Wrinkling Of Films Research Articles

All-Solution-Processing of Nanoporous and Wrinkled Electrodes on Polymer Substrates for Use in Electrochemical Biosensing

Multi-lengthscale electrodes that combine features from the nanometer to millimeter scale are of tremendous interest for use in electrochemical biosensors due to their increased analytical sensitivity and their enhanced interaction with biomolecular analytes. In this paper, we present a benchtop rapid prototyping approach based on all-solution-processing for creating multi-lengthscale electrodes on polymer substrates. Multi-lengthscale electrodes are created by combining xurography, nanoparticle self-assembly, electroless deposition, and polymer-induced thin film wrinkling. Xurography is used to develop a mask for implementing the electrode configuration on polymeric substrates with micrometer resolution. A nanoparticle seed layer is deposited with the aid of molecular linkers to direct the electroless deposition processes to desired locations on the substrate. Electroless deposition is used to create a uniform and conductive thin film on a shrinkable polymer substrate. Thin film wrinkling induced using a shape memory polymer is then used to develop tunable wrinkles with minimum feature sizes in the micro/nanoscale. It is observed that the structural parameters of the wrinkled film such as pore density, wrinkle wavelength, and wrinkle peak-to-valley values are controllable by varying the electroless deposition duration. This structural variability is then used to tune the functional parameters of the electrodes such as sheet resistance and electroactive surface area. The combination of electroless deposition with polymer-induced thin film wrinkling results in thin films having a sheet resistance of 0.39±0.04 Ω/□, which is comparable to films of the same thickness deposited by sputtering. We used the electrodes developed here for enzyme-free glucose detection due to their high surface area induced by the combination of porosity and wrinkling. The results demonstrate that the sensitivity of these electrodes can be tuned by varying the electroless deposition duration. The optimal porous and wrinkled electrodes demonstrate a sensitivity of 23 µA/mM.cm2 for enzyme free glucose detection, which is 18 times larger than the sensitivity measured with planar electrodes. In sum, an all-solution-processing method is developed, which allows electrodes of various configurations, with tunable features from the nanometer to millimeter scale to be created in a matter of hours. The structural tunability offered by this fabrication method, allows the functional parameters of the electrodes such as sheet resistance, electro-active surface area, and analytical sensitivity to be tuned. The application of the porous and wrinkled electrodes created in this work to enzyme-free glucose detection is demonstrated.

Geometric Prepatterning-Based Tuning of the Period Doubling Onset Strain During Thin-Film Wrinkling

Wrinkling of thin films is an easy-to-implement and low-cost technique to fabricate stretch-tunable periodic micro and nanoscale structures. However, the tunability of such structures is often limited by the emergence of an undesirable period-doubled mode at high strains. Predictively tuning the onset strain for period doubling via existing techniques requires one to have extensive knowledge about the nonlinear pattern formation behavior. Herein, a geometric prepatterning-based technique is introduced that can be implemented even with limited system knowledge to predictively delay period doubling. The technique comprises prepatterning the film/base bilayer with a sinusoidal pattern that has the same period as the natural period of the system. This technique has been verified via physical and computational experiments on the polydimethylsiloxane (PDMS)/glass bilayer system. It is observed that the onset strain can be increased from the typical value of 20% for flat films to greater than 30% with a modest prepattern aspect ratio (2·amplitude/period) of 0.15. In addition, finite element simulations reveal that (i) the onset strain increases with increasing prepattern amplitude and (ii) the delaying effect can be captured entirely by the prepattern geometry. Therefore, one can implement this technique even with limited system knowledge, such as material properties or film thickness, by simply replicating pre-existing wrinkled patterns to generate prepatterned bilayers. Thus, geometric prepatterning is a practical scheme to increase the operating range of stretch-tunable wrinkle-based devices by at least 50%.

