Dielectric elastomer composites: analytical and numerical non-convex homogenization methods and applications

With the practical objective of shedding light on promising experimental results that have recently identified dielectric elastomer composites as potential enablers of new technologies (essentially, as the next generation of soft sensors and actuators), this work puts forth analytical and numerical methods to determine the macroscopic elastic dielectric behavior of this class of soft electroactive materials directly in terms of their microscopic behavior. The macroscopic behavior of dielectric elastomer composites is first investigated within the classical asymptotic context of small deformations and moderate electric fields. Specifically, by a combination of analytical and numerical techniques, rigorous homogenization solutions are constructed for dielectric elastomer composites with general (possibly anisotropic) classes of two-phase particulate microstructures. Aimed at identifying what types of filler particles lead to enhanced elastic dielectric behaviors, these solutions are deployed to examine dielectric elastomers filled with stiff high-permittivity particles, high-permittivity particles that are liquid-like in mechanical behavior, and vacuous pores. In addition to generalizing the fundamental purely elastic and purely dielectric solutions of Eshelby and Maxwell to the coupled and nonlinear realm of electroelastostatics, the above-outlined rigorous asymptotic solutions turn out to be essential in the development of corresponding homogenization solutions for finite deformations and finite electric fields. Indeed, it is shown that they can be utilized as building blocks for the derivation of a general approximate homogenization solution for non-Gaussian dielectric elastomers filled with nonlinear elastic dielectric particles that may exhibit polarization saturation. By construction, this approximate solution is exact in the limit of small deformations and moderate electric fields. For finite deformations and finite electric fields, its accuracy is assessed by direct comparisons with full-field hybrid finite-element simulations, as well as with numerical solutions generated via a new WENO finite-difference scheme developed specifically for this class of problems. With the object of scrutinizing recent experimental results, the specializations of the proposed solution to various cases wherein the filler particles are of poly- and mono-disperse sizes and exhibit different types of elastic dielectric behaviors are discussed in detail. Stark disagreement between the theoretical results outlined above and a plurality of experimental results indicates that the basic point of view that dielectric elastomer composites can be idealized as two-phase particulate elastic dielectric composites is fundamentally incomplete, especially for cases involving stiff filler particles which (as opposed to what the theory predicts) have been reported to exhibit extreme enhancements in their electrostriction capabilities. It is posited that such extreme enhancements are the manifestation of interphasial phenomena. In particular, the presence of interphasial free charges that oscillate rapidly in space at the length scale of the microstructure of elastic dielectric composites is shown to have a significant and even dominant effect on their macroscopic response, possibly leading to extreme behaviors ranging from unusually large permittivities and electrostriction coefficients to metamaterial-type properties featuring negative permittivities. These results suggest a promising strategy to design deformable dielectric composites --- such as electrets and dielectric elastomer composites --- with exceptional electromechanical properties

Homogenization of elastomers filled with liquid inclusions: The small-deformation limit

This paper presents the derivation of the homogenized equations that describe the macroscopic mechanical response of elastomers filled with liquid inclusions in the setting of small quasistatic deformations. The derivation is carried out for materials with periodic microstructure by means of a two-scale asymptotic analysis. The focus is on the non-dissipative case when the elastomer is an elastic solid, the liquid making up the inclusions is an elastic fluid, the interfaces separating the solid elastomer from the liquid inclusions are elastic interfaces featuring an initial surface tension, and the inclusions are initially $n$-spherical ($n=2,3$) in shape. Remarkably, in spite of the presence of local residual stresses within the inclusions due to an initial surface tension at the interfaces, the macroscopic response of such filled elastomers turns out to be that of a linear elastic solid that is free of residual stresses and hence one that is simply characterized by an effective modulus of elasticity $\bar{\textbf{L}}$. What is more, in spite of the fact that the local moduli of elasticity in the bulk and the interfaces do not possess minor symmetries (due to the presence of residual stresses and the initial surface tension at the interfaces), the resulting effective modulus of elasticity $\bar{\textbf{L}}$ does possess the standard minor symmetries of a conventional linear elastic solid, that is, $\bar{L}_{ijkl}=\bar{L}_{jikl}=\bar{L}_{ijlk}$. As a first application, numerical results are worked out and analyzed for the effective modulus of elasticity of isotropic suspensions of incompressible liquid $2$-spherical inclusions of monodisperse size embedded in an isotropic incompressible elastomer

Cavitation in rubber: an elastic instability or a fracture phenomenon?

