Indentation of ultrathin elastic films and the emergence of asymptotic isometry

We study the indentation of a thin elastic film floating at the surface of a liquid. We focus on the onset of radial wrinkles at a threshold indentation depth and the evolution of the wrinkle pattern as indentation progresses far beyond this threshold. Comparison between experiments on thin polymer films and theoretical calculations shows that the system very quickly reaches the Far from Threshold (FT) regime, in which wrinkles lead to the relaxation of azimuthal compression. Furthermore, when the indentation depth is sufficiently large that the wrinkles cover most of the film, we recognize a novel mechanical response in which the work of indentation is transmitted almost solely to the liquid, rather than to the floating film. We attribute this unique response to a nontrivial isometry attained by the deformed film, and discuss the scaling laws and the relevance of similar isometries to other systems in which a confined sheet is subjected to weak tensile loads.Comment: 5 pages, close to published versio

Highly deformable actuators made of dielectric elastomers clamped by rigid rings

In the nascent field of soft machines, soft materials are used to create devices that actuate robots, sense environment, monitor health, and harvest energy. The soft materials undergo large deformation in response to external stimuli, often leading to instability that is usually undesirable but sometimes useful. Here, we study a dielectric elastomer membrane sandwiched between two soft conductors, rolled into a hollow tube, pre-stretched in the hoop direction, and fixed at the ends of the tube to two rigid rings. This structure functions as an electromechanical transducer when the two rings are subject to a mechanical force and the two conductors are subject to an electrical voltage. We formulate a computational model by using a variational principle and calculate the large and inhomogeneous deformation by solving a nonlinear boundary-value problem. We demonstrate that large actuation strains are achievable when the height-to-radius ratio of the tube is small and the hoop pre-stretch is large. The model provides a tool to analyze various modes of instability and optimize the electromechanical performance.Engineering and Applied Science

Large, Uni-directional Actuation in Dielectric Elastomers Achieved By Fiber Stiffening

Cylindrical actuators are made with dielectric elastomer sheets stiffened with fibers in the hoop direction. When a voltage is applied through the thickness of the sheets, large actuation strains are achievable in the axial direction, with or without pre-straining and mechanical loading. For example, actuation strains of 35.8% for a cylinder with a prestrain of 40%, and 28.6% for a cylinder without pre-strain have been achieved without any optimization. Furthermore, the actuation strain is independent of the aspect ratio of the cylinder, so that both large strains and large displacements are readily actuated by using long cylinders.Physic

Dielectric elastomer generator with equi-biaxial mechanical loading for energy harvesting

Dielectric elastomer generators (DEGs) are attractive candidates for harvesting electrical energy from mechanical work since they comprise relatively few moving parts and large elastomer sheets can be mass produced. Successfully demonstrations of the DEG prototypes have been reported from a diverse of energy sources, including ocean waves, wind, flowing water and human movement. The energy densities achieved, however, are still small compared with theoretical predictions. We show that significant improvements in energy density (550 J/kg with an efficiency of 22.1%), can be achieved using an equi-biaxial mechanical loading configuration, one that produces uniform deformation and maximizes the capacitance changes. Analysis of the energy dissipations indicates that mechanical losses, which are caused by the viscous losses both within the acrylic elastomer and within the thread materials used for the load transfer assembly, limits the energy conversion efficiency of the DEG. Addressing these losses is suggested to increase the energy conversion efficiency of the DEG. © (2013) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.Engineering and Applied Science

Maximizing the Energy Density of Dielectric Elastomer Generators Using Equi-Biaxial Loading

Dielectric elastomer generators (DEGs) for harvesting electrical energy from mechanical work have been demonstrated but the energy densities achieved are still small compared with theoretical predictions. We show that significant improvements in energy density (560 J/kg with a power density of 280 W/kg and an efficiency of 27%) can be achieved using equi-biaxial stretching, a mechanical loading configuration that maximizes the capacitance changes. We demonstrate the capacitance of dielectric elastomers subjected to equi-biaxial stretches is proportional to the fourth power of the stretch. Quantification of the individual energy contributions indicates that attaining higher conversion efficiencies is limited by viscous losses within the acrylic elastomer, suggesting that still higher conversion efficiencies with other elastomers should be attainable with our novel mechanical loading design.Engineering and Applied SciencesPhysic

Optimizing the Electrical Energy Conversion Cycle of Dielectric Elastomer Generators

Engineering and Applied Science

Spontaneous and Deterministic Three-Dimensional Curling of Pre-Strained Elastomeric Bi-Strips

Three dimensional curls (“hemi-helices”) consisting of multiple, periodic and alternating helical sections of opposite chiralities, separated by perversions, are one of a variety of complex shapes that can be produced by a simple generic process consisting of pre-straining one elastomeric strip, joining it side-by-side to another and then releasing the bi-strip. The initial wavelength of the hemi-helix and the number of perversions are determined by the strip cross-section, the constitutive behavior of the elastomer and the value of the pre-strain. The hemi-helix has no net twist. Topologically, the perversions separate regions of the hemi-helix deforming principally by bending from those where twisting dominates.Engineering and Applied Science

Structural Transition from Helices to Hemihelices

Helices are amongst the most common structures in nature and in some cases, such as tethered plant tendrils, a more complex but related shape, the hemihelix forms. In its simplest form it consists of two helices of opposite chirality joined by a perversion. A recent, simple experiment using elastomer strips reveals that hemihelices with multiple reversals of chirality can also occur, a richness not anticipated by existing analyses. Here, we show through analysis and experiments that the transition from a helical to a hemihelical shape, as well as the number of perversions, depends on the height to width ratio of the strip's cross-section. Our findings provides the basis for the deterministic manufacture of a variety of complex three-dimensional shapes from flat strips

The Thickness and Stretch Dependence of the Electrical Breakdown Strength of an Acrylic Dielectric Elastomer

The performance of dielectric elastomer actuators is limited by electrical breakdown. Attempts to measure this are confounded by the voltage-induced thinning of the elastomer. A test configuration is introduced that avoids this problem: A thin sheet of elastomer is stretched, crossed-wire electrodes are attached, and then embedded in a stiff polymer. The applied electric field at breakdown, $E_{B}$, is found to depend on both the deformed thickness, h, and the stretch applied, $\lambda$. For the acrylic elastomer investigated, the breakdown field scales as $E_{B}$  =  51  h$^{ − 0.25 }$ $\lambda$$^{0.63}$. The test configuration allows multiple individual tests to be made on the same sheet of elastomer.Engineering and Applied Science

A smooth cascade of wrinkles at the edge of a floating elastic film

The mechanism by which a patterned state accommodates the breaking of translational symmetry by a phase boundary or a sample wall has been addressed in the context of Landau branching in type-I superconductors, refinement of magnetic domains, and compressed elastic sheets. We explore this issue by studying an ultrathin polymer sheet floating on the surface of a fluid, decorated with a pattern of parallel wrinkles. At the edge of the sheet, this corrugated profile meets the fluid meniscus. Rather than branching of wrinkles into generations of ever-smaller sharp folds, we discover a smooth cascade in which the coarse pattern in the bulk is matched to fine structure at the edge by the continuous introduction of discrete, higher wavenumber Fourier modes. The observed multiscale morphology is controlled by a dimensionless parameter that quantifies the relative strength of the edge forces and the rigidity of the bulk pattern.Comment: 4 pages, 4 figure