Rapid prototyping of all-solution-processed multi-lengthscale electrodes using polymer-induced thin film wrinkling

Three-dimensional electrodes that are controllable over multiple lengthscales are very important for use in bioanalytical systems that integrate solid-phase devices with solution-phase samples. Here we present a fabrication method based on all-solution-processing and thin film wrinkling using smart polymers that is ideal for rapid prototyping of tunable three-dimensional electrodes and is extendable to large volume manufacturing. Although all-solution-processing is an attractive alternative to vapor-based techniques for low-cost manufacturing of electrodes, it often results in films suffering from low conductivity and poor substrate adhesion. These limitations are addressed here by using a smart polymer to create a conformal layer of overlapping wrinkles on the substrate to shorten the current path and embed the conductor onto the polymer layer. The structural evolution of these wrinkled electrodes, deposited by electroless deposition onto a nanoparticle seed layer, is studied at varying deposition times to understand its effects on structural parameters such as porosity, wrinkle wavelength and height. Furthermore, the effect of structural parameters on functional properties such as electro-active surface area and surface-enhanced Raman scattering is investigated. It is found that wrinkling of electroless-deposited thin films can be used to reduce sheet resistance, increase surface area, and enhance the surface-enhanced Raman scattering signal.

Temperature dependent evolution of wrinkled single-crystal silicon ribbons on shape memory polymers.

Shape memory polymers (SMPs) can remember two or more distinct shapes, and thus can have a lot of potential applications. This paper presents combined experimental and theoretical studies on the wrinkling of single-crystal Si ribbons on SMPs and the temperature dependent evolution. Using the shape memory effect of heat responsive SMPs, this study provides a method to build wavy forms of single-crystal silicon thin films on top of SMP substrates. Silicon ribbons obtained from a Si-on-insulator (SOI) wafer are released and transferred onto the surface of programmed SMPs. Then such bilayer systems are recovered at different temperatures, yielding well-defined, wavy profiles of Si ribbons. The wavy profiles are shown to evolve with time, and the evolution behavior strongly depends on the recovery temperature. At relatively low recovery temperatures, both wrinkle wavelength and amplitude increase with time as evolution progresses. Finite element analysis (FEA) accounting for the thermomechanical behavior of SMPs is conducted to study the wrinkling of Si ribbons on SMPs, which shows good agreement with experiment. Merging of wrinkles is observed in FEA, which could explain the increase of wrinkle wavelength observed in the experiment. This study can have important implications for smart stretchable electronics, wrinkling mechanics, stimuli-responsive surface engineering, and advanced manufacturing.

Wrinkling of milk skin is mediated by evaporation.

Wrinkling of thin films and membranes can occur due to various mechanisms such as growth and/or mismatch between the mechanical properties of the film and substrate. However, the physical origins of dynamic wrinkling in soft membranes are still not fully understood. Here we use milk skin as a tractable experimental system to investigate the physics of wrinkle formation in a thin, poroelastic film. Upon heating milk, a micron-thick hydrogel of denatured proteins and fat globules forms at the air-water interface. Over time, we observe an increase in the total length of wrinkles. By confocal imaging and profilometry, we determine that the composition and thickness of the milk skin appears to be homogeneous over the length scale of the wrinkles, excluding differences in milk skin composition as a major contributor to wrinkling. To explain the physical origins of wrinkle growth, we describe theory that considers the milk skin as a thin, poroelastic film where pressure is generated by the evaporative-driven flow of solvent across the film; this imparts in-plane stresses in the milk skin, which cause wrinkling. Viscous effects can explain the time-dependent growth of wrinkles. Our theoretical predictions of the effects of relative humidity on the total length of wrinkles over time are consistent with our experimental results. Our findings provide insight into the physics of the common phenomenon of milk skin wrinkling, and identify hydration gradients as another physical mechanism that can drive morphological instabilities in soft matter.

Programmable, reversible and repeatable wrinkling of shape memory polymer thin films on elastomeric substrates for smart adhesion.