It is by now well established that loading conditions with sufficiently large triaxialities can induce the sudden -appearance of internal cavities within elastomeric (and other soft) solids. The occurrence of such a phenomenon, commonly referred to as cavitation, can be attributed to the growth of pre-existing defects into finite sizes. In the first part of this discussion, I will present a new theory within the context of nonlinear elasticity to study the phenomenon of cavitation in rubber that contrary to earlier approaches: (i) allows to consider general 3D loading conditions with arbitrary triaxiality; (ii) applies to general classes of nonlinear elastic solids; and (iii) incorporates direct information on the initial shape, spatial distribution, and mechanical properties of the underlying defects at which cavitation can initiate. The basic idea is to first cast cavitation in elastomeric solids as the homogenization problem of nonlinear elastic materials containing random distributions of zero-volume cavities, or defects. Then, by means of a novel iterated homogenization procedure, exact solutions are constructed for such a problem. These include solutions for the change in size of the underlying cavities as a function of the applied loading conditions, from which the onset of cavitation – corresponding to the event when the initially infinitesimal cavities suddenly grow into finite sizes – can be readily determined. In the second part of the discussion, I will confront the theory with a variety of cavitation experiments with the objective of establishing whether the phenomenon of cavitation is an elastic instability (and hence depends only on the elastic properties of the rubber), or, on the other hand, a fracture process (and hence depends on the fracture properties of the rubber). REFERENCES [1] Lefèvre, V., Ravi-Chandar, K., Lopez-Pamies, O. Cavitation in rubber: An elastic instability or a fracture phenomenon? International Journal of Fracture. 2014. Submitted. [2] Lopez-Pamies, O., Nakamura T., Idiart, M.I. Cavitation in elastomeric solids: I – A defect growth theory. Journal of the Mechanics and Physics of Solids. 2011, 59, 1464–1487. [3] Lopez-Pamies, O., Nakamura T., Idiart, M.I. Cavitation in elastomeric solids: II – Onset-of-cavitation surfaces for Neo-Hookean materials. Journal of the Mechanics and Physics of Solids. 2011, 59, 1488–1505

Representações sociais sobre hipertensão arterial e o cuidado: o discurso do sujeito coletivo

OBJETIVO: Analisar as representações sociais da equipe de saúde sobre a hipertensão arterial e a maneira como desenvolvem os cuidados específicos aos hipertensos. MÉTODOS: Utilizou-se a entrevista semiestruturada com 21 profissionais de saúde que atuavam em Unidades Básicas de Saúde, a técnica do discurso do sujeito coletivo para organização dos dados e interpretação pela teoria das representações sociais. RESULTADOS: As representações dos profissionais revelaram um distanciamento entre as ações preconizadas pelo programa de controle e as que se realizam nas unidades de saúde. CONCLUSÕES: Os profissionais reconhecem a hipertensão como uma doença grave, com grandes consequências, porém as dificuldades estruturais por parte dos serviços, aliadas à desmotivação do usuário para participação em ações de promoção em saúde, não favorecem um cuidado em saúde voltado para as necessidades dos portadores de hipertensão. Tais resultados indicam a necessidade de ações interdisciplinares na prática assistencial que impactem positivamente na oferta e qualidade dos cuidados.OBJETIVO: Analizar las representaciones sociales del equipo de salud sobre la hipertensión arterial y la manera cómo desarrollan los cuidados específicos a los hipertensos. MÉTODOS: Se utilizó la entrevista semiestructurada con 21 profesionales de salud que actuaban en Unidades Básicas de Salud, la técnica del discurso del sujeto colectivo para la organización de los datos e interpretación por la teoría de las representaciones sociales. RESULTADOS: Las representaciones de los profesionales revelaron un distanciamiento entre las acciones preconizadas por el programa de control y las que se realizan en las unidades de salud. CONCLUSIONES: Los profesionales reconocen a la hipertensión como una enfermedad grave, con grandes consecuencias, sin embargo las dificultades estructurales por parte de los servicos, aliadas a la desmotivación del usuario para participar en acciones de promoción en salud, no favorecen a un cuidado en salud volcado a las necesidades de los portadores de hipertensión. Tales resultados indican la necesidad de acciones interdisciplinaris en la práctica asistencial que impacten positivamente en el ofrecimiento y calidad de los cuidados.OBJECTIVE: To analyze the social representations of the health team about arterial hypertension and the manner that develops specific care with hypertensive patients. METHODS: We used semi-structured interviews with 21 health professionals who worked in the Basic Health Units. The technique of collective subject discourse was used for organization and interpretation of the data, using the theory of social representation. RESULTS: The representations of health professionals showed a gap between the actions recommended by the control program and those taking place in health facilities. CONCLUSIONS: Professionals recognized hypertension as a serious disease, with great consequences, but the structural difficulties for part of the services, together with the user's motivation to participate in activities to promote health, did not favor health care focused on the needs of those suffering with hypertension. These results indicate the need for interdisciplinary actions in care practice that impact positively on the delivery and quality of care

