Interfacial Sliding and Buckling of Monolayer Graphene on a Stretchable Substrate

The nonlinear mechanical response of monolayer graphene on polyethylene terephthalate (PET) is characterised using in‐situ Raman spectroscopy and atomic force microscopy. While interfacial stress transfer leads to tension in graphene as the PET substrate is stretched, retraction of the substrate during unloading imposes compression in the graphene. Two interfacial failure mechanisms, shear sliding under tension and buckling under compression, are identified. Using a nonlinear shear‐lag model, the interfacial shear strength is found to range between 0.46 and 0.69 MPa. The critical strain for onset of interfacial sliding is ～0.3%, while the maximum strain that can be transferred to graphene ranges from 1.2% to 1.6% depending on the interfacial shear strength and graphene size. Beyond a critical compressive strain of around ?0.7%, buckling ridges are observed after unloading. The results from this work provide valuable insight and design guidelines for a broad spectrum of applications of graphene and other 2D nanomaterials, such as flexible and stretchable electronics, strain sensing, and nanocomposites.     

Detection of buckling in steel pipeline and column by the distributed Brillouin sensor

We conducted a strain characterization experiment to monitor steel pipe and column buckling for the first time using a distributed Brillouin sensor system. Two specimens (steel pipe and column) were prepared by locally thinning the inner wall to initiate buckling. An axial load was applied to the specimens and increased while compressive strain was measured by both Brillouin sensor and strain gauges. With the Brillouin sensor, we observed compression on the whole specimens while elongation was detected in the neighborhood of the thinned wall at onset of the buckling. Both tension and compression are measured simultaneously from the same spectrum. This capability to extract both informations at the same time makes the Brillouin sensor a unique tool for structural health monitoring. The buckling was identified and localized thanks to this original approach.     

Mechanical Buckling: Mechanics,Metrology, and Stretchable Electronics

Mechanical buckling usually means catastrophic failure in structural mechanics systems. However, controlled buckling of thin films on compliant substrates has been used to advantage in diverse fields such as micro‐/nanofabrication, optics, bioengineering, and metrology as well as fundamental mechanics studies. In this Feature Article, a mechanical buckling model is presented, which sprang, in part, from the buckling study of high‐quality, single‐crystalline nanomaterials. To check the mechanical‐buckling phenomenon down to the nano‐/molecular scale, well‐aligned single‐walled carbon nanotube arrays and cross linked carbon‐based monolayers are transferred from growth substrate onto elastomeric substrate and then they are buckled into well‐defined shapes that are amenable to quantitative analysis. From this nano‐ or molecular‐scale buckling, it is shown that the mechanical moduli of nanoscale materials can easily be determined, even using a model based on continuum mechanics. In addition, buckling phenomena can be utilized for the determination of mechanical moduli of organic functional materials such as poly(3‐hexylthiophene) (P3HT) and P3HT/6,6‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) composite, which are widely used for organic transistors and organic photovoltaics. The results provide useful information for the realization of flexible and/or stretchable organic electronics. Finally, the fabrication and applications of “wavy, stretchable” single‐crystal Si electronics on elastomeric substrates are demonstrated.     

Size Effects on the Mechanical Properties of Nanoporous Graphene Networks

It is essential to understand the size scaling effects on the mechanical properties of graphene networks to realize the potential mechanical applications of graphene assemblies. Here, a “highly dense‐yet‐nanoporous graphene monolith (HPGM)” is used as a model material of graphene networks to investigate the dependence of mechanical properties on the intrinsic interplanar interactions and the extrinsic specimen size effects. The interactions between graphene sheets could be enhanced by heat treatment and the plastic HPGM is transformed into a highly elastic network. A strong size effect is revealed by in situ compression of micro‐ and nanopillars inside electron microscopes. Both the modulus and strength are drastically increased as the specimen size reduces to ≈100 nm, because of the reduced weak links in a small volume. Molecular dynamics simulations reveal the deformation mechanism involving slip‐stick sliding, bending, buckling of graphene sheets, collapsing, and densification of graphene cells. In addition, a size‐dependent brittle‐to‐ductile transition of the HPGM nanopillars is discovered and understood by the competition between volumetric deformation energy and critical dilation energy.     

Durable Ultraflexible Organic Photovoltaics with Novel Metal‐Oxide‐Free Cathode

Flexible and stretchable organic photovoltaics (OPVs) are promising as a power source for wearable devices with multifunctions ranging from sensing to locomotion. Achieving mechanical robustness and high power conversion efficiency for ultraflexible OPVs is essential for their successful application. However, it is challenging to simultaneously achieve these features by the difficulty to maintain stable performance under a microscale bending radius. Ultraflexible OPVs are proposed by employing a novel metal‐oxide‐free cathode that consists of a printed ultrathin metallic transparent electrode and an organic electron transport layer to achieve high electron‐collecting capabilities and mechanical robustness. In fact, the proposed ultraflexible OPV achieves a power conversion efficiency of 9.7% and durability with 74% efficiency retention after 500 cycles of deformation at 37% compression through buckling. The proposed approach can be applied to active layers with different morphologies, thus suggesting its universality and potential for high‐performance ultraflexible OPV devices.     

