Materials: Polydimethylsiloxane (PDMS) elastomer kits with the base/curing agent (Sylgard 184) were purchased from Dow Corning. Commercialized compact disk-recordable (CD-R) and digital versatile disk-recordable (DVD-R) were purchased from the market.

Fabrication of PDMS stamps from CD/DVD: PDMS was used to duplicate the surface structure of CD/DVD by replica molding. CD/DVD with recording layer was used as the original template in the replication step. The mixture of PDMS base and its curing agent (10:1,mass ratio) was cast onto the recording layer of the CD/DVD templates,followed with thorough degassing. After heated at 70 ºC for 4 h,the flexible negative stamps were obtained by peeling of the cured PDMS sheet from the CD/DVD template.

Fabrication of the secondary wrinkle patterns: Rectangular specimens (flat PDMS and PDMS stamps) were cut into 4 cm× 1 cm with a razor blade and mounted onto a home-made manual screw-driven stage. Uniaxial strain ranging from 5% to 30% was exerted by mechanical uniaxial stretching. Then the prestrained specimens undergo oxygen plasma (OP) treatment (Harrick PDC 32G) at a pressure of 0.02 mbar for 10-30 min. Finally,the pre-strain in the OP-exposed PDMS substrate was slowly released to induce the surface wrinkling.

Characterization: Atomic force microscope (AFM) images were obtained in tapping mode on an Agilent 5500 AFM/SPM microscope with silicon cantilevers (NC-W,the typical frequency of 285 kHz). Optical microscope images were recorded using an inverted Observer A1 microscope (Zeiss,Germany) equipped with a charge-coupled device camera. 3. Results and discussion

Scheme 1 shows our combined strategy to fabricate hierarchically composite patterns. It is composed of three steps: (I) fabrication of PDMS stamps (②) by replication of the structured CD/DVD templates with PDMS (①) to obtain the primary patterns; (II) OP exposure of the pre-strained PDMS stamps to generate the SiO_x/PDMS bilayer; (III) release of the pre-strain to induce surface wrinkling for formation of the secondary wrinkle patterns, resulting in the hierarchical structures with different relative orientations (③ and ④). In order to successfully realize the replication,thorough degassing needs to remove trapped air and promote PDMS pre-copolymer to fill the microstructures of the CD/ DVD templates. Fig. 1a shows the atomic force microscope (AFM) image of the as-replicated PDMS stamp. It is seen that the PDMS stamp has highly ordered line-to-space microstructures. The corresponding cross-sectional profile indicates that the stamp replicated from CD has a pitch of~1.2 μm and a height of ~200 nm (Fig. 1b),while the stamp from DVD has a pitch of ~750 nm and a height of ~100 nm (Fig. S1 in Supporting information).

In the following steps,the replicated PDMS stamp that is uniaxially stretched for a given prestrain (ε_pre) is exposed to oxygen plasma (OP),followed with slow release of the prestrain. During the OP treatment,the PDMS surface is oxidized and converted into a rigid thin silicate-like (SiO_x) layer,which has been well characterized previously ^{[29, 30, 31]}. Consequently,the film/ substrate system composed of the SiO_x/PDMS bilayer is formed with the thickness and modulus of the oxidized layer being dependent on the OP exposure time (t_OP) ^[32]. During the prestrain relaxation,the compressive stress (σ) is increasingly induced due to the different stiffness between the SiO_x layer and the underlying PDMS substrate. Once σ is higher than the bilayer-defined critical stress (σ_c),the surface wrinkling takes place with sinusoidal waves perpendicular to the prestrain direction. It is noted that the strain release rate has a major impact on the overall quality of wrinkle patterns ^[17]. For example,releasing the strain at a high rate always results in structural defects coupled with weak regularity. In order to avoid these undesired disadvantages,the pre-strain is released at quite a slow rate in our experiment.

Just as expected,two-level well-organized composite patterns are obtained (Fig. 2). It is easy to differentiate them based on the periodicity and orientation. For the primary ones,they correspond to the lined gratings of the replicated PDMS stamp with the constant periodicity of ~1.2 μm (marked with yellow line in Fig. 2a₁-e₁). As for the highly oriented secondary ones (marked with green line in Fig. 2a₁-e₁),the orientation is perpendicular to the pre-strain direction. Furthermore,the wavelength is sensitive to t_OP. These results indicate that the secondary patterns should come from the wrinkle morphologies. Therefore,the relative orientation between the two-level microstructures is related to the angle between the lined gratings and the pre-strain direction, which provides us a simple route to finely tune the hierarchical microstructures.

The corresponding AFM information is shown in Fig. 2a₂-e₂ and a₃-e₃. Unexpectedly,a clear decrease in the height for the primary stamp patterns is observed with the increase of the OP exposure duration. For example,the height decreases gradually from the initial 200 nm to the final 100 nm,when t_OP = 30min. It is known that PDMS is a siloxane polymer comprising Si-O-Si backbones. During OP exposure,carbon and hydrogen are etched and oxidized SiO_x is formed at the same time ^{[29, 30, 31]}. With respect to the grooves and ridges of the stamp with the line-tospace geometry,they should be etched simultaneously with the same velocity. In other words,the height between grooves and ridges should be kept constant after the OP exposure.We assume that the exposure-induced unexpected decrease maybe results from the differences in the distance and exposed area for the ridges and grooves of the primary patterns related to the OP resource. Certainly,more experiments need to be carried out to reveal the detailed physics.