Patterns of neighborhood environment attributes related to physical activity across 11 countries: A latent class analysis

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Perceived neighborhood environment and physical activity in 11 countries: Do associations differ by country?

Background: Increasing empirical evidence supports associations between neighborhood environments and physical activity. However, since most studies were conducted in a single country, particularly western countries, the generalizability of associations in an international setting is not well understood. The current study examined whether associations between perceived attributes of neighborhood environments and physical activity differed by country. Methods: Population representative samples from 11 countries on five continents were surveyed using comparable methodologies and measurement instruments. Neighborhood environment × country interactions were tested in logistic regression models with meeting physical activity recommendations as the outcome, adjusted for demographic characteristics. Country-specific associations were reported. Results: Significant neighborhood environment attribute × country interactions implied some differences across countries in the association of each neighborhood attribute with meeting physical activity recommendations. Across the 11 countries, land-use mix and sidewalks had the most consistent associations with physical activity. Access to public transit, bicycle facilities, and low-cost recreation facilities had some associations with physical activity, but with less consistency across countries. There was little evidence supporting the associations of residential density and crime-related safety with physical activity in most countries. Conclusion: There is evidence of generalizability for the associations of land use mix, and presence of sidewalks with physical activity. Associations of other neighborhood characteristics with physical activity tended to differ by country. Future studies should include objective measures of neighborhood environments, compare psychometric properties of reports across countries, and use better specified models to further understand the similarities and differences in associations across countries

Adjuvant hyperthermic intraperitoneal chemotherapy (HIPEC) in patients with colon cancer at high risk of peritoneal carcinomatosis; the COLOPEC randomized multicentre trial

Background: The peritoneum is the second most common site of recurrence in colorectal cancer. Early detection of peritoneal carcinomatosis (PC) by imaging is difficult. Patients eventually presenting with clinically apparent PC have a poor prognosis. Median survival is only about five months if untreated and the benefit of palliative systemic chemotherapy is limited. Only a quarter of patients are eligible for curative treatment, consisting of cytoreductive surgery and hyperthermic intraperitoneal chemotherapy (CR/HIPEC). However, the effectiveness depends highly on the extent of disease and the treatment is associated with a considerable complication rate. These clinical problems underline the need for effective adjuvant therapy in high-risk patients to minimize the risk of outgrowth of peritoneal micro metastases. Adjuvant hyperthermic intraperitoneal chemotherapy (HIPEC) seems to be suitable for this purpose. Without the need for cytoreductive surgery, adjuvant HIPEC can be performed with a low complication rate and short hospital stay. Methods/Design: The aim of this study is to determine the effectiveness of adjuvant HIPEC in preventing the development of PC in patients with colon cancer at high risk of peritoneal recurrence. This study will be performed in the nine Dutch HIPEC centres, starting in April 2015. Eligible for inclusion are patients who underwent curative resection for T4 or intra-abdominally perforated cM0 stage colon cancer. After resection of the primary tumour, 176 patients will be randomized to adjuvant HIPEC followed by routine adjuvant systemic chemotherapy in the experimental arm, or to systemic chemotherapy only in the control arm. Adjuvant HIPEC will be performed simultaneously or shortly after the primary resection. Oxaliplatin will be used as chemotherapeutic agent, for 30 min at 42-43 degrees C. Just before HIPEC, 5-fluorouracil and leucovorin will be administered intravenously. Primary endpoint is peritoneal disease-free survival at 18 months. Diagnostic laparoscopy will be performed routinely after 18 months postoperatively in both arms of the study in patients without evidence of disease based on routine follow-up using CT imaging and CEA. Discussion: Adjuvant HIPEC is assumed to reduce the expected 25 % absolute risk of PC in patients with T4 or perforated colon cancer to a risk of 10 %. This reduction is likely to translate into a prolonged overall survival