3D‐Architected Soft Machines with Topologically Encoded Motion

The limited range of mechanical responses achievable by materials compatible with additive manufacturing hinders the 3D printing of continuum soft robots with programmed motion. This paper describes the rapid design and fabrication of low‐density, 3D‐architected soft machines (ASMs) by combining Voronoi tessellation and additive manufacturing. On tendon‐based actuation, ASMs deform according to the topologically encoded buckling of their structure to produce a wide range of motions (contraction, twisting, bending, and cyclic motion). ASMs exhibiting densities as low as 0.094 g cm^?3 (≈8% of bulk polymer) can be rapidly built by the stereolithographic 3D printing of flexible photopolymers or the injection molding of elastomers. The buckling of ASMs can be programmed by inducing gradients in the thickness of their flexible beams or by the localized enlargement of the Voronoi cells to generate complex motions such as multi‐finger gripping or quadrupedal locomotion. The topological architecture of these low‐density soft robots confers them with the stiffness necessary to recover their original shape even after ultrahigh compression (400%) and extension (500%). ASMs expand the range of mechanical properties currently achievable by 3D printed or molded materials to enable the fabrication of soft machines with auxetic mechanical metamaterial properties.     

Video aggregation: adapting video traffic for transport overbroadband networks by integrating data compression and statisticalmultiplexing

Future broadband integrated services networks based on the asynchronous transfer mode (ATM) technology are expected to carry information from a large variety of different services and applications. This paper investigates video aggregation, a concept that integrates compression and statistical multiplexing of video information for transport over a communication network. We focus on the transmission of a group of video sessions as a bundle, the practical examples of which include entertainment-video broadcast and video-on-demand (VoD). In this situation, the advantage of constant bit-rate (CBR) transport (which facilitates simple network management and operation) and the advantage of variable bit-rate (VBR) video compression (which yields smoother image quality) can be achieved simultaneously. We show that it is better to integrate compression and statistical multiplexing before the bundle of video traffic enters the network than performing them as independent processes. We present experimental results which indicate the advantages of video aggregation in terms of superior image quality and efficient bandwidth usage     

Carbon Nanotube Fiber Based Stretchable Conductor

Carbon nanotube (CNT) based continuous fiber, a CNT assembly that could potentially retain the superb properties of individual CNTs on a macroscopic scale, belongs to a fascinating new class of electronic materials with potential applications in electronics, sensing, and conducting wires. Here, the fabrication of CNT fiber based stretchable conductors by a simple prestraining‐then‐buckling approach is reported. To enhance the interfacial bonding between the fibers and the poly(dimethylsiloxane) (PDMS) substrate and thus facilitate the buckling formation, CNT fibers are first coated with a thin layer of liquid PDMS before being transferred to the prestrained substrate. The CNT fibers are deformed into massive buckles, resulting from the compressive force generated upon releasing the fiber/substrate assembly from prestrain. This buckling shape is quite different from the sinusoidal shape observed previously in otherwise analogous systems. Similar experiments performed on carbon fiber/PDMS composite film, on the other hand, result in extensive fiber fracture due to the higher fiber flexural modulus. Furthermore, the CNT fiber/PDMS composite film shows very little variation in resistance (≈1%) under multiple stretching‐and‐releasing cycles up to a prestrain level of 40%, indicating the outstanding stability and repeatability in performance as stretchable conductors.     

Visualizing Strain Evolution and Coordinated Buckling within CNT Arrays by In Situ Digital Image Correlation

Spatial mapping of strain fields within compressed carbon nanotube (CNT) array columns is achieved using digital image correlation (DIC) analysis of in situ scanning electron microscopy (SEM) image sequences. Full‐field displacement and strain maps are generated based upon the motion of the constituent CNTs, which serve as a traceable high‐contrast speckle pattern for DIC analysis. The deformation modes and CNT array buckling characteristics vary systematically as a function of column aspect ratio, including bending, crushing, and bottom‐up buckle accumulation behaviors. In spite of disparate appearing deformation modes, strain maps indicate that CNT array buckling consistently initiates at 5% local principal strain (?₂) for all columns. The ability to quantitatively assess the deformation modes and buckling behavior of CNT arrays at the nanoscale will enable their improved design for high‐strain electrical contacts, compliant thermal interfaces, force sensors, energy‐absorbing foams, or other applications.     

