Wrinkling Of Films Research Articles

Spontaneous hierarchical wrinkling of metal films sputter-deposited on liquid-like gel substrates

Complex wrinkle patterns are ubiquitous in nature and are beneficial for a wide range of practical applications. In this work, we report on the spontaneous hierarchical wrinkling of metal films sputter-deposited on liquid-like gel substrates. It is found that the metal atoms and clusters can penetrate into the gel surface layer at the early stage of sputtering. The volume-expansion-induced compressive stress forces the gel surface to generate highly deformed ridge structures. Subsequently developed wrinkles are strongly dependent on the ridge structure, but are insensitive to the film thickness. Tiny wrinkles nested inside the preformed wrinkling can be also observed, resulting in the spontaneous formation of multiscale hierarchical structures. The morphological characteristics, evolutional behaviors and underlying mechanisms of the hierarchical structures are discussed in detail.

Boundary curvature effect on the wrinkling of thin suspended films

A relation between the boundary curvature κ and the wrinkle wavelength λ of a thin suspended film under boundary confinement is demonstrated. Experiments were performed with nanocrystalline diamond films of approximate thickness 184 nm grown on glass substrates. By removing portions of the substrates after growth, suspended films with circular boundaries of radius 30–811 μm were fabricated. Due to residual stresses, the portions of the film bonded to the substrate are of approximate compressive prestrain 11×10−4 and the suspended portions of the film are azimuthally wrinkled at their boundary. Measurements show that λ decreases monotonically with κ, and a simple model that is in line with this trend is proposed. The model can be applied to design devices with functional wrinkles and can be adapted to gain insight into other systems such as plant leaves. A method for measuring residual compressive strain in thin films, which complements standard strain characterization methods, is also described.

Application of a Laser Cutter to Pattern Wrinkles on Polymer Films

Wrinkling of thin films is a simple way to fabricate microscale topographical structures without the use of expensive lithographic processes or clean rooms. Such wrinkles have applications in surfa...

The emergence of local wrinkling or global buckling in thin freestanding bilayer films

Periodic wrinkling of a rigid capping layer on a deformable substrate provides a useful method for templating surface topography for a variety of novel applications. Many experiments have studied wrinkle formation during the compression of a rigid film on a relatively soft pre-strained elastic substrate, and most have focused on the regime where the substrate thickness can be considered semi-infinite relative to that of the film. As the relative thickness of the substrate is decreased, the bending stiffness of the film dominates, causing the bilayer to transition to either local wrinkling or a global buckling instability. In this work optical microscopy was used to study the critical parameters that determine the emergence of local wrinkling or global buckling of freestanding bilayer films consisting of a thin rigid polymer capping layer on a pre-strained elastomeric substrate. The thickness ratio of the film and substrate as well as the pre-strain were controlled and used to create a buckling phase diagram which describes the behaviour of the system as the ratio of the thickness of the substrate is decreased. A simple force balance model was developed to understand the thickness and strain dependences of the wrinkling and buckling modes, with excellent quantitative agreement being obtained with experiments using only independently measured material parameters.

Thickness-dependent wrinkling of PDMS films for programmable mechanochromic responses

We report a remarkable thickness-dependent wrinkling behavior of oxygen plasma-treated polydimethylsiloxane (PDMS) films, in which an energy barrier separates the wrinkling mechanics into two regimes. For thick films, the film wrinkles with a constant periodicity which can be precisely predicted by the classic nonlinear finite mechanics. Reducing the film thickness below 1 mm leads to nonuniform wrinkles with an increasing periodicity which gives rise to random scattering and transparency changes under mechanical strains. By tuning the film thickness, we were able to control both the quality and size of the periodic wrinkles and further design mechanochromic devices featuring brilliant structural colors and programmable colorimetric responses. This work sheds light on the fundamental understanding of the wrinkling mechanics of bilayer systems and their intriguing mechanochromic applications.

Wrinkling prediction, formation and evolution in thin films adhering on polymeric substrata

Wrinkling has recently attracted an increasing interest by suggesting a number of unforeseeable applications in many emerging material science and engineering fields. If guided and somehow designed, wrinkles could be in fact used as an alternative printing way for realizing complex surface geometries and thus employed as an innovative bottom-up process in the fabrication of nano- and micro-devices. For these reasons, the prediction of wrinkles of films adhering on flat as well as on structured substrata is a challenging task, genesis and development of the phenomenon being not yet completely understood both when thin membranes are coupled with soft supports and in cases where the geometry of the surfaces are characterized by complex three-dimensional profiles. Here we investigate the experimental formation of new intriguing and somehow unforeseeable wrinkled patterns achieved on periodic structures, by showing prediction through a new hybrid analytical-numerical strategy capable to overcome some common obstacles encountered in modeling film wrinkling on flat and 3D-shaped substrata. The proposed approach, which drastically reduces the computational effort, furnishes a helpful way for predicting both qualitative and quantitative results in terms of wrinkling patterns, magnitude and wavelength, by also allowing to follow the onset of film instabilities and the progressive evolution of the phenomenon until its final stage.

