@article{ZhuKolloscheLuetal.2012, author = {Zhu, Jian and Kollosche, Matthias and Lu, Tongqing and Kofod, Guggi and Suo, Zhigang}, title = {Two types of transitions to wrinkles in dielectric elastomers}, series = {Soft matter}, volume = {8}, journal = {Soft matter}, number = {34}, publisher = {Royal Society of Chemistry}, address = {Cambridge}, issn = {1744-683X}, doi = {10.1039/c2sm26034d}, pages = {8840 -- 8846}, year = {2012}, abstract = {A membrane of a dielectric elastomer coated with compliant electrodes may form wrinkles as the applied voltage is ramped up. We present a combination of experiment and theory to investigate the transition to wrinkles using a clamped membrane subject to a constant force and a voltage ramp. Two types of transitions are identified. In type-I transition, the voltage-stretch curve is N-shaped, and flat and wrinkled regions coexist in separate areas of the membrane. The type-I transition progresses by nucleation of small wrinkled regions, followed by the growth of the wrinkled regions at the expense of the flat regions, until the entire membrane is wrinkled. By contrast, in type-II transition, the voltage-stretch curve is monotonic, and the entire flat membrane becomes wrinkled with no nucleation barrier. The two types of transitions are analogous to the first and the second order phase transitions. While the type-I transition is accompanied by a jump in the vertical displacement, type-II transition is accompanied by a continuous change in the vertical displacement. Such transitions may enable applications in muscle-like actuation and energy harvesting, where large deformation and large energy of conversion are desired.}, language = {en} } @article{McCarthyRisseKatekomoletal.2009, author = {Mc Carthy, Denis N. and Risse, Stefan and Katekomol, Phisan and Kofod, Guggi}, title = {The effect of dispersion on the increased relative permittivity of TiO2/SEBS composites}, issn = {0022-3727}, doi = {10.1088/0022-3727/42/14/145406}, year = {2009}, abstract = {Polymer composites are currently suggested for use as improved dielectric materials in many applications. Here, the effect of particle size and dispersion on the electrical properties of composites of rutile TiO2 and poly(styrene- ethylene-butadiene-styrene) (SEBS) are investigated. Both 15 and 300 nm particles are mixed with SEBS, with amounts of sorbitan monopalmitate surfactant from 0 to 3.3 vol\%, and their dielectric and mechanical properties are measured. Composites with the 300 nm TiO2 particles result in increases of 170\% in relative permittivity over the pure polymer, far above those predicted by standard theories, such as Bruggeman (140\%) and Yamada (114\%), and improving dispersion with surfactant has little effect. The composites with 15 nm particles showed surprisingly large relative permittivity increases (350\%), but improving the dispersion by the addition of any surfactant causes the relative permittivity to decrease to 240\% of the pure polymer value. We suggest that the increase is due to the formation of a highly conductive layer in the polymer around the TiO2 particles.}, language = {en} } @article{KolloscheKofodSuoetal.2015, author = {Kollosche, Matthias and Kofod, Guggi and Suo, Zhigang and Zhu, Jian}, title = {Temporal evolution and instability in a viscoelastic dielectric elastomer}, series = {Journal of the mechanics and physics of solids}, volume = {76}, journal = {Journal of the mechanics and physics of solids}, publisher = {Elsevier}, address = {Oxford}, issn = {0022-5096}, doi = {10.1016/j.jmps.2014.11.013}, pages = {47 -- 64}, year = {2015}, abstract = {Dielectric elastomer transducers are being developed for applications in stretchable electronics, tunable optics, biomedical devices, and soft machines. These transducers exhibit highly nonlinear electromechanical behavior: a dielectric membrane under voltage can form wrinkles, undergo snap-through instability, and suffer electrical breakdown. We investigate temporal evolution and instability by conducting a large set of experiments under various prestretches and loading rates, and by developing a model that allows viscoelastic instability. We use the model to classify types of instability, and map the experimental observations according to prestretches and loading rates. The model describes the entire set of experimental observations. A new type of instability is discovered, which we call wrinkle-to-wrinkle transition. A flat membrane at a critical voltage forms wrinkles and then, at a second critical voltage, snaps into another state of winkles of a shorter wavelength. This study demonstrates that viscoelasticity is essential to the understanding of temporal evolution and instability of dielectric elastomers. (C) 2014 Elsevier Ltd. All rights reserved.