Wrinkle Patterns Research Articles

Facile and robust strategy to antireflective photo-curing coating through self-wrinkling

We reported a facile and bio-inspired strategy for obtaining antireflective (AR) coating through polymerization-induced self-wrinkling. Upon irradiation of light, the complex wrinkle micro-patterns with different morphologies were generated spontaneously on the surface of coating during photo-cross-linking, which enables the photo-curing coating can decrease reflection. The resulting photo-curing coating exhibits a high transmittance over 90% and low reflection below 5%∼8%, with an efficiency anti-reflection of 4% ∼ 7% compared to the flat blank coating. The successful application of these AR coatings with wrinkles pattern to encapsulate the thin film solar cells results in appreciable photovoltaic performance improvement of more than 4%∼8%, which benefits from the decrease of the light reflection and increase of optical paths in the photoactive layer by the introduction of wrinkling pattern. Furthermore, the efficiency improvements of the solar cells are more obvious, with a remarkable increase of 8.5%, at oblique light incident angle than that with vertical light incident angle

Multidirectional Wrinkle Patterns Programmed by Sequential Uniaxial Strain with Conformal yet Nontraceable Masks.

Surface wrinkling is a promising route to control the mechanical, electrical, and optical properties of materials in a wide range of applications. However, previous artificial wrinkles are restricted to single or random orientation and lacks selectivity. To address this challenge, this study presents multidirectional wrinkle patterns with high selectivity and orientation through sequential uniaxial strain with conformal polymeric shadow masks. The conformal but nontraceable polymeric stencil with microapertures are adhered to a flat substrate prior to oxidation, which forms discrete and parallel wrinkles in confined domains without any contamination. By fully investigating the process, this study displays compound topography of wrinkles consisting of wrinkle islands and surrounding secondary wrinkles on the same surface. With this topography, various diffusion properties are presented: from semi-transparent yet diffusive films to multidirectional diffusive films, which will be available for new types of optical diffuser applications.

Multi-Responsive Wrinkling Patterns by the Photoswitchable Supramolecular Network

Multiresponsive reversible wrinkling patterns provide an effective approach to dynamically tuning the properties of surface on-demand to realize a smart surface; however, their fabrication remains challenging. In this study, we report a simple yet robust method to fabricate multiresponsive wrinkles based on a supramolecular polymer network composed of copolymer (P4VP-PS-PnBA) and carboxyl containing anthracene (AN-COOH), which can be cross-linked dynamically through reversible photodimerization of anthracene (AN) and the hydrogen bond between carboxyl and pyridine groups. The wrinkle pattern can be generated and erased selectively by UV radiation of different wavelengths due to reversible dimerization of AN. The resulting wrinkles have an extremely sensitive response to hydrogen chloride (HCl) gas and can be erased by HCl with a concentration of 5 ppm in the atmosphere. The generation/elimination process responsive to light and HCl could be cycled many times without damaging characteristic wrinkles, which...

Wrinkling patterns in metal films sputter deposited on viscoelastic substrates

Metal molybdenum (Mo) films have been deposited on viscoelastic gel (for comparison also on elastic PDMS) substrates by magnetron sputtering technique and the wrinkling patterns are investigated by using optical microscopy, atomic force microscopy and scanning electron microscopy. For elastic PDMS substrates, the Mo films can spontaneously form labyrinth wrinkles, whose sizes increase linearly with the film thickness. For viscoelastic gel substrates, two wrinkle patterns with different wavelengths, namely 1st (with small wavelength) and 2nd (with large wavelength) wrinkling, can be observed in the films. They are originated from the surface instability of gel during sputtering and the thermal compression after deposition, respectively. The 2nd wrinkle wavelength increases with the film thickness whereas the 1st wrinkle wavelength is thickness independent. The structural characteristics, morphological evolutions and physical mechanisms of these wrinkling patterns have been discussed and analyzed based on the stress theory.