Micrometer‐Scale Porous Buckling Shell Actuators Based on Liquid Crystal Networks

Micrometer‐scale liquid crystal network (LCN) actuators have potential for application areas like biomedical systems, soft robotics, and microfluidics. To fully harness their power, a diversification in production methods is called for, targeting unconventional shapes and complex actuation modes. Crucial for controlling LCN actuation is the combination of macroscopic shape and molecular‐scale alignment in the ground state, the latter becoming particularly challenging when the desired shape is more complex than a flat sheet. Here, one‐step processing of an LCN precursor material in a glass capillary microfluidic set‐up to mold it into thin shells is used, which are stretched by osmosis to reach a diameter of a few hundred micrometers and thickness on the order of a micrometer, before they are UV crosslinked into an LCN. The shells exhibit radial alignment of the director field and the surface is porous, with pore size that is tunable via the osmosis time. The LCN shells actuate reversibly upon heating and cooling. The decrease in order parameter upon heating induces a reduction in thickness and expansion of surface area of the shells that triggers continuous buckling in multiple locations. Such buckling porous shells are interesting as soft cargo carriers with capacity for autonomous cargo release.     

Buckling‐Based Strong Dry Adhesives Via Interlocking

High‐aspect‐ratio shape‐memory polymer (SMP) pillar arrays are investigated as a new type of dry adhesive based on buckling and interlocking mechanism. When two identical SMP pillar arrays are engaged at 80 °C, above the glass transition temperature at a preload larger than the critical buckling threshold, the pillars are deformed and become interweaved and/or indented with each other. After cooling to room temperature, strong pull‐off forces are observed in the normal and shear directions, both of which are much larger than those from pillar‐to‐flat surface and flat‐to‐flat surface contact. From finite element anaylsis (FEA) and comparison of measured and calculated adhesion values using different contact mechanics models, it is shown that interweaved pillars are the main source that contributes to the pillar‐to‐pillar adhesion and the indented pillars set the lower limit, whereas the probability of interdigitation is very low. Further, it is found that interweaved pillars are primarily responsible for the decreased adhesion strength and increased anisotropy when the pillar spacing became larger. Finally, it is shown that the bonded pillars can be easily separated after reheating to 80 °C due to significant drop of modulus of SMPs.     

Transforming One‐Dimensional Nanowalls to Long‐Range Ordered Two‐Dimensional Nanowaves: Exploiting Buckling Instability and Nanofibers Effect in Holographic Lithography

Two‐dimensional nanowaves with long‐range order are fabricated by exploiting swelling‐induced buckling of one‐dimensional (1D) nanowalls with nanofibers formed in‐between during holographic lithography of the negative‐tone photoresist SU‐8. The 1D film goes through a constrained swelling in the development stage, and becomes buckled above the critical threshold. The degree of lateral undulation can be controlled by tuning the pattern aspect ratio (height/width) and exposure dosage. At a high aspect ratio (e.g., 6) and a high exposure dosage, nanofibers (30–50 nm in diameter) are formed between the nanowalls as a result of overlapping of low crosslinking density regions. By comparing experimental results with finite‐element analysis, the buckling mechanism is investigated, which confirms that the nanofibers prevent the deformed nanowalls from recovery to their original state, thus, leading to long‐range ordered two‐dimensional (2D) wavy structures. The film with nanowaves show weaker reflecting color under an ambient light and lower transmittance compared to the straight nanowalls. Using double exposure through a photomask, patterns consisting of both nanowaves and nanowalls for optical display are created.     

Biasing Buckling Direction in Shape‐Programmable Hydrogel Sheets with Through‐Thickness Gradients

A photocrosslinkable poly( N , N ′‐diethylacrylamide) copolymer allows for the photolithographic fabrication of hydrogel sheets with nonuniform crosslinking density and swelling ratio. Using this material system, different 3D shapes with nonzero Gaussian curvature K are successfully programmed by prescribing a “metric” defined by in‐plane variations in swelling. However, this methodology does not control the direction of buckling adopted by each positive K feature, and therefore cannot controllably select between different isometric shapes defined by a single metric. Here, by introducing gradients in swelling through the thickness of the gel sheet by tuning the absorption of the UV‐light used for crosslinking, a preferential buckling direction is locally specified for each feature by the direction of UV exposure. By also controlling the strength of coupling between neighboring features, this is shown to be an effective method to program buckling direction of each unit within a canonical corrugated surface shape.     