Programmable, reversible and repeatable wrinkling of shape memory polymer (SMP) thin films on elastomeric polydimethylsiloxane (PDMS) substrates is realized, by utilizing the heat responsive shape memory effect of SMPs. The dependencies of wrinkle wavelength and amplitude on program strain and SMP film thickness are shown to agree with the established nonlinear buckling theory. The wrinkling is reversible, as the wrinkled SMP thin film can be recovered to the flat state by heating up the bilayer system. The programming cycle between wrinkle and flat is repeatable, and different program strains can be used in different programming cycles to induce different surface morphologies. Enabled by the programmable, reversible and repeatable SMP film wrinkling on PDMS, smart, programmable surface adhesion with large tuning range is demonstrated.

A new Fourier-related double scale analysis for wrinkling analysis of thin films on compliant substrates

In this paper, a new Fourier-related double scale approach is presented to study the wrinkling of thin films on compliant substrates. By using the method of Fourier series with slowly variable coefficients, the 1D microscopic model proposed by Yang et al. (2015) is transformed into a 1D macroscopic film/substrate model whose mesh size is independent on the wrinkling wavelength. Numerical tests prove that the new model improves computational efficiency significantly with accurate results, especially when dealing with wrinkling phenomena with vast wavenumbers. Besides, we propose a strategy to efficiently trace the wrinkling pattern corresponding to the lowest critical load by accounting for several harmonics of Fourier series in this new model. The established nonlinear system is solved by the Asymptotic Numerical Method (ANM), which has advantages of efficiency and reliability for stability analyses.

Preparation of self-supporting Au thin films on perforated substrate by releasing from water-soluble sacrificial layer

A self-supporting thin film is useful as a target material for laser-driven ion acceleration experiments. In this study, 100-nm-thick sputtered gold (Au) thin films were released from substrates using water-soluble sacrificial layers, and the released films were subsequently scooped up on perforated substrates. Au thin films were deposited by DC plasma sputtering on the sacrificial layers. In the releasing test, sodium chloride (NaCl) was shown to be most suitable as a sacrificial layer for Au thin films. In addition, sputtered Au thin films with thicknesses of 50 and 150 nm were deposited onto NaCl sacrificial layers, released on water, and scooped up on perforated substrates. Self-supporting Au thin films were obtained for all film thicknesses, but wrinkles and cracks appeared in the 50 nm film.

Energy Absorption Behavior of Polyurea Under Laser-Induced Dynamic Mixed-Mode Loading

The energy absorption response of polyurea thin films is investigated under laser-induced dynamic mixed-mode loading. By exploiting the substrate geometry, the arrival times and the convenience of two simultaneously detected waves (longitudinal and shear), the transient shear loading of thin films is achieved under mixed-mode conditions. Polyurea films are fabricated on fused silica prismatic (90°–45°–45°) substrates by spin coating a polyurea mixture diluted with tetrahydrofuran solvent. A high amplitude shear wave loading of the polyurea films is attained by the mode-conversion of a laser generated compressive stress wave at an oblique face of the prism substrate. Mixed-mode dynamic shear experiments are conducted on 100 nm thick aluminum and 2 μm thick polyurea films over a range of laser loading fluences (energy/unit area). The free surface displacement, surface velocity and energy fluence histories are inferred from the shear portion of high-speed interferometric measurements. On comparing input–output velocities and fluence parameters, test results show velocity retardation and significant energy absorption (50–65 %) by the polyurea films. Postmortem spallation profiles reveal film wrinkling, tearing and a directional failure pattern, indicating shear as the dominant failure mode under the mixed-mode conditions.