Elastic stabilization of wrinkles in thin films by auxetic microstructure

Thin elastic sheets and membranes are known to wrinkle when they are stretched — the associated physics is highly non-linear. The unusual behavior exhibited by thin films upon stretching when they possess auxetic structure, i.e. when their apparent Poisson’s ratio is negative, is reported here. Wrinkling is now suppressed within the bulk of auxetic films when tensioned, whereas localized creases confined to the clamps, which decay away exponentially, appear. These edge wrinkles are characterized for their amplitude and wavelength experimentally, theoretically, and computationally, which show excellent agreement with expected trends. The scaling for amplitude, wavelength and decay rate upon film properties and tension is obtained using simple analyses based on kinematic mismatch resulting from lateral Poisson’s expansion.

Turning Catalysts on by Light‐Induced Stress: When Red Means Go

AbstractWe report the activation of platinum catalyst thin films for oxygen reduction reaction in perchloric acid electrolytes when the films are subjected to mechanical stress. The catalysts films were deposited by magnetron sputtering on photosensitive chalcogenide As20Se80 semiconductor glass layer. When platinum films are illuminated by red laser light the glass under semitransparent platinum film exhibits photo‐induced flow. The material flow underneath platinum film induces mechanical stress resulting in film wrinkling and significant increase in its catalytic activity.

Tunable surface wrinkling on shape memory polymers with application in smart micromirror

Surfaces with tunable topological features enable important applications, such as optical devices, precision metrology, adhesion, and wetting. In this study, we demonstrate a facile method to fabricate and control the surface morphologies by combining thin film wrinkling and thermal expansion. This approach utilizes self-assembled surface wrinkling induced by shape recovery of shape memory polymers (SMPs) and localized thermal expansion caused by Joule heating. Recovering the prestrain in the SMP substrate induces global wrinkling of the thin film on the substrate. Joule heating in the SMP by a heating wire embedded in the substrate induces thermal expansion of the substrate in a localized area, which leads to the disappearance of the wrinkling pattern. This effect is reversed when heating is stopped, leading to reversible and repeatable tuning of the surface morphology in a controllable localized surface region. With metal coating, the SMP surface can be switched from specular to diffuse reflectance in response to external Joule heating. Finally, we demonstrate a smart micromirror device with its diffuse reflectance tunable between 13.5% and 81.9% in the visible light region. This approach provides a method to modulate surface diffusivity by controlling its surface morphologies, with potential applications in optical display and optical microelectromechanical systems devices.

Localized wrinkling of metal films on elastic and liquid substrates

Global and localized wrinkling phenomena in films/substrates have been widely studied by means of experiment and theoretical analyses in the past two decades. They have shown broad application prospect in microstructure manufacture. The global wrinkling patterns with well-defined wavelengths can be readily fabricated in the polymer films, the metal films and the surface of modified elastic substrates by O2 plasma or UVO, and they have attracted much attention. For the localized wrinkling patterns, however, most previous studies were limited to polymer films, which while metal films were not properly considered. Recent studies have shown that wrinkles can be localized by constrained edges and cracks in metal films deposited on the elastic and liquid substrates. In this review, we focus on the formation and evolution mechanisms of these localized wrinkle patterns, and we aim to provide some useful guidance for future studies on designing the localized wrinkling patterns with real-time controllable dimension, orientation and morphology in the films/substrates.

Impact of Substrate Characteristics on Stretchable Polymer Semiconductor Behavior.

Stretchable conductive polymer films are required to survive not only large tensile strain but also stay functional after the reduction in applied strain. In the deformation process, the elastomer substrate that is typically employed plays a critical role in response to the polymer film. In this study, we examine the role of a polydimethylsiloxane (PDMS) elastomer substrate on the ability to achieve stretchable PDPP-4T films. In particular, we consider the adhesion and near-surface modulus of the PDMS tuned through UV/ozone (UVO) treatment on the competition between film wrinkling and plastic deformation. We also consider the role of PDMS tension on the stability of films under cyclic strain. We find that increasing the near-surface modulus of the PDMS and maintaining the PDMS in tension throughout the cyclic strain process promote plastic deformation over film wrinkling. In addition, the UVO treatment increases film adhesion to the PDMS resulting in a significantly reduced film folding and delamination. For a 20 min UVO-treated PDMS, we show that a PDPP-4T film root-mean-square roughness is consistently below 3 nm for up to 100 strain cycles with a strain range of 40%. In addition, although the film is plastically deforming, the microstructural order is largely stable as probed by grazing incidence X-ray scattering and UV-visible spectroscopy. These results highlight the importance of neighboring elastomer characteristics on the ability to achieve stretchable polymer semiconductors.