基于某伺服转台的摇臂设计分析

桁架是工程及建筑中常见的一种结构形式,由于其在结构中的重要作用,设计后必须对其进行力学分析。以桁架的结构形式设计了一个伺服转台摇臂,利用ANSYS Workbench软件对其进行强度分析、屈曲分析以及模态分析,得到合理的结构。通过强度分析确定摇臂的结构形式,然后结合屈曲分析得到摇臂的合理尺寸,最后再对摇臂进行模态分析。     

Fabrication of a Macroporous Microwell Array for Surface‐Enhanced Raman Scattering

Here, a colloidal templating procedure for generating high‐density arrays of gold macroporous microwells, which act as discrete sites for surface‐enhanced Raman scattering (SERS), is reported. Development of such a novel array with discrete macroporous sites requires multiple fabrication steps. First, selective wet‐chemical etching of the distal face of a coherent optical fiber bundle produces a microwell array. The microwells are then selectively filled with a macroporous structure by electroless template synthesis using self‐assembled nanospheres. The fabricated arrays are structured at both the micrometer and nanometer scale on etched imaging bundles. Confocal Raman microscopy is used to detect a benzenethiol monolayer adsorbed on the macroporous gold and to map the spatial distribution of the SERS signal. The Raman enhancement factor of the modified wells is investigated and an average enhancement factor of 4 × 10⁴ is measured. This demonstrates that such nanostructured wells can enhance the local electromagnetic field and lead to a platform of ordered SERS‐active micrometer‐sized spots defined by the initial shape of the etched optical fibers. Since the fabrication steps keep the initial architecture of the optical fiber bundle, such ordered SERS‐active platforms fabricated onto an imaging waveguide open new applications in remote SERS imaging, plasmonic devices, and integrated electro‐optical sensor arrays.     

Competition between the buckling-driven delamination and wrinkling in compressed thin coatings

The competition between two common failure modes of a thin coating under in-plane compression, the surface wrinkling and the buckling-driven delamination, is studied to assess the critical strain when the mechanical instability may occur at given geometrical and material parameters. A buckling map is constructed based on results of a finite element analysis, which relates the critical applied strain for the onset of instability to the interface adhesion and elastic properties of materials. An approximate scaling relation is derived for the energy release rate of buckling-driven delamination of a coating deposited on a compliant substrate.     

Foam‐Like Behavior in Compliant,Continuously Reinforced Nanocomposites

In the pursuit of advanced polymer composites, nanoscale fillers have long been championed as promising candidates for structural reinforcement. Despite progress, questions remain as to how these diminutive fillers influence the distribution of stresses within the matrix and, in turn, influence bulk mechanical properties. The dynamic mechanical behavior of elastomer‐impregnated forests of carbon nanotubes (CNTs) has revealed distinct orientation‐dependent behavior that sheds light on these complicated interactions. When compressed along the axis of the fillers, the composite will mimic open‐cell foams and exhibit strain softening for increasing amplitudes due to the collective Euler buckling of the slender nanotubes. In contrast, the same material will behave similarly to the neat polymer when compressed orthogonal to the alignment direction of the nanotubes. However, in this orientation the material is incapable of achieving the same ultimate compressive strain due to the role that the embedded nanotubes play in augmenting the effective cross‐link density of the polymer network. Both of these responses are recoverable, robust, and show little dependency on the diameter and wall‐number of the included CNTs. Such observations give insight into the mechanics of polymer/nanoparticle interactions in nanocomposite structures under strain, and the thoughtful control of such coordinated buckling behavior opens the possibility for the development of foam‐like materials with large Poisson ratios.     

Buckled Conductive Polymer Ribbons in Elastomer Channels as Stretchable Fiber Conductor

Conductors that can sustain large strains without change in resistance are highly needed for wearable electronic systems. Here, the fabrication of highly stretchable coaxial fiber conductors through self‐buckling of conductive polymer ribbons inside thermoplastic elastomer channels, using a “solution stretching–drying–buckling” process, is reported. The unique hierarchically buckled and conductive core in the axial direction makes the resistance of the fiber very stable, with less than 4% change when applying as much as 680% strain. These fibers can then be directly used as stretchable electrical interconnects or wearable heaters.     

Robust Watermarking Scheme Based on Radius Weight Mean and Feature‐Embedding Technique

In this paper, the radius weight mean (RWM) and the feature‐embedding technique are used to present a novel watermarking scheme for color images. Simulations validate that the stego‐images generated by the proposed scheme are robust against most common image‐processing operations, such as compression, color quantization, bit truncation, noise addition, cropping, blurring, mosaicking, zigzagging, inversion, (edge) sharpening, and so on. The proposed method possesses outstanding performance in resisting high compression ratio attacks: JPEG2000 and JPEG. Further, to provide extra hiding storage, a steganographic method using the RWM with the least significant bit substitution technique is suggested. Experiment results indicate that the resulting perceived quality is desirable, whereas the peak signal‐to‐noise ratio is high. The payload generated using the proposed method is also superior to that generated by existing approaches.