One‐Step Spontaneous Formation of Dual Wrinkling on Uniform‐Sized Microparticles Induced by Surface

AbstractThis study demonstrates the wrinkle formation on biodegradable polymer‐blend microparticles prepared from an emulsion‐solvent evaporation method and investigates the formed patterns. A labyrinthine pattern is obtained for uniform‐sized microparticles, owing to the considerable size reduction during solvent evaporation. Changing the radius of the organic droplets dispersed in the aqueous solution switches the wrinkle pattern from labyrinth to bi‐phase. For the first time, the dual wrinkling structure is prepared; both labyrinthine texture and hexagonal dimple structures are spontaneously formed on the same microparticle surface. The former pattern is due to the surface instability from blends of hydrophobic polymer and amphiphilic block copolymer, while the latter is due to a mechanism similar that of the breath figure formed with organic phase change materials during solidification of microparticles. The general applicability of this approach is demonstrated on other pairs of polymer blends.

Crack‐Free, Soft Wrinkles Enable Switchable Anisotropic Wetting

AbstractSoft skin layers on elastomeric substrates are demonstrated to support mechano‐responsive wrinkle patterns that do not exhibit cracking under applied strain. Soft fluoropolymer skin layers on pre‐strained poly(dimethylsiloxane) slabs achieved crack‐free surface wrinkling at high strain regimes not possible by using conventional stiff skin layers. A side‐by‐side comparison between the soft and hard skin layers after multiple cycles of stretching and releasing revealed that the soft skin layer enabled dynamic control over wrinkle topography without cracks or delamination. We systematically characterized the evolution of wrinkle wavelength, amplitude, and orientation as a function of tensile strain to resolve the crack‐free structural transformation. We demonstrated that wrinkled surfaces can guide water spreading along wrinkle orientation, and hence switchable, anisotropic wetting was realized.

Crack-Free, Soft Wrinkles Enable Switchable Anisotropic Wetting.

Soft skin layers on elastomeric substrates are demonstrated to support mechano-responsive wrinkle patterns that do not exhibit cracking under applied strain. Soft fluoropolymer skin layers on pre-strained poly(dimethylsiloxane) slabs achieved crack-free surface wrinkling at high strain regimes not possible by using conventional stiff skin layers. A side-by-side comparison between the soft and hard skin layers after multiple cycles of stretching and releasing revealed that the soft skin layer enabled dynamic control over wrinkle topography without cracks or delamination. We systematically characterized the evolution of wrinkle wavelength, amplitude, and orientation as a function of tensile strain to resolve the crack-free structural transformation. We demonstrated that wrinkled surfaces can guide water spreading along wrinkle orientation, and hence switchable, anisotropic wetting was realized.

Spontaneous biaxial pattern generation and autonomous wetting switching on the surface of gold/shape memory polystyrene bilayer

Abstract In this paper, shape memory effect induced initiation and evolution of surface patterns (wrinkles and cracks) were studied on the surface of gold/shape memory polystyrene (PS) bilayer, alongside with their impacts on autonomous surface wetting effects. Surface wrinkling was generated as a result of in-plane compression in the gold film where the thermal-induced shape memory effect occurred on the foundation layer. Cracks were generated on gold surface when the wrinkle pattern was further developed at a higher strain. The in-plane surface morphological bifurcation was observed when the crack patterns were developed perpendicular to the wrinkles direction, which is induced by biaxial stress transformation within the gold thin film because of the lateral Poisson's effect. The experimental mechanics investigations describe the relationships of the initiation/evolvement of surface morphology upon gold/shape memory PS bilayer with respects to various settings, such as the thicknesses of gold films, the applied strain on polymer layer, etc. The associated impact on surface wetting condition brought by the generated biaxial pattern on gold surface was studied. The water contact angle fluctuates within a narrow range according to the pre-strain for the samples after heating under the same plasma treatment times, which indicates that the biaxial pattern (cracks and wrinkles) in this paper have a little effect on the hydrophobicity of the gold film surface when the heated samples were treated by plasma for same times. After the surface plasma treatment, the surface hydrophilicity of the samples after post-annealing is significantly higher than that of the sample after deposition. And the contact angle decreases steadily as the air plasma treatment time is increased, the controllable surface hydrophobicity of gold coated PS bilayer can be achieved by tuning the plasma treatment time.

Controlled wrinkling analysis of thin films on gradient substrates

The paper investigates continuously changing wrinkle patterns of thin films bonded to a gradient substrate. Three types of gradient substrates including exponential, power-law, and symmetry models are considered. The Galerkin method is used to discretize the governing equation of film bonded to gradient substrates. The wavelength and the normalized amplitude of the wrinkles for substrates of various material gradients are obtained. The numerical simulation based on the finite element method (FEM) is used to evolve the wrinkle patterns. The result agrees well with that of the analytical model. It is concluded that localization of wrinkle patterns strongly depends on the material gradient. The critical membrane force depends on both the minimum value of wrinkle stiffness and the gradient of wrinkle stiffness when the wrinkle stiffness is at its minimum. This work provides a better understanding for local wrinkle formation caused by gradient substrates.