Thin film wrinkling by strain mismatch on 3D surfaces

Wrinkling of thin films in micro- and nanoscale has found wide applications in stretchable electronics, optical gratings, material characterization, and biology. However, most wrinkling work is in planar fashion and only limited work on 3D wrinkling has been reported. In this paper, we present experimental demonstration of 3D Cr film wrinkling on PDMS micro-ridges induced by strain mismatch. We first fabricated PDMS micro-ridges using replication from a silicon mold, prepared by conventional microfabrication processes. While pre-stretching the PDMS specimen along the ridge direction, thin Cr film was deposited through physical sputtering. After releasing the prestrain, two wrinkling waves formed on the two inclined surfaces of the PDMS ridges with the same wavelength but opposite phases. These two wrinkling waves twist the top of the ridge into a wavy shape, forming wrinkling waves oscillating in the direction parallel to the substrate plane. In contrast, 2D wrinkling waves oscillate in the direction perpendicular to the substrate plane. Using finite element simulations, we demonstrate that the 180° phase shift between the two sides is favored by the dominant buckling modes. While the 3D wrinkling wavelength is consistent with 2D theoretical prediction, wrinkles on the ridge feature non-uniform wrinkling wave amplitude along the width of the inclined surface. This 3D wrinkling work opens new opportunities in manufacturing curved-shape mold for contact printing and increases surface to volume ratio for wetting and battery applications.

Spontaneous wrinkling of soft matter by energetic deposition of Cr and Au

Wrinkling of stiff thin films deposited on compliant substrates is an effect that has been broadly investigated. However, wrinkling consequent to metal ion implantation has been less studied. In the work described here, we have explored the sub-micron wrinkling phenomena that spontaneously occur when metal ions (Au and Cr) are implanted with energy of a few tens of electron volts (49 eV for Au and 72 eV for Cr) into a compliant material (PDMS). This very low energy ion implantation was performed using a Filtered Cathodic Vacuum Arc technique, a process often referred to as energetic deposition or energetic condensation. For comparison, Au and Cr depositions with similar doses were also done using a sputtering technique (with lower particle energy of approximately 2 eV), and no wrinkle formation was then observed. In this way, we can discuss the role of ion energy in wrinkle formation. Depth profiles of the implanted material were calculated using the Tridyn computer simulation code for each metal, for several implantation doses. UV-vis absorption spectroscopy analysis confirmed the presence of metal nanoparticles. Atomic Force Microscopy imaging with spectral processing was used to compare the wrinkle morphology for each case investigated.

(Invited) A Versatile Fabrication Platform for Rapid Prototyping of Biosensors

Background: Currently, translating biosensing platforms from the laboratory to the market is a lengthy process. One approach to expediting technology translation in this area is to develop rapid prototyping processes that enable an idea to be implemented into a sensor prototype rapidly and inexpensively. Furthermore, this process needs to be scalable to enable mid-volume and large-volume manufacturing for in-field testing and commercialization. In this work, we have developed a versatile rapid prototyping toolbox for creating tunable materials and integrating them into functional devices. Methods: biosensors are multi-lengthscale devices that are often optimized in the nm lengthscale for enhancing surface reaction kinetics and signal generation, in the micrometer lengthscale for addressing mass transport demands, and in the mm lengthscale for interfacing with the fluidic and electrical connections of the outside world. We have created a rapid prototyping toolbox based on craft cutting, self-assembly, electrochemical deposition, and wrinkling of thin films on shrink memory polymers to fabricate multi-lengthscale biosensors using tools available on the laboratory benchtop in a matter of hours. The benchtop processes developed here allow us to tune structural parameters such as roughness, aspect ratio, adhesion, porosity, and minimum feature sizes without the need for semiconductor cleanrooms. Results: We sought to create application-specific materials that translate structural tunability to functional tunability. For this purpose, we have studied the effect of electrode porosity, roughness, and surface area on the sensitivity of electrochemical biosensors. Through this study, we have found that by tuning the electrode morphology and porosity, we are able to enhance the observed electrocatalysis at electrode surfaces. These rapid surface reaction kinetics have enabled us to perform enzyme-free glucose detection in clinically-relevant concentrations of glucose. In addition to direct electrochemical sensing, we combined these newly-developed wrinkled materials with a DNA recognition layer to understand the role of electrode morphology on the sensitivity of DNA sensors. Through these experiments we find that significantly larger signal changes are observed when DNA is displayed on wrinkled surfaces with sub-10 nm pores. In order to create the optimal materials for surface-enhanced Raman spectroscopy (SERS), we used our newly developed rapid prototyping toolbox and combined layer-by-layer self-assembly of gold nanoparticles with substrate-mediated thin film wrinkling. This allowed us to precisely tune the nanoparticle size and separations from the sub-10nm to the sub-micrometer lengthscales. Due to the rapid prototyping nature of this method, we were able to develop application-specific SERS substrates without the need for numerical modeling. Conclusions: In conclusion, we have developed a rapid prototyping tools box that is designed to address the lengthscale engineering needs of the area of biosensing. Using this method, it is possible to rapidly screen through multiple structural parameters to manipulate biosensor sensitivity and signal transduction efficiency.

