Stacked dielectric tubes with electromechanically controlled radii

This work examines the features and capabilities of a tube comprised of stacked cylindrical dielectric layers separated by flexible electrodes with the applicative aim of electrically controlling the inner radius. The study begins with the analysis of the non-homogenous electromechanical response of a single-layer dielectric tube according to well-established coupled models. Two boundary conditions are examined - traction free boundaries and a fixed outer radius. The advantages and limitations of each boundary conditions are discussed. It is shown that dielectric tubes subjected to traction free boundaries experience instabilities, an effect that can be avoided by fixing the outer surface. Next, the electromechanical behavior of a stacked cylindrical actuator comprised of dielectric tubes that are mechanically connected in series and electrically connected in parallel is determined under the two boundary conditions discussed above. It is shown that the stacking of cylindrical layers increases the range of available inner radii at the cost of design limitations. Interestingly, it is found that mounting layers on a stacked cylindrical actuator may lead to instabilities even if the outer radius is fixed

Electromechanical Interplay in Deformable Dielectric Elastomer Networks

A systematic, statistical-mechanics-based analysis of the response of dielectric elastomers to coupled electromechanical loading is conducted, starting from the monomer level through the polymer chain and ending with closed-form expressions for the polarization and stress fields. It is found that the apparent response at the macrolevel is dictated by four microscopic parameters—the monomer type and polarizability and the chain length and density. Our analysis further reveals a new electrostrictive effect that either reinforces or opposes the polarization-induced deformation. The validity of the results is attested through comparisons with well-established experimental measurements of both the polarization field and the electrostrictive stress

Multiscale analysis of the coupling between mechanics and electrostatics in polymer chain networks

Electroactive polymers (EAPs) are materials capable of undergoing large deformations when stimulated by an electric field. At the present, there are models describing the polymers uncoupled electrostatic response under the influence of an electric field at both the macroscopic and the microscopic levels. Similarly, there are models describing the polymers reaction to purely mechanical loadings, macroscopically as well as through their molecular microstructure. The connection between the micro- and the macroanalyses shed light on the overall response of polymers and provide tools for optimizing their performances. In recent years, the electro-mechanical coupling in EAPs has been characterized and modeled at the macroscopic-continuum level. To the best of our knowledge, the corresponding analysis at the molecular microscopic level is not available yet. Our studies [1–2] is aimed towards understanding and analyzing the relation between the structure of EAPs and the forces and stresses that develop due to electrostatic excitations. To this end we introduce a multiscale model that assumes known geometries of the chains before and after the deformation. In addition, a variational approach is used leading to the development of an expression for the internally stored electrical enthalpy in the polymer and the corresponding stresses that develop. In a way of an example a polymer with specific chain structure under constant electric excitation and axial deformation is examined. The results are compared with a common phenomenological model as well as with experimental findings. REFERENCES [1] Cohen, N., deBotton, G. The electromechanical response of polymer networks with long-chain molecules. Math. Mech. Solids. 2014 (to appear). [2] Cohen, N., deBotton, G. Multiscale analysis of the electromechanical response of dielectric elastomers. Eur. J. Mech. A-Solids, 2014

Stacked dielectric tubes with electromechanically controlled radii

This work examines the features and capabilities of a tube comprised of stacked cylindrical dielectric layers separated by flexible electrodes with the applicative aim of electrically controlling the inner radius. The study begins with the analysis of the non-homogenous electromechanical response of a single-layer dielectric tube according to well-established coupled models. Two boundary conditions are examined - traction free boundaries and a fixed outer radius. The advantages and limitations of each boundary conditions are discussed. It is shown that dielectric tubes subjected to traction free boundaries experience instabilities, an effect that can be avoided by fixing the outer surface. Next, the electromechanical behavior of a stacked cylindrical actuator comprised of dielectric tubes that are mechanically connected in series and electrically connected in parallel is determined under the two boundary conditions discussed above. It is shown that the stacking of cylindrical layers increases the range of available inner radii at the cost of design limitations. Interestingly, it is found that mounting layers on a stacked cylindrical actuator may lead to instabilities even if the outer radius is fixed

Lifestyle travellers: Backpacking as a way of life

Scholarship on backpackers speculates some individuals may extend backpacking to a way of life. This article empirically explores this proposition using lifestyle consumption as its framing concept and conceptualises individuals who style their lives around the enduring practice of backpacking as ‘lifestyle travellers’. Ethnographic interviews with lifestyle travellers in India and Thailand offer an emic account of the practices, ideologies and social identity that characterise lifestyle travel as a distinctive subtype within backpacking. Departing from the drifter construct, which (re)constitutes this identity as socially deviant, the concept of lifestyle allows for a contemporary appraisal of these individuals’ patterns of meaningful consumption and wider insights into how ongoing mobility can lead to different ways of understanding identities and relating to place. Keywords: lifestyle consumption; backpacker; mobility; drifter; identit

Electromechanical Interplay in Deformable Dielectric Elastomer Networks

A systematic, statistical-mechanics-based analysis of the response of dielectric elastomers to coupled electromechanical loading is conducted, starting from the monomer level through the polymer chain and ending with closed-form expressions for the polarization and stress fields. It is found that the apparent response at the macrolevel is dictated by four microscopic parameters—the monomer type and polarizability and the chain length and density. Our analysis further reveals a new electrostrictive effect that either reinforces or opposes the polarization-induced deformation. The validity of the results is attested through comparisons with well-established experimental measurements of both the polarization field and the electrostrictive stress

Minimum Neighboring Degree Realization in Graphs and Trees

We study a graph realization problem that pertains to degrees in vertex neighborhoods. The classical problem of degree sequence realizability asks whether or not a given sequence of n positive integers is equal to the degree sequence of some n-vertex undirected simple graph. While the realizability problem of degree sequences has been well studied for different classes of graphs, there has been relatively little work concerning the realizability of other types of information profiles, such as the vertex neighborhood profiles. In this paper we introduce and explore the minimum degrees in vertex neighborhood profile as it is one of the most natural extensions of the classical degree profile to vertex neighboring degree profiles. Given a graph G = (V,E), the min-degree of a vertex v ? V, namely MinND(v), is given by min{deg(w) ? w ? N[v]}. Our input is a sequence ? = (d_?^{n_?}, ?d?^{n?}), where d_{i+1} > d_i and each n_i is a positive integer. We provide some necessary and sufficient conditions for ? to be realizable. Furthermore, under the restriction that the realization is acyclic, i.e., a tree or a forest, we provide a full characterization of realizable sequences, along with a corresponding constructive algorithm. We believe our results are a crucial step towards understanding extremal neighborhood degree relations in graphs

Giant Superelastic Piezoelectricity in Flexible Ferroelectric BaTiO3BaTiO_3 Membranes

Mechanical displacement in commonly used piezoelectric materials is typically restricted to linear or biaxial in nature and to a few percent of the material dimensions. Here, we show that free-standing BaTiO$_3$ membranes exhibit non-conventional electromechanical coupling. Under an external electric field, these superelastic membranes undergo controllable and reversible 'sushi-rolling-like' 180$^\circ$ folding-unfolding cycles. This crease-free folding is mediated by charged ferroelectric domains, leading to a giant > 3.8 and 4.6 $\mu$m displacements for a 30-nm thick membrane at room temperature and 60$^\circ$C, respectively. Further increasing the electric field above the coercive value changes the fold curvature, hence augmenting the effective piezoresponse. Finally, it is found that the membranes fold with increasing temperature followed by complete immobility of the membrane above the Curie temperature, allowing us to model the ferroelectric-domain origin of the effect