Step-and-repeat stamping method for the generation of large-area microscale wrinkle patterns

We proposed a step-and-repeat stamping method for fabricating large-area microscale wrinkles with a Polydimethylsiloxane (PDMS) mold. First, an original wrinkle pattern was created by controlling the thickness of a photocurable resin (NOA-68T), which was exposed to ultraviolet light within a short time and then fully cured at a temperature of 30 °C for approximately 10 h. Microscale wrinkles could be obtained after two successive steps. PDMS was cast in a container with the fabricated wrinkle pattern and then cured at 75 °C for 2 h. A PDMS mold with a wrinkle pattern was then obtained after separating the master wrinkle pattern from the cured PDMS. A Selfassembled monolayer (SAM) was coated onto the surface of the PDMS mold to realize effective wrinkle replication on PDMS. Finally, the wrinkle pattern was stamped directly onto the large-area substrate within a significantly reduced processing time. The contact angles on the wrinkled surfaces of the resin layer and the PDMS mold were 47.8 % and 36.3 % larger than those on the corresponding flat surfaces, respectively. The proposed simple and cost-effective method of generating a large wrinkled area can be applied to surface modification in various engineering applications, including cell culture, microfluidics, biomedical devices, and flexible electronics.

Metallization of microscale wrinkles on a curved surface by contact and electro-replication method

We suggest a novel experimental technique to fabricate microscale metallic wrinkles on a curved surface to clear duplication of form using a contact and electro-replication process. As the first step, in order to fabricate polymeric master wrinkle patterns, a UV-curable adhesive was deposited on a substrate with 180 μm in thickness, and then it was weakly polymerized by short UV exposure of 5 s for generating the gradient of material properties along the direction of layer thickness. Then, wrinkles were generated on a skin of the weakly polymerized layer by compressive forces that occurred during a thermal post-processing. After that, the master was replicated using a PDMS mold. The metallic wrinkles were reproduced on a curved metallic surface using the replicated mold in a nickel-based electroforming system. For understanding the major process parameters of electro-replication, a Taguchi method was utilized. Through this work, the metallic wrinkles were well reproduced and we believe that the proposed method is a promising technology to fabricate metallic wrinkles that have rich potential applications such as heat transfer enhancement and surface modification of metallic structures.

Wrinkles in Polytetrafluoroethylene on Polystyrene: Persistence Lengths and the Effect of Nanoinclusions.

We characterize wrinkling on the surfaces of prestrained polystyrene sheets coated with thin polytetrafluoroethylene skins using a combination of mechanical strain measurements and 3D finite element simulations. The simulations show that wrinkle wavelength increases with skin thickness, in agreement with a well-known continuum model and recent experiments. The wrinkle amplitudes also increase with strain. Nanoinclusions, such as holes and patterned lines, influence wrinkle patterns over limited distances, and these distances are shown to scale with the wrinkle wavelengths. Good agreement between experimental and simulated influence distances is observed. The inclusions provide strain relief, and they behave as if they are attracting adjacent material when the sheets are under strain. The wrinkles have stiffnesses in much the same way as do polymers (but at different length scales), a property that is quantified for polymers using persistence lengths. We show that the concept of persistence length can be useful in characterizing the wrinkle properties that we have observed. However, the calculated persistence lengths do not vary systematically with thickness and strain, as interactions between neighboring wrinkles produce confinement that is analogous to the kinetic confinement of polymers.

Application of nanomechanical response of wrinkled thin films in surface feature generation

Wrinkle patterns on the elastomeric substrate are gaining significant attention these days because of its wide variety of applications in the area of stretchable electronics, bacterial adhesion, surface modifications, etc. Development of controlled patterns on these wrinkled thin film surfaces can lead to different possible outcomes such as surface area enhancement, generation of rough surface topology, formation of template for nano-pillars, etc. which can further diversify and strengthen the application of wrinkled thin films. On a wrinkled surface, due to varying stress distribution and material plasticity along the slope, the indentation responses are expected to be a function of the position of indentation on the slope. In this work, this property is applied to generate controlled patterns on wrinkled polymeric thin film using nanoindentation (NI) technique. Synthesis of polymeric nano-pillars of uniform and varying heights and generation of enhanced surface area on flat and wrinkled surfaces are discussed here. Besides the inherent mechanical properties of a film material, the mechanical response of a thin film to indentation also depends on its thickness and the interfacial strength of adhesion with the substrate. These responses are studied for different thicknesses of wrinkled film with an objective to determine the necessary indentation characteristics for achieving target surface area enhancement and required nano-pillar heights. The height and the height distribution of these pillars can be varied by varying the film thickness, the applied indentation load, and the location of indentation.

