Bilayer Strips Research Articles

Modeling the Stimulus-Responsive Behaviors of Fiber-Reinforced Soft Materials

Hydrogels can change their size upon swelling. The swelling ratio is the same for all directions in the stress-free state. Dielectric elastomers can reduce their thickness and expand the area upon an electric field. Similarly, the expansion ratio in the plane is also the same for different directions. This isotropic shape change effect limits the function of these soft materials in certain circumstances. To address this issue, recent works have shown that the incorporation of fibers into the polymer matrix can induce an anisotropic response upon external stimulus. In this work, we develop multi-field coupling models for both fiber-reinforced hydrogels and dielectric elastomers. For the former, the change in free energy is caused by the stretching of polymer chains and fibers and the mixing of solvents and polymer networks. The Fickian-type law is adopted for the solvent diffusion. The free energy density for the latter consists of a mechanical part, considering the deformation of both polymer matrix and fibers, and an electric polarization component. Gauss’s law is adopted to obtain the distribution of the electric field. The multi-field models are then implemented for finite element analysis. We consider the stimulus-responsiveness of bilayer strips with an active layer and a passive layer. Without fibers, the strips bend upon the external stimulus. In contrast, the shape changes to helix shapes, which can be further tuned by changing the distribution of fibers. The work provides an efficient design tool for self-folding structures based on stimulus-responsive polymers.

Elastic/viscoelastic polymer bilayers: a model-based approach to stretch-responsive constructs.

The use of polymers in the fabrication of bilayers for stimuli-responsive systems is well-known, yet viscoelasticity and viscoelastic models representing bilayer behavior have received surprisingly little attention. Of particular recent interest to us are simple polymeric bilayers in which one material, such as styrene-ethylene-propylene-styrene (SEPS) or styrene-isobutylene-styrene (SIBS), shows typical rubbery elastic response upon extension and retraction, and the other, an unvulcanized, low-Tg polymer such as butyl rubber (butyl), exhibits a viscoelastic response. When such a bilayer strip is extended to a fixed strain and held for several seconds followed by sudden release of this strain, rapid curling is observed, achieving a maximum curvature within 1 second, with a gradual uncurling, typically taking 300-600 seconds to eventually return to a flat strip. Attention has been directed to modeling the observed bilayer behavior. We compare predicted curvature and relaxation time constants from finite element analysis (FEA) simulations using Maxwell, Zener, Generalized Maxwell, and Parallel Rheological Framework (PRF) viscoelastic models to the experimentally measured values. We find that the Generalized Maxwell model predicts curvature over time with the lowest overall mean absolute scaled error (MASE) of 0.519, corresponding to a 4.9% difference from the second lowest error model and a 76.8% difference from the highest error model. Building upon an understanding of the material mechanics in simple bilayer strips, more complex bilayer systems can be designed. Samples of cross and weave geometries were fabricated from bilayer films and initial testing demonstrates how these materials can be used in potential applications.

Study of plane strain compression of composite sheets

In the plane strain compression of multi-layers metal, each layer of metal shows a different degree of deformation due to the differences in material properties. Also, the deformation in different positions of a metal layer is always inhomogeneous. In this study, the effects of material ratios, friction between the metal layers and the amount of compression on the deformation behaviors of a bi-layer metal strip under plane strain compression are investigated. 2D plane strain finite element simulation models of AISI-1008/AL6061-T6 bi-layer metal strips are used. The results indicate that the strips show different deformation behaviors under different conditions, and non-smooth interfaces caused by the inhomogeneous material flow during the deformation process are detected in most cases.

Paper-Based Bi-Material Cantilever Actuator Bending Behavior and Modeling.

In this paper, the behavior of the Bi-Material Cantilever (B-MaC) response deflection upon fluidic loading was experimentally studied and modeled for bilayer strips. A B-MaC consists of a strip of paper adhered to a strip of tape. When fluid is introduced, the paper expands while the tape does not, which causes the structure to bend due to strain mismatch, similar to the thermal loading of bi-metal thermostats. The main novelty of the paper-based bilayer cantilevers is the mechanical properties of two different types of material layers, a top layer of sensing paper and a bottom layer of actuating tape, to create a structure that can respond to moisture changes. When the sensing layer absorbs moisture, it causes the bilayer cantilever to bend or curl due to the differential swelling between the two layers. The portion of the paper strip that gets wet forms an arc, and as the fluid advances and fully wets the B-MaC, the entire B-MaC assumes the shape of the initial arc. This study showed that paper with higher hygroscopic expansion forms an arc with a smaller radius of curvature, whereas thicker tape with a higher Young's modulus forms an arc with a larger radius of curvature. The results showed that the theoretical modeling could accurately predict the behavior of the bilayer strips. The significance of paper-based bilayer cantilevers lies in their potential applications in various fields, such as biomedicine, and environmental monitoring. In summary, the novelty and significance of paper-based bilayer cantilevers lie in their unique combination of sensing and actuating capabilities using a low-cost and environmentally friendly material.

