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Cephalopod skin is capable of fast color changing enabled by tunable skin transparency as well as structure color. Under this inspiration, herein, a flexible surface with unique hierarchical structure that integrates both transparency change in chemical color (optical scattering) and structure coloration (optical interfering) is developed by harnessing wrinkling instability, thanks to the interfacial Au catalysis in soft lithography. As a result, a hierarchical structure in terms of wrinkled film overlaid by nano‐dome array is obtained in the flexile surface. Experiments find that subject to biaxial strains from 0% to 60%, the hierarchical surface first experiences a transition from nontransparent to transparent owing to the flattening of the wrinkles and then exhibits iridescence structure color shifting from blue to red. The switchable and dynamical tunable mechanochromic characteristics are demonstrated in a smart window, offering potentials for developing flexible devices with optical multiple functionality. A flexible surface with hierarchical structure that integrates both chemical color (scattering in micro‐scale) and structure color change (interfering in nanoscale) is fabricated, by a one‐step soft lithography to harness wrinkling instability. Subject to biaxial strain, the hierarchical surface first experiences a transition from blurring to transparent and then exhibits iridescence in structure color redshift, offering a dynamic and switchable optical performance.

Wrinkling is a significant challenge associated with the forming of tubes via diameter reduction. The work reported herein employed elastoplastic principles to assess the external pressure diameter reduction forming process by generating a strain diagram showing the occurrence of critical instability. This diagram can be used to effectively predict the appearance of wrinkling defects during forming. The Donnell linear buckling theory together with a bilinear material model was used to derive an expression for the critical external pressure leading to wrinkling instability, employing constant tube end conditions and a uniform external pressure, and the effects of forming conditions and material parameters on wrinkling were explored. During experimental trials, AA6061 tubes were formed via diameter reduction in conjunction with varying heat treatment conditions using the solid granule medium forming process. A Vialux portable mesh strain tester was employed to collect relevant data to verify the critical instability points, and the effects of various factors on resistance to wrinkling were investigated. An analysis of the experimental results demonstrates that the conclusions of the theoretical analysis are correct.

Periodically patterned surfaces can cause special surface properties and are employed as functional building blocks in many devices, yet remaining challenges in fabrication. Advancements in fabricating structured polymer surfaces for obtaining periodic patterns are accomplished by adopting “top‐down” strategies based on self‐assembly or physico‐chemical growth of atoms, molecules, or particles or “bottom‐up” strategies ranging from traditional micromolding (embossing) or micro/nanoimprinting to novel laser‐induced periodic surface structure, soft lithography, or direct laser interference patterning among others. Thus, technological advances directly promote higher resolution capabilities. Contrasted with the above techniques requiring highly sophisticated tools, surface instabilities taking advantage of the intrinsic properties of polymers induce surface wrinkling in order to fabricate periodically oriented wrinkled patterns. Such abundant and elaborate patterns are obtained as a result of self‐organizing processes that are rather difficult if not impossible to fabricate through conventional patterning techniques. Focusing on oriented wrinkles, this review thoroughly describes the formation mechanisms and fabrication approaches for oriented wrinkles, as well as their fine‐tuning in the wavelength, amplitude, and orientation control. Finally, the major applications in which oriented wrinkled interfaces are already in use or may be prospective in the near future are overviewed. Advances in the fabrication of periodic patterns on polymer surfaces taking advantage of anisotropic wrinkling are summarized. This review thoroughly describes the formation mechanisms and fabrication approaches for oriented wrinkles, as well as their fine‐tuning in the wavelength, amplitude, and orientation control. Finally, the major applications in which oriented wrinkled interfaces are already employed or may be prospective are overviewed.

Fabrication of Hierarchically‐Patterned Surfaces In article number 2300039, So Nagashima, Ko Suzuki, and colleagues introduce a bottom‐up method for fabricating bio‐inspired hierarchical patterns with tunable anisotropic wetting properties. This method exploits the surface instability of bilayers comprising a gold nanofilm on a substantially pre‐stretched elastomer substrate. The wetting anisotropy can be finely tuned by simply adjusting the magnitude of substrate pre‐stretches and film thickness.

When a thin film bonded to a thick compliant substrate is subject to in-plane compression, wrinkles can develop if the critical state for instability is reached. In various applications the film/substrate system may also take the form of cylindrical fibers, where an axial compressive loading can trigger axisymmetric wrinkles. A straightforward computational approach to simulate such wrinkling behavior is needed for material design, to either prevent wrinkling or to exploit its benefits for flexible device applications. In this work a comprehensive numerical study is undertaken by employing the finite element method. The embedded imperfection approach used previously for planar structures is now applied to the cylindrical film/substrate system to trigger axisymmetric wrinkles. Uniform and reversible elastic wrinkles can be directly predicted, and the wrinkle geometries are verified by available elastic analytical solutions. The effect of initial imperfection location is also examined. We further study plastic yielding and viscoelasticity of the thin film, and examine how the wrinkle configuration may be influenced by inelastic deformation. Yielding transforms the uniform wrinkles into localized deep folds, and this new surface feature is irrecoverable upon unloading. A viscoelastic thin film results in rate-dependent instability behavior, with very slow strain rates favoring localized waveforms. This study has developed a robust computational approach for the prediction of wrinkle initiation and growth in cylindrical structures with a thin film bonded to a soft core. It is demonstrated that material nonlinearity and rate dependency can be incorporated into the model to greatly expand the simulation capabilities.