A simple fabrication process for stepwise gradient wrinkle pattern with spatially-controlled wavelength based on sequential oxygen plasma treatment

Though a gradient surface wrinkle pattern has drawn much interest recently, the previous efforts have been limited to the generation of continuous gradient patterns of surface wrinkle with gradually varying wavelengths. Here, we report a simple fabrication process for a stepwise gradient wrinkle pattern which has spatially-controlled discrete wavelengths in distinct regions with clear boundary on a polydimethylsiloxane (PDMS) substrate. The stepwise gradient wrinkle pattern is achieved based on the sequential treatment of oxygen plasma for the PDMS surface using a glass plate as an effective blocking mask for the plasma, resulting in the modulation of plasma dose on the specific regions of the PDMS surface. We fabricated a stepwise gradient wrinkle pattern on three regions of uniform feature size with 540, 840, and 1240nm in wavelength and 60, 110, and 220nm in depth. The utility of the stepwise gradient wrinkle pattern as a platform for screening global surface properties induced by a number of wrinkle patterns was evaluated by assessing the difference of anisotropic wetting behavior and transmittance depending on wrinkle feature sizes. This fabrication method can be easily implemented in microfluidic and optical applications investigating effects of various wrinkle groove patterns.

Analytical study of wrinkling in thin-film-on-elastomer system with finite substrate thickness

Surface wrinkling, a film bonded on a pre-strained elastomeric substrate can form periodic wrinkling patterns, is a common phenomenon in daily life. In existing theoretical models, the film is much thinner than the substrate so that the substrate can be considered to be elastomeric with infinite thickness. In this paper, the effect of finite substrate thickness is analyzed theoretically for free boundary condition cases. Based on the minimum potential energy principle, a theoretical model is established, and the wave length and amplitude of the wrinkling pattern are obtained. When the thickness of the substrate is more than 200 times larger than the thickness of the film, the results of this study agree well with the results obtained from the previous models for infinite substrate thickness. However, for thin substrates, the effect of finite substrate thickness becomes significant. The model given in this paper accurately describes the effect of finite substrate thickness, providing important design guidelines for future thin-film-on-substrate systems such as stretchable electronic devices.

Controlled fabrication of nanoscale wrinkle structure by fluorocarbon plasma for highly transparent triboelectric nanogenerator

In this paper, we report a novel nanoscale wrinkle-structure fabrication process using fluorocarbon plasma on poly(dimethylsiloxane) (PDMS) and Solaris membranes. Wrinkles with wavelengths of hundreds of nanometers were obtained on these two materials, showing that the fabrication process was universally applicable. By varying the plasma-treating time, the wavelength of the wrinkle structure could be controlled. Highly transparent membranes with wrinkle patterns were obtained when the plasma-treating time was <125 s. The transmittances of these membranes were >90% in the visible region, making it difficult to distinguish them from a flat membrane. The deposited fluorocarbon polymer also dramatically reduced the surface energy, which allowed us to replicate the wrinkle pattern with high precision onto other membranes without any surfactant coating. The combined advantages of high electron affinity and high transparency enabled the fabricated membrane to improve the performance of a triboelectric nanogenerator. This nanoscale, single-step, and universal wrinkle-pattern fabrication process, with the functionality of high transparency and ultra-low surface energy, shows an attractive potential for future applications in micro- and nanodevices, especially in transparent energy harvesters.