Wrinkling of structured thin films via contrasted materials.

Regular surface patterns induced by the wrinkling of thin films have received intense attention in both science and engineering. We investigate the wrinkling of structured thin films that consist of two types of materials arranged in periodic patterns. A mechanical model is proposed to understand the physics of the wrinkling, and a set of scaling laws for the wrinkle wavelength are obtained. Periodic wrinkles are generated in the local regions of structured films via in-plane contrasted elastic modulus between heterogeneous materials. The wrinkle morphology and location can be tailored by designing structured thin films in a controllable way. Our findings provide the basis for understanding the wrinkling of structured thin films and for the manufacture of regular surface patterns via wrinkling.

Wrinkling of extensional thin films through modified large deflection equations analytically and experimentally

The stretch-induced wrinkling of thin films is solved through the modified von Kármán large deflection equations by first selecting the suitable deformation expressions that satisfy boundary conditions. Then, adopting the principle of minimum potential energy we obtain the deformations of simply supported rectangular thin films. The obtained significant deflections are nonlinearly elastic and of the lowest order of infinite solutions. The parameters of aspect ratio, the thickness and material of thin films are studied analytically and numerically. The highlighted results of wrinkle amplitude and load are in good agreement with experiments. The methodology also indicates the limit load impending plasticity and predicts the applied load precisely for each wrinkle. Further, it can be extended to the variety of multifunctional orthotropic and multi-layered thin films.

Buckling and Wrinkling of Thin Films and Membranes

An analysis of the results of thin films and membranes study shows their considerable promise due to a number of unique physical and physical-mechanical properties. The buckling (wrinkling, folding and wrinklon formation) is quite a typical phenomenon and can lead to a substantial change of physical properties extending the application field of thin films and membranes. The variation of the elastic strain of a two-dimensional material is an effective method of changing its geometry and properties, and it can stimulate the formation of wrinkles and influence on the parameters such as wavelength, amplitude and orientation. This is due to the fact that the physical system is in weakly stable (easily altered by external factors) state which is close to the stability limit. At the same time, various structural states of the system are thermodynamically similar but structurally different. The studies carried out on thin carbon films (in the limit of graphene-like structures) have shown some nontrivial results indicating unique physical and physical-mechanical properties of thin films and membranes. The promising areas of further research are proposed.

Selective Alignment of Molecular Glass Wrinkles by Engineered Magnetic Field Landscapes

A new approach for aligning wrinkles in thin viscoelastic polymethylmethacrylate films is established by sandwiching them between ferromagnetic layers. Experiments prove that a contribution from an engineered anisotropic 1D periodic magnetic stray field pattern can be decisive for the alignment of otherwise randomly oriented wrinkles at commensurate periods of the magnetic template pattern and the wrinkles. Alternatively, random wrinkling is observed under similar conditions, but when periodic magnetic stray fields are missing or not commensurate with the wrinkle wavelength. The possibility to stabilize two distinct types of wrinkling patterns in the same material system paves the way toward mechanically reconfigurable functional elements, enabling exciting possibilities for stretchable electronics, magnetic field sensors, and tuneable gratings for optoelectronics and photonics.