Reversible Surface Patterning by Dynamic Crosslink Gradients: Controlling Buckling in 2D.

Harnessing the self-organization of soft materials to make complex, well-ordered surface patterns in a noninvasive manner is challenging. The wrinkling of thin films provides a compelling strategy to achieve this. Despite much attention, however, a simple, single-step, reversible method that gives rise to controlled, two-dimensional (2D) ordered, continuous, and discontinuous patterns has proven to be elusive. Here a novel, robust method is described to achieve this using an ultraviolet-light-sensitive anthracene-containing polymer thin film. The origin of the patterns is the local buckling of the thin film, where the control over the topology is given by laterally patterning out-of-plane gradients in the crosslink density of the film. The underlying buckling mechanics and formation of the surface features are well-described by finite element analysis. By illuminating the film with a photomask, local and long-range patterns that can be both continuous and discontinuous are able to be written. Furthermore, the patterning is fully reversible over multiple cycles. The results demonstrate a simple strategy for erasable storage of information in a surface topography that has applications in memory, anticounterfeiting, and plasmonics.

Programmable localized wrinkling of thin films on shape memory polymers with application in nonuniform optical gratings

Shape memory polymers (SMPs) can remember different shapes and can be recovered to their permanent shapes from temporary shapes with appropriate stimuli, such as heat, humidity, and electrical field. Using programmed thermal responsive SMPs as substrates, we demonstrate a self-assembly fabrication method for programmable surface wrinkling within a highly confined area that is accurately controllable. Different from global wrinkling reported in most of the literature, Joule heating through a heating wire embedded in the SMP substrate leads to temperature increase and thus recovery in a highly confined area of the SMP substrate, inducing localized wrinkling of the stiff thin film on SMPs. The patterns show good sinusoidal profiles, with the wrinkling wavelength and amplitude decreasing gradually with the distance from the heat source. The surface wrinkling area can be accurately tuned by controlling the heat input, such as power and duration. Based on this unique surface wrinkling phenomenon, we demonstrate a nonuniform reflective optical grating device, whose peak wrinkling wavelength and amplitude decrease gradually away from the heat source. This study offers a simple method to fabricate programmable localized wrinkling patterns, with potential applications in surface engineering, advanced manufacturing, optical gratings, and other demanding areas.

Effect of the Orientation and Bending Stiffness of Nanopatterned Films on Wrinkling

The production of hierarchical structures has an important role for applications such as superhydrophobic surfaces, structural colors, and energy devices. Microscale wrinkling of surfaces decorated by nanoscale patterns can be one of the most efficient methods to realize hierarchical structures. Here, we investigate wrinkling of films embedding anisotropic nanopatterned surfaces prepared by nanoimprint lithography. After transferring nanoimprinted polystyrene thin films onto stretched elastomeric substrates, we achieve microscale wrinkles by releasing the strain. We examine the anisotropic wrinkles of well-defined nanolines with various widths, heights, and spacing ratios and propose a model that considers only bending stiffness of the patterned film. We compare the approach with a model that considers inplane stiffness and confirm that our scheme matches well with that of nanopatterns with thin residual layers.

Surface wrinkling of anisotropic films bonded on a compliant substrate

Material anisotropy regulates the instabilities of film–substrate systems at different length scale. In this paper, we investigate the surface wrinkling and morphological evolution of an orthotropic thin film resting on a compliant substrate. Under different loading conditions, the system may buckle into various surface patterns, e.g., stripe, checkerboard, and herringbone, which are analyzed by using the Föppl–von Kármán plate theory. The Fourier spectral method is employed to simulate the morphological evolutions of surface patterns under different loading biaxialities. We find that both loading biaxiality and material anisotropy affect the characteristics of surface wrinkling patterns and their evolutions. Stripe and checkerboard modes often emerge at the critical buckling and they tend to transform into herringbone and labyrinth patterns during postbuckling. Phase diagrams are established to reveal the dependence of surface patterns on material anisotropy, Poisson's effect, and loading biaxiality. This study may help design diverse functional surfaces and deepen our understanding of the morphogenesis of some soft biological tissues and organs.

Smoothening of wrinkles in CVD-grown hexagonal boron nitride films.