Strategies for Developing Shape-Shifting Behaviours and Potential Applications of Poly (N-vinyl Caprolactam) Hydrogels.

Stimuli-responsive hydrogels are one type of smart hydrogel, which can expand/contract in water according to changes in the surrounding environment. However, it is difficult to develop flexible shapeshifting behaviours by using a single hydrogel material. This study exploited a new method to utilise single and bilayer structures to allow hydrogel-based materials to exhibit controllable shape-shifting behaviours. Although other studies have demonstrated similar transformation behaviours, this is the first report of such smart materials developed using photopolymerised N-vinyl caprolactam (NVCL)-based polymers. Our contribution provides a straightforward method in the fabrication of deformable structures. In the presence of water, the bending behaviours (vertex-to-vertex and edge-to-edge) were achieved in monolayer squares. By controlling the content and combination of the NVCL solutions with elastic resin, the bilayer strips were prepared. The expected reversible self-bending and self-helixing behaviours were achieved in specific types of samples. In addition, by limiting the expansion time of the bilayer, the layered flower samples exhibited predictable self-curving shape transformation behaviour in at least three cycles of testing. These structures displayed the capacity of self-transformation, and the value and functionality of the produced components are reflected in this paper.

Ratchetlike motion of helical bilayers induced by boundary constraints.

We show that application of boundary constraints generates unusual folding behaviors in responsive (swellable) helical bilayer strips. Unlike the smooth folding trajectories typical of free helical bilayers, the boundary-constrained bilayers exhibit intermittent folding behaviors characterized by rapid, steplike movements. We experimentally study bilayer strips as they swell and fold, and we propose a simple model to explain the emergence of ratchetlike behavior. Experiments and model predictions are then compared to simulations, which enable calculation of elastic energy during swelling. We investigate the dependence of this steplike behavior as a function of elastic boundary condition strength, strip length, and strip shape; interestingly, "V-shape" strips with the same boundary conditions fold smoothly.

Development of a Material Design Space for 4D-Printed Bio-Inspired Hygroscopically Actuated Bilayer Structures with Unequal Effective Layer Widths.

(1) Significance of geometry for bio-inspired hygroscopically actuated bilayer structures is well studied and can be used to fine-tune curvatures in many existent material systems. We developed a material design space to find new material combinations that takes into account unequal effective widths of the layers, as commonly used in fused filament fabrication, and deflections under self-weight. (2) For this purpose, we adapted Timoshenko’s model for the curvature of bilayer strips and used an established hygromorphic 4D-printed bilayer system to validate its ability to predict curvatures in various experiments. (3) The combination of curvature evaluation with simple, linear beam deflection calculations leads to an analytical solution space to study influences of Young’s moduli, swelling strains and densities on deflection under self-weight and curvature under hygroscopic swelling. It shows that the choice of the ratio of Young’s moduli can be crucial for achieving a solution that is stable against production errors. (4) Under the assumption of linear material behavior, the presented development of a material design space allows selection or design of a suited material combination for application-specific, bio-inspired bilayer systems with unequal layer widths.

Actuated bending of a long cantilevered glassy nematic bilayer strip: failure of the beam model

Cantilevered long glassy nematic bilayer strips bend in response to light, and thus are attractive for actuation applications. The objective of this work is to examine whether the deformation can be predicted by the one-dimensional beam model as in usual practice. To do so, we consider a bilayer with orthogonal director orientations in the upper and lower layers and analyze its bending in the framework of the classical plate theory. We find that the deflection is much larger than that given by the beam model, because the spontaneous bending in the width direction increases the curvature along the long axis. Therefore, the one-dimensional beam model fails to describe the bending behavior of the cantilevered bilayer strips.

Photoresponse of current-biased superconductor/normal metal strip with large ratio of resistivities

We study the perspective of thin bilayer strips made of highly resistive (in the normal state) superconductor (S) and low resistive normal metal (N) as a detector of electromagnetic radiation. A small minigap in the quasiparticle spectra, induced in the N layer due to proximity induced superconductivity from the S layer and the ability of low resistive N layer to carry a larger superconducting current than the host dirty superconductor leads to steep temperature dependence of critical current of SN strip in wide temperature ranges. It results in higher sensitivity of current-biased SN strip to the electromagnetic radiation in comparison with S strip. We present experimental results on several microns wide MoN/Cu and MoN strips, which confirm this statement in the bolometric regime. We also discuss possible advantages of single photon counting by narrow SN strips.