}, language = {en} } @article{RisseKussmaulKruegeretal.2012, author = {Risse, Sebastian and Kussmaul, Bjoern and Kr{\"u}ger, Hartmut and Kofod, Guggi}, title = {Synergistic improvement of actuation properties with compatibilized high permittivity filler}, series = {Advanced functional materials}, volume = {22}, journal = {Advanced functional materials}, number = {18}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1616-301X}, doi = {10.1002/adfm.201200320}, pages = {3958 -- 3962}, year = {2012}, abstract = {Electroactive polymers can be used for actuators with many desirable features, including high electromechanical energy density, low weight, compactness, direct voltage control, and complete silence during actuation. These features may enable personalized robotics with much higher ability to delicately manipulate their surroundings than can be achieved with currently available actuators; however, much work is still necessary to enhance the electroactive materials. Electric field-driven actuator materials are improved by an increase in permittivity and by a reduction in stiffness. Here, a synergistic enhancement method based on a macromolecular plasticizing filler molecule with a combination of both high dipole moment and compatibilizer moieties, synthesized to simultaneously ensure improvement of electromechanical properties and compatibility with the host matrix is presented. Measurements show an 85\% increase in permittivity combined with 290\% reduction in mechanical stiffness. NMR measurements confirm the structure of the filler while DSC measurements confirm that it is compatible with the host matrix at all the mixture ratios investigated. Actuation strain measurements in the pure shear configuration display an increase in sensitivity to the electrical field of more than 450\%, confirming that the filler molecule does not only improve dielectric and mechanical properties, it also leads to a synergistic enhancement of actuation properties by simple means.}, language = {en} } @article{KolloscheStoyanovLaflammeetal.2011, author = {Kollosche, Matthias and Stoyanov, Hristiyan and Laflamme, Simon and Kofod, Guggi}, title = {Strongly enhanced sensitivity in elastic capacitive strain sensors}, series = {Journal of materials chemistry}, volume = {21}, journal = {Journal of materials chemistry}, number = {23}, publisher = {Royal Society of Chemistry}, address = {Cambridge}, issn = {0959-9428}, doi = {10.1039/c0jm03786a}, pages = {8292 -- 8294}, year = {2011}, abstract = {Strain sensors based on dielectric elastomer capacitors function by the direct coupling of mechanical deformations with the capacitance. The coupling can be improved by enhancing the relative permittivity of the dielectric elastomer. Here, this is carried out through the grafting of conducting polymer (poly-aniline) to the elastomer backbone, leading to molecular composites. An enhancement in capacitance response of 46 times is observed. This could help to extend the possible range of miniaturization towards even smaller device features.}, language = {en} } @article{CarpiAndersonBaueretal.2015, author = {Carpi, Federico and Anderson, Iain and Bauer, Siegfried and Frediani, Gabriele and Gallone, Giuseppe and Gei, Massimiliano and Graaf, Christian and Jean-Mistral, Claire and Kaal, William and Kofod, Guggi and Kollosche, Matthias and Kornbluh, Roy and Lassen, Benny and Matysek, Marc and Michel, Silvain and Nowak, Stephan and Pei, Qibing and Pelrine, Ron and Rechenbach, Bjorn and Rosset, Samuel and Shea, Herbert}, title = {Standards for dielectric elastomer transducers}, series = {Smart materials and structures}, volume = {24}, journal = {Smart materials and structures}, number = {10}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {0964-1726}, doi = {10.1088/0964-1726/24/10/105025}, pages = {25}, year = {2015}, abstract = {Dielectric elastomer transducers consist of thin electrically insulating elastomeric membranes coated on both sides with compliant electrodes. They are a promising electromechanically active polymer technology that may be used for actuators, strain sensors, and electrical generators that harvest mechanical energy. The rapid