Hexagonal boron nitride (h-BN) is an ideal substrate for two-dimensional (2D) materials because of its unique electrically insulating nature, atomic smoothness and low density of dangling bonds. Although mechanical exfoliation from bulk crystals produces the most pristine flakes, scalable fabrication of devices is still dependent on other more direct synthetic routes. To date, the most utilized method to synthesize large-area h-BN films is by chemical vapor deposition (CVD) using catalytic metal substrates. However, a major drawback for such synthetic films is the manifestation of thermally-induced wrinkles, which severely disrupt the smoothness of the h-BN films. Here, we provide a detailed characterization study of the microstructure of h-BN wrinkles and demonstrate an effective post-synthesis smoothening route by thermal annealing in air. The smoothened h-BN film showed an improved surface smoothness by up to 66% and resulted in a much cleaner surface due to the elimination of polymer residues with no substantial oxidative damage to the film. The unwrinkling effect is attributed to the hydroxylation of the h-BN film as well as the substrate surface, resulting in a reduction in adhesion energy at the interface. Dehydroxylation occurs over time under ambient conditions at room temperature and the smoothened film can be restored back with the intrinsic properties of h-BN. This work provides an efficient route to achieve smoother h-BN films, which are beneficial for high-performance 2D heterostructure devices.

On the influence of PDMS (polydimethylsiloxane) substrate surface energy in wrinkling of DLC (diamond-like carbon) thin films

We have explored the influence of surface energy of the PDMS (polydimethylsiloxane) substrate on the wrinkling of diamond-like carbon thin films. The surface energy of PDMS can be tuned by exposure to oxygen plasma or by shallow-implantation of gold. The result is an increase in the wrinkling wavelength and amplitude. By means of a tri-layer wrinkling model, we discuss whether the major contribution to the wavelength variation is via the surface energy of the substrate or via the mechanical properties of the interface layer resulting from the surface treatment. We conclude that the surface energy of the substrate is an important property that must be considered in order to provide a complete description of wrinkling phenomena.

Fault-Tolerant Electro-Responsive Surfaces for Dynamic Micropattern Molds and Tunable Optics

Electrically deformable surfaces based on dielectric elastomers have recently demonstrated controllable microscale roughness, ease of operation, fast response, and possibilities for programmable control. Potential applications include marine anti-biofouling, dynamic pattern generation, and voltage-controlled smart windows. Most of these systems, however, exhibit limited durability due to irreversible dielectric breakdown. Lowering device voltage to avoid this issue is hindered by an inadequate understanding of the electrically-induced wrinkling deformation as a function of the deformable elastic film thickness. Here we report responsive surfaces that overcome these shortcomings: we achieve fault-tolerant behavior based on the ability to self-insulate breakdown faults, and we enhance fundamental understanding of the system by quantifying the critical field necessary to induce wrinkles in films of different thickness and comparing to analytical models. We also observe new capabilities of these responsive surfaces, such as field amplification near local breakdown sites, which enable actuation and wrinkle pattern formation at lower applied voltages. We demonstrate the wide applicability of our responsive, fault-tolerant films by using our system for adjustable transparency films, tunable diffraction gratings, and a dynamic surface template/factory from which various static micropatterns can be molded on demand.

Topographical and Electrical Stimulation of Neuronal Cells through Microwrinkled Conducting Polymer Biointerfaces

The development of smart biointerfaces combining multiple functions is crucial for triggering a variety of cellular responses. In this work, wrinkled organic interfaces based on the conducting polymer poly(3,4-ethylene dioxythiophene) doped with poly(styrene sulfonate) are developed with the aim to simultaneously convey electrical and topographical stimuli to cultured cells. The surface wrinkling of thin films on heat-shrink polymer sheets allows for rapid patterning of self-assembled anisotropic topographies characterized by micro/sub-microscale aligned wrinkles. The developed interfaces prove to support the growth and differentiation of neural cells (SH-SY5Y, human neuroblastoma) and are remarkably effective in promoting axonal guidance, by guiding and stimulating the neurite growth in differentiating cells. Electrical stimulation with biphasic pulses delivered through the conductive wrinkled interface is found to further promote the neurite growth, demonstrating the suitability of such interfaces as platforms for conveying multiple stimuli to cells and tissues.

Wrinkling of thin films on a microstructured substrate

ABSTRACTSurface wrinkling of thin films on substrates offers an effective strategy to create controllable surface patterns of wide application. In this article, both theoretical analysis and numerical simulations are performed to study the surface wrinkling of thin films bonded on a microstructured soft substrate under compression. We consider two typical kinds of substrates that have different mechanical properties. One possesses a periodic array of micropillars, and the other has a period of alternating unbonded—bonded regions at the film—substrate interface. The characteristics of surface wrinkling and postbuckling evolution of films are revealed. It is found that the interfacial microstructures modulate the normal mechanical properties of the substrate and, thus, tailor the buckling behavior of the thin film atop it. Under lateral compression, the film on the substrate shows periodic arrays of micropillars exhibiting sinusoidal wrinkling first, and then with further compression, the wrinkles give way to a deep fold accompanied by Euler buckling of the micropillars and the decay of neighboring wrinkles. For the system with periodic unbonded—bonded regions at the film—substrate interface, wrinkling of the film is characterized by a relative shift in the normal direction. This study offers potential applications in the fabrication of tunable surface morphologies.