Surface wrinkling instability in thin films attached to a compliant substrate is a well-recognized form of deformation under mechanical loading. The influence of the loading history on the formation of instability patterns has not been studied. In this work, the effects of the deformation history involving different loading sequences were investigated via comprehensive large-scale finite element simulations. We employed a recently developed embedded imperfection technique which is capable of direct numerical predictions of the surface instability patterns and eliminates the need for re-defining the imperfection after each analysis step. Attention was devoted to both uniaxial compression and biaxial compression. We show that, after the formation of wrinkles, the surface patterns could still be eliminated upon complete unloading of the elastic film–substrate structure. The loading path, however, played an important role in the temporal development of wrinkle configurations. With the same final biaxial state, different deformation histories could lead to different surface patterns. The finding brings about possibilities for creating variants of wrinkle morphologies controlled by the actual deformation path. This study also offers a mechanistic rationale for prior experimental observations.

We report the formation of antisymmetric wrinkling patterns in films on ridged substrates induced by the buckling instability of the substrates via finite element simulations and experiments. Our simulated results reveal that the uniaxial compression along the ridge can trigger both the wrinkling instability of the film and the lateral buckling instability of the ridge. The latter could change the wrinkles from a symmetric pattern to an antisymmetric pattern in a range of film-substrate modulus ratio and aspect ratio of the ridge profile, as validated by the experimental observations. A three-dimensional phase diagram with four buckling patterns, i.e., sole ridge buckling pattern, antisymmetric wrinkling pattern with different wavelengths from ridge buckling, symmetric wrinkling pattern without ridge buckling, and antisymmetric wrinkling pattern with the same wavelength as ridge buckling, is built with respect to the uniaxial compression, modulus ratio, and aspect ratio. The results not only elucidate how and when the interplay between the wrinkling instability and the ridge instability results in the formation of the antisymmetric wrinkling pattern but also offer a way to generate controllable complex wrinkling patterns.

This paper introduces a method for the bottom‐up fabrication of bio‐inspired hierarchical patterns having tunable anisotropic wetting properties. The method exploits the surface instability of bilayers comprising a gold nanofilm attached to a substantially prestretched elastomer substrate. Upon film formation, highly aligned wrinkles spontaneously form on the surface owing to the surface instability driven by the compressive residual stress in the film. Thereafter, uniaxial compressive strain is applied to the film by prestretch relaxation of the substrate, which generates an array of high‐aspect‐ratio ridges on the surface. Consequently, hierarchical patterns comprising unidirectionally aligned ridges covered with wrinkles are obtained. Water droplets placed on surfaces having the aforementioned hierarchical patterns show direction‐dependent contact angles, resulting in elongated shapes, which indicates the presence of anisotropic wetting properties. The magnitude of wetting anisotropy can be tuned by simple control of the applied compressive strain and film thickness. Surface instability of bilayers comprising a gold nanofilm attached to a substantially prestretched elastomer substrate is exploited for the bottom‐up fabrication of bio‐inspired high‐aspect‐ratio hierarchical patterns having anisotropic wetting properties. The magnitude of wetting anisotropy can be finely tuned by simply adjusting the film thickness and magnitude of the substrate prestretch.

The bilayered structure of hard thin film on soft substrate can lose stability and form specific patterns, such as wrinkles or creases, on the surface, induced by external stimuli. For bilayer hydrogels, the surface morphology caused by the instability is usually controlled by the solvent-induced swelling/shrinking and mechanical force. Here, two important issues on the instability of bilayer hydrogels, which were not considered in the previous studies, are focused on in this study. First, the upper layer of a hydrogel is not necessarily too thin. Thus we investigated how the thickness of the upper layer can affect the surface morphology of bilayer hydrogels under compression through both finite element (FE) simulation and theoretical analysis. Second, a hydrogel can absorb water molecules before the mechanical compression. The effect of the pre-absorption of water before the mechanical compression was studied through FE simulations and theoretical analysis. Our results show that when the thickness of the upper layer is very large, surface wrinkles can exist without transforming into period doublings. The pre-absorption of the water can result in folds or unexpected hierarchical wrinkles, which can be realized in experiments through further efforts.

As a category of polymeric materials, soft dielectrics, such as most elastomers and rubber-like materials, have shown great potential for extensive applications in various fields. Owing to their intriguing electromechanical coupling behaviors, the morphological instabilities in soft dielectrics have been an active research field in recent years. In this work, the recent progress in experimental and theoretical research on their electromechanical morphological instabilities is reviewed, especially regarding the theoretical aspect. First, we revisit the theoretical framework for the electroelasticity of soft dielectrics. Then, the typical configurations of soft dielectric membranes used to generate two typical types of surface instabilities, namely wrinkles and creases, are introduced. Three commonly used modeling approaches (i.e., the stress balance method, the incremental method, and the energy method) for surface instabilities are reviewed with specific examples. Moreover, discussions on the difference between these methods and the corresponding critical loading conditions are presented. Furthermore, this review also covers the relation and transition between wrinkling and creasing phenomena.