Computational analysis of hygromorphic self-shaping wood gridshell structures.

Bi-layered composites capable of self-shaping are of increasing relevance to science and engineering. They can be made out of anisotropic materials that are responsive to changes in a state variable, e.g. wood, which swells and shrinks by changes in moisture. When extensive bending is desired, such bilayers are usually designed as cross-ply structures. However, the nature of cross-ply laminates tends to prevent changes of the Gaussian curvature so that a plate-like geometry of the composite will be partly restricted from shaping. Therefore, an effective approach for maximizing bending is to keep the composite in a narrow strip configuration so that Gaussian curvature can remain constant during shaping. This represents a fundamental limitation for many applications where self-shaped double-curved structures could be beneficial, e.g. in timber architecture. In this study, we propose to achieve double-curvature by gridshell configurations of narrow self-shaping wood bilayer strips. Using numerical mechanical simulations, we investigate a parametric phase-space of shaping. Our results show that double curvature can be achieved and that the change in Gaussian curvature is dependent on the system’s geometry. Furthermore, we discuss a novel architectural application potential in the form of self-erecting timber gridshells.

An autonomous shading system based on coupled wood bilayer elements

Climate Adaptive Building Shells can help to reach the aimed and required global reduction of energy consumption in the building sector. By being implemented as, e.g., façade shading systems, their adaptability to environmental changes can improve energy efficiency and indoor comfort of buildings. Autonomous, humidity-driven wood bilayers are proposed as an alternative to motor-driven façade shading elements. Due the hygro-responsiveness of the wood material, the changes of relative humidity during day and night as well as the drying effect of direct solar radiation can be utilized for inducing cyclic programmed shape changes of wood bilayers for aperture opening and closing of such adaptive façade shading systems. The kinetics of such autonomous shape changes of wood bilayers have been analyzed at small scale, but the application-relevant upscaling remains a challenge. So far, the proposed solutions do not allow maintaining a sufficiently high rate of shape change and the required mechanical stability of the wood bilayers at the same time. Here, we present the coupling of two wood bilayers as one possible solution for the implementation as shading elements. By coupling, the rate of aperture opening and closing can be amplified without increasing the (limited) rate of shape change of the single wood bilayers. The coupling and the resulting combination of shape change and rotation are characterized for five different typologies of lever arm configuration. Based on first studies with bilayer strips, an upscaling in width is conducted and its implementation for a humidity-driven shading system is presented by means of a demonstrator.

Polarized surface-enhanced Raman spectroscopy of suspended carbon nanotubes by Pt-Re nanoantennas

We present optical nanoantennas designed for applications that require processing temperatures larger than 800{\deg}C. The antennas consist of arrays of Re/Pt bilayer strips fabricated with a lift-off-free technique on top of etched trenches. Reflectance measurements show a clear plasmonic resonance at approximately 670 nm for light polarized orthogonal to the strip axis. The functionality of the antennas is demonstrated by growing single-walled carbon nanotubes (CNTs) on top of the antenna arrays and measuring the corresponding Raman signal enhancement of individual CNTs. The results of the measurements are quantitatively discussed in light of numerical simulations which highlight the impact of the substrate.

An analytical solution of the ECAE process of bi-layer strip using a Bezier type streamline

Co-extrusion of strips of different strength and ductility is a process in the production of bimetal rods of desirable physical and mechanical properties. Equal channel angular extrusion (ECAE) is an innovative process for applying severe plastic deformation to materials. The present study is concerned with the ECAE of different bi-layer strips. Developing a kinematically admissible velocity field using a Bezier-type streamline is the aim of present research. In this analysis, using the mentioned velocity field and the upper bound theorem, the extrusion force as well as the strain distribution in the deformation zone of the ECAE process are predicted. The solution takes into account the die profile, friction conditions, strength, arrangement and thickness ratio of the layer materials. Experimental results of ECAE of aluminum/copper and aluminum/mild steel bi-layer strips showed a good agreement with the predicted results.

Numerical Analysis on 2-D Standing Spin Waves in Microstructured Exchange-Coupled Hard/Soft Bilayer Strips

Geometrically confined 2-D standing spin waves (SSWs) excited in exchange-coupled hard/soft bilayer strips have been systematically investigated by micromagnetic simulation. The resonance frequency of the SSW was found to be tunable in a wide frequency range by adjusting the thickness ratio of the hard and soft layers. The dependence of the resonance frequency on various structural and material parameters has been systematically investigated.