Wrinkle pattern on PLLA induced by stress of polymer-metal bilayer

This paper is focused on study the effect of the PLLA modification with Ar plasma followed by deposition of thin layers of metal (Au, Ag) and studying of thermal stress influence on the properties of the PLLA substrate. Particular attention was devoted to changes of surface morphology and roughness which was studied with atomic force microscopy and laser confocal microscopy. The wettability of the surface was characterized by measurement of contact angle and the subsequent calculation of the surface energy. The chemical composition was determined by XPS spectroscopy. The main aim was devoted to construction of wrinkle-like surface pattern on the basis of polymer-metal bilayer stress. After heat treatment of PLLA (non-treated or treated with plasma) deposited with thin metal layers the wrinkle-like structure was formed. Depending on the thickness of deposited layers, the structure exhibited partial orderliness and preferred direction of the structure across the whole surface was observed for some combination of input parameters. Surface energy of PLLA-metal system and in particular its polar and nonpolar component, as well as the concentration of the metal on the surface is strongly influenced by the pretreatment of substrate with plasma. By plasma treatment of PLLA with subsequent deposition of metal nanolayer and consequent thermal treatment materials useful in tissue engineering can be constructed, for example cell carriers or manufacture of integrated circuits based on biopolymers can be enhanced.

Dynamic pattern of wrinkles in a dielectric elastomer.

A membrane of a dielectric elastomer may undergo electromechanical phase transition from the flat to wrinkled state, when the applied voltage reaches a critical value. The wrinkled region is observed to expand at the expense of the flat region during the phase transition. In this paper, we report on a dynamic pattern of wrinkles in a circular membrane of a dielectric elastomer. During phase transition, both the flat and wrinkled regions move interchangeably in the membrane. The radial prestretch is found to significantly affect electromechanical phase transition. For example, a membrane with a small prestretch can exhibit a dynamic pattern of wrinkles, which is essentially related to snap-through instability. However, a membrane with a large prestretch undergoes continuous phase transition, without exhibiting a dynamic pattern. An analytical model is developed to interpret these experimental phenomena. Finite element simulations are performed to predict the wrinkle morphology, especially the coexistence of flat and wrinkled regions. Both the theoretical calculations and finite element simulations are qualitatively consistent with the experiments. Additionally, we observe another type of electromechanical behavior involving a dynamic pattern of wrinkles with different wavelengths. The membrane first undergoes continuous transition from the flat to wrinkled state, followed by discontinuous transition from one wrinkled state to another. These results may inspire new applications for dielectric elastomers such as on-demand patterning of wrinkles for microfluidics and stretchable electronics.

Controlling wrinkles and assembly patterns in dried graphene oxide films using lyotropic graphene oxide liquid crystals

ABSTRACTWe fabricated graphene oxide (GO) films on glass substrates by blade coating a lyotropic GO liquid crystal dispersion. Substrate temperature and blading speed were precisely controlled to manipulate the surface morphologies of GO films. The temperature and blade speed influenced the drying rate of film and the amount GO dispersion supplied. By controlling these parameters, film-thickness modulation and three types of surface wrinkle patterns were selectively achieved. We also plotted the wrinkle patterns diagram as functions of the film fabrication conditions. The films exhibited different optical anisotropies depending on wrinkle patterns. GO films with controlled wrinkles can be used as electrodes for supercapacitor applications owing to the large surface areas.

Self-Supported Crack-Free Conducting Polymer Films with Stabilized Wrinkling Patterns and Their Applications.

Self-supported conducting polymer films with controlled microarchitectures are highly attractive from fundamental and applied points of view. Here a versatile strategy is demonstrated to fabricate thin free-standing crack-free polyaniline (PANI)-based films with stable wrinkling patterns. It is based on oxidization polymerization of pyrrole inside a pre-wrinkled PANI film, in which the wrinkled PANI film is used both as a template and oxidizing agent for the first time. The subsequently grown polypyrrole (PPy) and the formation of interpenetrated PANI/PPy networks play a decisive role in enhancing the film integrity and the stability of wrinkles. This enhancing effect is attributed to the modification of internal stresses by the interpenetrated PANI/PPy microstructures. Consequently, a crack-free film with stable controlled wrinkles such as the wavelength, orientation and spatial location has been achieved. Moreover, the wrinkling PANI/PPy film can be removed from the initially deposited substrate to become free-standing. It can be further transferred onto target substrates to fabricate hierarchical patterns and functional devices such as flexible electrodes, gas sensors, and surface-enhanced Raman scattering substrates. This simple universal enhancing strategy has been extended to fabrication of other PANI-based composite systems with crack-free film integrity and stabilized surface patterns, irrespective of pattern types and film geometries.