Gal Debotton

We report that a dielectric polymer chain, constrained at both ends, sharply collapses when exposed to a high electric field. The chain collapse is driven by nonlocal dipolar interactions and anisotropic polarization of monomers, a characteristic of real polymers that prior theories were unable to incorporate. Once collapsed, a large number of chain monomers accumulate at the center location between the chain ends, locally increasing the electric field and polarization by orders of magnitude. The chain collapse is sensitive to the orientation of the applied electric field and chain stretch. Our findings not only offer new ways for rapid actuation and sensing but also provide a pathway to discover the critical physics behind instabilities and electrical breakdown in dielectric polymers.

An equibiaxially stretched thin neo-Hookean circular membrane with a hole at its center under plane-stress condition is analyzed within the framework of finite deformation elasticity. Initially, we introduce a novel form for the differential governing equation to the problem. This enables the introduction of a closed-form solution in the limit of infinite stretch. Comparison of this solution to corresponding finite element simulations reveals a neat agreement for stretch ratios larger than 2.5. In the practically important case of a small hole, at the circumference of the hole, the stress concentration factor is 4 and the tangential stretch ratio is twice the applied far-field stretch ratio. These values are double the corresponding ratios in the well-known limit of infinitesimal deformation.

Background: The behavior of shape memory alloys that admit large reversible deformations in response to thermal excitation has been extensively studied in recent years. Yet, the number of works dealing with springs made from these alloys is rather limited in spite of their attractiveness in various applications. Objective: To bridge this gap we designed and constructed an experimental system for characterizing the behavior of the springs. It enables precise control of the three state variables: temperature, elongation, and force. Methods: Control of the sample temperature is achieved by immersing it in a water-filled thermal bath, where the water temperature is adjusted using a thermoelectric Peltier device. A tension-compression motorized unit sets the spring elongation and a force gauge is used for measuring the force exerted on the spring. The data is continuously monitored and acquired with a self-coded LabVIEW program. An important aspect is the calibration procedure developed for identifying the spring load-free state and ensuring the repetitiveness of the measurements. Results: Experiments in which the elongation or the force were measured as a function of the temperature demonstrate the role of the phase transformations. Isothermal experiments enabled to characterize the variations of the force versus the elongation at different temperatures. Conclusions: The proposed system facilitates the execution of highly accurate experiments through which the complex history-dependent behavior of shape memory springs can be revealed and studied.

In the last decade, we witness an active search for an efficient, reliable and long-lasting power source for miniature devices and wireless sensors. One of the promising energy conversion methods for the development of such a power source is the direct-charge energy-harvesting method. This technique is implemented in a vacuum capacitor arrangement. In this setting, the kinetic energy of the emitted particles is converted into usable electrical energy without intermediate steps. In this work, we introduce a systematic analysis of the performance of this method in terms of appropriate transfer functions describing the main energy loss mechanisms that govern its efficiency. The individual contributions of these mechanisms are determined and integrated in order to evaluate the overall efficiency. To demonstrate the validity of our analysis, it is applied to a specific harvester for which experimental measurements are available. The predicted efficiency and volumetric power density are compared with the experimental findings under various operational configurations. Excellent agreement of our predicated performances with the experimental results is revealed. The analysis enables to predict the performance envelope of the harvester. Thus, it can be used for the design and optimization of direct charge β -radiation devices. In the specific device examined, the predicted highest efficiency is 13.5%.

Thin-wall composite spheres (TWCSs) are very common in both natural and artificial structures. Their response to mechanical loading was investigated in the past almost solely in the limit of infinitesimal deformations. We examine, within the framework of finite deformation elasticity, the mechanics of incompressible TWCSs with neo-Hookean core and shell phases subjected to general homogeneous displacement and traction boundary conditions. We derive explicit general forms for the displacement and the pressure fields in both phases in terms of a power series about the shear and the tension magnitudes and the shell volume fraction. The predictions of the analytical solutions are analyzed and compared with corresponding results of finite element simulations for TWCSs with different ratios between the phases shear moduli. In addition to an extension of the work of Weil and deBotton [22] from simple shear to general homogeneous boundary conditions, we modify the power series solution and provide a reliable solution for any combination of phases shear modulus. We demonstrate that a relatively small number of terms in the series is required for a good agreement with the numerical simulations up to a stretch ratio of 1.5 when considering the local fields, and up to a stretch ratio of 2 when considering the average fields. The analysis emphasizes the interaction between the shell and the core and reveals the different roles of the coating under different boundary conditions. We highlight interesting similarities and dissimilarities between the spatial distributions of the local stresses and the variations of the average stresses developing in TWCSs with stiff and soft shells, under displacement and traction boundary conditions.