Cerebral activations during number multiplication and comparison: a PET study

Positron emission tomography was used to examine the cerebral networks underlying number comparison and multiplication in eight normal volunteers. Cerebral blood flow was measured within anatomical regions of interest defined in each subject using magnetic resonance imaging. Three conditions were used: rest with eyes closed, mental multiplication of pairs of arabic digits and larger-smaller comparison of the same pairs. Both multiplication and comparison activated the left and right lateral occipital cortices, the left precentral gyrus, and the supplementary motor area. Beyond these common activations, multiplication activated also the left and right inferior parietal gyri, the left fusiform and lingual gyri, and the right cuneus. Relative to comparison, multiplication also yielded superior activity in the left lenticular nucleus and in Brodmann's area 8, and induced a hemispheric asymmetry in the activation of the precentral and inferior frontal gyri. Conversely, relative to multiplication, comparison yielded superior activity in the right superior temporal gyrus, the left and right middle temporal gyri, the right superior frontal gyrus, and the right inferior frontal gyrus. These results underline the role of bilateral inferior parietal regions in number processing and suggest that multiplication and comparison may rest on partially distinct networks

Enabling planetary science across light-years. Ariel Definition Study Report

Ariel, the Atmospheric Remote-sensing Infrared Exoplanet Large-survey, was adopted as the fourth medium-class mission in ESA's Cosmic Vision programme to be launched in 2029. During its 4-year mission, Ariel will study what exoplanets are made of, how they formed and how they evolve, by surveying a diverse sample of about 1000 extrasolar planets, simultaneously in visible and infrared wavelengths. It is the first mission dedicated to measuring the chemical composition and thermal structures of hundreds of transiting exoplanets, enabling planetary science far beyond the boundaries of the Solar System. The payload consists of an off-axis Cassegrain telescope (primary mirror 1100 mm x 730 mm ellipse) and two separate instruments (FGS and AIRS) covering simultaneously 0.5-7.8 micron spectral range. The satellite is best placed into an L2 orbit to maximise the thermal stability and the field of regard. The payload module is passively cooled via a series of V-Groove radiators; the detectors for the AIRS are the only items that require active cooling via an active Ne JT cooler. The Ariel payload is developed by a consortium of more than 50 institutes from 16 ESA countries, which include the UK, France, Italy, Belgium, Poland, Spain, Austria, Denmark, Ireland, Portugal, Czech Republic, Hungary, the Netherlands, Sweden, Norway, Estonia, and a NASA contribution

Potential of Core-Collapse Supernova Neutrino Detection at JUNO

JUNO is an underground neutrino observatory under construction in Jiangmen, China. It uses 20kton liquid scintillator as target, which enables it to detect supernova burst neutrinos of a large statistics for the next galactic core-collapse supernova (CCSN) and also pre-supernova neutrinos from the nearby CCSN progenitors. All flavors of supernova burst neutrinos can be detected by JUNO via several interaction channels, including inverse beta decay, elastic scattering on electron and proton, interactions on C12 nuclei, etc. This retains the possibility for JUNO to reconstruct the energy spectra of supernova burst neutrinos of all flavors. The real time monitoring systems based on FPGA and DAQ are under development in JUNO, which allow prompt alert and trigger-less data acquisition of CCSN events. The alert performances of both monitoring systems have been thoroughly studied using simulations. Moreover, once a CCSN is tagged, the system can give fast characterizations, such as directionality and light curve