Design of a Compact Biaxial Tensile Stage for Fabrication and Tuning of Complex Micro- and Nano-scale Wrinkle Patterns

Wrinkling of thin films is a strain-driven process that enables scalable and low-cost fabrication of periodic micro- and nano-scale patterns. In the past, single-period sinusoidal wrinkles have been applied for thin-film metrology and microfluidics applications. However, real-world adoption of this process beyond these specific applications is limited by the inability to predictively fabricate a variety of complex functional patterns. This is primarily due to the inability of current tools and techniques to provide the means for applying large, accurate, and nonequal biaxial strains. For example, the existing biaxial tensile stages are inappropriate because they are too large to fit within the vacuum chambers that are required for thin-film deposition/growth during wrinkling. Herein, we have designed a compact biaxial tensile stage that enables (i) applying large and accurate strains to elastomeric films and (ii) in situ visualization of wrinkle formation. This stage enables one to stretch a 37.5 mm long film by 33.5% with a strain resolution of 0.027% and maintains a registration accuracy of 7 μm over repeated registrations of the stage to a custom-assembled vision system. Herein, we also demonstrate the utility of the stage in (i) studying the wrinkling process and (ii) fabricating complex wrinkled patterns that are inaccessible via other techniques. Specifically, we demonstrate that (i) spatial nonuniformity in the patterns is limited to 6.5%, (ii) one-dimensional (1D) single-period wrinkles of nominal period 2.3 μm transition into the period-doubled mode when the compressive strain due to prestretch release of plasma-oxidized polydimethylsiloxane (PDMS) film exceeds ∼18%, and (iii) asymmetric two-dimensional (2D) wrinkles can be fabricated by tuning the strain state and/or the actuation path, i.e., the strain history. Thus, this tensile stage opens up the design space for fabricating and tuning complex wrinkled patterns and enables extracting empirical process knowledge via in situ visualization of wrinkle formation.

Nonlinear analysis of photo-induced wrinkling of glassy twist nematic films on compliant substrates

A kinetics approach is developed for the geometrically nonlinear analysis of photo-induced wrinkling of glassy twist nematic films on soft elastic substrates. In this way, the problem is reduced to finding the steady state of an overdamped evolution system according to a kinetic law, rather than directly solving the coupled nonlinear equations. This enables one to account for the complicated director distribution and obtain the precise wrinkling morphology of the film. Though the approach proposed here is for a twist nematic film, it can be extended to study glassy nematic films with other director distributions.

Wrinkling of YBa2Cu3O7−x film prepared by trifluoroacetate metal organic deposition

YBa2Cu3O7−x (YBCO) films with wrinkles have been fabricated on LaAlO3(100) substrates by trifluoroacetate metal organic deposition (TFA-MOD). The decomposition behavior of YBCO-TFA gel and the formation mechanism of wrinkles were analyzed by thermal analysis, Fourier transform infrared spectroscopy, and atomic force microscopy. The effects of wrinkles on the texture, microstructure and critical current density of the final crystallized film were also investigated by X-ray diffraction and scanning electron microscopy. The heating rate in the decomposition process was a crucial parameter to control the wavelength and wave amplitude of wrinkles on the surface of YBCO precursor film. Furthermore, the wrinkles of precursor films were transformed into the ridges of crystallized films during the crystallization process. Large ridges resulted in the degradation of texture of YBCO crystallized films. The decrease of critical current density (Jc) of YBCO crystallized films could be ascribed to the formation of ridges, the segregated phases and the degradation of texture. It suggested that the smooth morphology of YBCO precursor films would be beneficial to the enhancement of Jc value.

Surface Wrinkling Patterns of Film–Substrate Systems With a Structured Interface

Wrinkling of thin films resting on compliant substrates has emerged as a facile means to create well-ordered surface patterns. In this paper, both theoretical analysis and numerical simulations are presented to study the surface wrinkling of a film–substrate system with periodic interfacial structures. It is demonstrated that a variety of novel surface wrinkling patterns can be generated through the introduction of interfacial architectures. These surface patterns can be easily tuned by adjusting two geometric parameters: the lengths of the thin films in the thick and the thin regions. A phase diagram is established for the onset of different wrinkling morphologies with respect to the two geometric dimensions. This study offers a promising route for engineering the surfaces of materials endowed with tunable properties and functions.