Electrical oscillation in Pt/VO2 bilayer strips

We report on the observation of stable electrical oscillation in Pt/vanadium dioxide (VO2) bilayer strips, in which the Pt overlayer serves the dual purposes of heating up the VO2 and weakening the electric field in the VO2 layer. Systematic measurements in an ultrahigh vacuum nanoprobe system show that the oscillation frequency increases with the bias current and/or with decreasing device dimension. In contrast to most VO2-based oscillators reported to date, which are electrically triggered, current-induced Joule heating in the Pt overlayer is found to play a dominant role in the generation of oscillation in Pt/VO2 bilayers. A simple model involving thermally triggered transition of VO2 on a heat sink is able to account for the experimental observations. The results in this work provide an alternative view of the triggering mechanism in VO2-based oscillators.

Residual stress and mechanical property measurements in amorphous Si photovoltaic thin films

The mechanical reliability and efficiency of thin film photovoltaics attached to structural members depends on the initial state of residual stresses in the films. In this work, predictions for the mechanical and functional failure of photovoltaic films co-cured with carbon fiber composite laminates were made possible by quantifying the mean and gradient residual stresses and the failure properties of the individual layers in thin film inorganic photovoltaics consisting of an amorphous silicon (Si) p–n junction diode, a zinc oxide (ZnO) Transparent Conductive Oxide (TCO) layer (each 1μm thick), a Kapton™ layer, and a thick aluminum substrate. The mean residual stresses (−466±118) MPa in the Si monolayer and the Si/ZnO bilayer (−661±93) MPa were calculated from the geometrical details of straight and telephone cord type buckling delaminations induced to the p–n junction layer and its combination with the TCO layer, respectively. Curvature measurements provided the residual stress gradient of the Si monolayer as 274±20MPa/μm and the stress gradient profile in the Si/ZnO bilayer. The tensile strength of freestanding amorphous Si monolayer and Si/ZnO bilayer strips was measured as 425±75MPa and 109±23MPa, respectively. These microscale tension experiments also showed that there is weak adhesion between the Si and the mechanically weak ZnO layers. The aforementioned experimental results were employed to predict the onset of fragmentation of the ZnO layer and the initiation of functional degradation of the photovoltaic films that were co-cured with 0° carbon fiber composite laminates, at 0.3% and 0.9% applied strain, respectively, which was in very good agreement with experimental measurements at the composite level.

Bond strength evaluation of roll bonded bi-layer copper alloy strips in different rolling conditions

Roll bonding is a solid-state welding process to join similar and dissimilar metals and alloys and is widely used as a manufacturing process. In this study, bi-layer copper alloy strips were roll bonded at warm and cold conditions; and then the effects of parameters that create successful bonds, such as the amount of deformation by rolling and rolling temperature on the bond strength were investigated. For this purpose, faying surfaces were degreased and scratch brushed, being fractured coherently after entering roll gap at different reductions. As deformation proceeded and the roll pressure increased, these cracks quickly expanded into fissures. This process allowed the bond to be established between the underlying materials of the base metal layers, termed virgin metal, which were extruded through the cracks and fissures at the scratch brushed regions. The peel strengths of the bonds were measured and were found to increase when the rolling temperature or the thickness reduction are increased.

Dependence of force produced by polypyrrole-based artificial muscles on ionic species involved

Artificial muscles have been fabricated in the form bilayer strips using an insulating polymer layer and polypyrrole (PPy) conducting polymer film, and the force produced by them during redox processes have been investigated. This study reports the effects of anions in the polymerization electrolyte and the cations in the cycling electrolyte on the force generated by such muscles. The size of anions used during polymerization of PPy films have greater effect on the force produced by the muscles made with such films. Higher forces are obtained with larger surfactant anions. Size of cations in the cycling electrolytes also plays a major role in determining the force exerted by the muscles.

Bending bilayer strips built from polyaniline for artificial electrochemical muscles

Electrically conducting conjugated polymers such as polyaniline (PANi) usually show gel properties with volume changes in the range 1-100% in response to some external stimuli. The bending beam method provides an effective and sensitive way to detect and make use of these volume changes. Bipolymer laminate strips (2.5*0.2 cm2), made of a PANi layer (10 mu m thick) and a substrate polyimide layer (45 mu m thick) bound together, were prepared. These strips bend, corresponding to volume changes in the PANi layer, during electrochemical redox of the PANi in aqueous solutions. The extension (<or=1.0 cm) and velocity (<or=48 cm min-1) of the movement of the strips can be controlled by applying appropriate potentials and currents. The authors obtained significant and reversible movement back and forth in seconds.