Fig. 3 shows the plot of the wrinkle wavelength and amplitude as a function of t_OP. We see the wrinkle wavelength in the hierarchical composite structures is almost linear to t_OP. For the amplitude,it is also close to the linear dependence. Indeed,at a small prestrain applied,the wrinkling wavelength (l0) and amplitude (A₀) are determined by ^{[14, 15]}:

In which,the critical wrinkle strain (ε_c) and stress (σ_c) are defined as:

where h is the rigid film thickness,E is Young’s modulus,v is Poisson’s ratio,ε_pre is the applied prestrain. Subscripts f and s refer to the rigid film and soft substrate,respectively. Eqs. (1) and (2) show that the wrinkle wavelength and amplitude can be finely tailored by simple control over the film thickness because of their linear dependence. This theoretical relation is also supported by our experimental results shown in Fig. 3. It is noted that that the thickness of the as-formed oxidized SiO_x layer is roughly linear to t_OP before the saturated thickness arrives at ^{[27, 32, 33]}. Although the accurate thickness and mechanical properties of the oxidized layer are unknown to some extent,the above result suggests that we can precisely control the secondary wrinkle wavelength with t_OP. For comparison,the flat PDMS substrate as well as the DVDreplicated stamp (Figs. S2 and S3 in Supporting information) is also employed for the controlled surface wrinkling. Here we see that under the same conditions,the wrinkle wavelength from the flat PDMS substrate is bigger than that from the CD/DVD replicated PDMS stamps (Fig. 3 and Fig. S2),although in the three cases,the wrinkle wavelength increases approximately linearly with t_OP. This result is strongly indicative of the influence of the first-level surface patterns on the secondary wrinkle morphologies,which will be discussed in detail subsequently.

It is accepted that in the case of a large strain employed,the wavelength and amplitude should be written as ^[34]:

where λ₀ is the wavelength in Eq. (1) and ξ = 5ε_pre(1 + ε_pre)/32. This implies that,besides the plasma exposure time,the wrinkle wavelength can be adjusted by the prestrain. Just as expected from Eqs. (4) and (5),the wrinkle wavelength decreases while amplitude increases non-linearly with the increase of the prestrain (Fig. 4). In the current case,the wrinkle wavelength decreases from ~4.2 μm to ~3.2 μm,while the amplitude increases from ~500 nm to ~780 nm,when the prestrain changes from 5% to 30% (Fig. 4).

Apart from the above wrinkle wavelength and amplitude,we can manipulate the spatial orientation between the replicated lined patterns and the induced wrinkles to gain highly organized composite patterns. Since the secondary wrinkle orientation is vertical to the compressive stress and the prestrain direction. Thus we can conveniently control the initial angle (θ) between the lined patterns of the PDMS stamp and the compression direction. Fig. 5 shows AFM height images and the corresponding cross-sectional profiles of the resultant dual length-scale composite patterns from different θ. When θ is ~0º (Fig. 5a),~45º (Fig. 5b),and ~75º (Fig. 5c),the wavelength of the induced wrinkle patterns (λ) is roughly 5 μm,4.7 μm and 4.55 μm,respectively. In principle, arbitrary θ can be conveniently realized. We can see that λ decreases as θ increases. This dependence of the wrinkle wavelength on θ is in accord with the reported result of Stafford et al. ^[26],although the induced stress field in the film/substrate system might be different when the primary physical patterns are respectively from the patterned film and the patterned substrate.

4. Conclusion

We fabricate hierarchical micro/nano composite structures by combining surface wrinkling and template replication. Since the soft/rigid bilayer is essentially from the same PDMS,there is no clear boundary in the bilayer,which results in the well-defined hierarchical structures without any delamination. The primary patterns are from the PDMS stamps,which are replicated from the commercialized CD/DVD. The secondary oriented morphologies are from surface wrinkling of the PDMS stamps,which can be precisely controlled by changing plasma exposure time and the prestrain. Besides,we realize the tunable spatial orientation of the well-organized composite structures by varying the angle between the lined stamp patterns and the prestrain direction. As proved,the CD/DVD-based template replication coupled with surface wrinkling provides a simple,low-cost,and high-throughput method to pattern surfaces with well-organized hierarchical composite structures,which have a great potential for applications in the related fields (e.g.,wetting and friction ^{[28, 35, 36]},structured templates for replication and direction of material deposition/ growth ^{[17, 37, 38, 39]}).

Acknowledgments

Financial supports from the Natural Science Foundation of China (Nos. 21074090 and 21374076), and Tianjin Research Program of Application Foundation and Advanced Technology (No. 13JCYBJC17100) are gratefully acknowledged.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cclet.2014.10.003.