development of this field calls for the first standards, collecting guidelines on how to assess and compare the performance of materials and devices. This paper addresses this need, presenting standardized methods for material characterisation, device testing and performance measurement. These proposed standards are intended to have a general scope and a broad applicability to different material types and device configurations. Nevertheless, they also intentionally exclude some aspects where knowledge and/or consensus in the literature were deemed to be insufficient. This is a sign of a young and vital field, whose research development is expected to benefit from this effort towards standardisation.}, language = {en} } @article{StoyanovKolloscheRisseetal.2013, author = {Stoyanov, Hristiyan and Kollosche, Matthias and Risse, Sebastian and Wache, Remi and Kofod, Guggi}, title = {Soft conductive elastomer materials for stretchable electronics and voltage controlled artificial muscles}, series = {Advanced materials}, volume = {25}, journal = {Advanced materials}, number = {4}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {0935-9648}, doi = {10.1002/adma.201202728}, pages = {578 -- 583}, year = {2013}, abstract = {Block copolymer elastomer conductors (BEC) are mixtures of block copolymers grafted with conducting polymers, which are found to support very large strains, while retaining a high level of conductivity. These novel materials may find use in stretchable electronics. The use of BEC is demonstrated in a capacitive strain sensor and in an artificial muscle of the dielectric elastomer actuator type, supporting more than 100\% actuation strain and capacity strain sensitivity up to 300\%.}, language = {en} } @article{LaflammeKolloscheConnoretal.2012, author = {Laflamme, S. and Kollosche, Matthias and Connor, Jerome J. and Kofod, Guggi}, title = {Soft capacitive sensor for structural health monitoring of large-scale systems}, series = {Structural control \& health monitorin}, volume = {19}, journal = {Structural control \& health monitorin}, number = {1}, publisher = {Wiley-Blackwell}, address = {Hoboken}, issn = {1545-2263}, doi = {10.1002/stc.426}, pages = {70 -- 81}, year = {2012}, abstract = {Structural integrity of infrastructures can be preserved if damage is diagnosed, localized, and repaired in time. During the past decade, there has been a considerable effort to automate the process of structural health monitoring, which is complicated by the inherent large size of civil structures. Hence, a need has arisen to develop new approaches that enable more effective health monitoring. In this paper, a new sensing technique for damage localization on large civil structures is proposed. Specifically, changes in strain are detected using a capacitance sensor built with a soft, stretchable dielectric polymer with attached stretchable metal film electrodes. A change in strain causes a measurable change in the capacitance of the sensor, which can be directly monitored when the sensor is fixed to a structure. The proposed method is shown here to permit an accurate detection of cracks. The proposed system deploys a layer of dielectric polymer on the surface of a structural element, and regularly monitors any change in capacitance, giving in turn information about the structural state. The smart material is composed of inexpensive silicone elastomers, which make the monitoring system a promising application for large surfaces. Results from tests conducted on small- scale specimens showed that the technology is capable of detecting cracks, and tests conducted on large- size specimens demonstrated that several sensor patches organized on a sensor sheet are capable of localizing a crack. The sensor strain also exhibits a high correlation with the loss of stiffness.}, language = {en} } @article{KofodPaajanenBauer2006, author = {Kofod, Guggi and Paajanen, Mika and Bauer, Siegfried}, title = {Self-organized minimum-energy structures for dielectric elastomer actuators}, issn = {0947-8396}, doi = {10.1007/s00339-006-3680-3}, year = {2006}, language = {en} } @article{WacheMcCarthyRisseetal.2015, author = {Wache, Remi and McCarthy, Denis N. and Risse, Sebastian and Kofod, Guggi}, title = {Rotary Motion Achieved by New Torsional Dielectric Elastomer Actuators Design}, series = {IEEE ASME transactions on mechatronics}, volume = {20}, journal = {IEEE ASME transactions on mechatronics}, number = {2}, publisher = {Inst. of Electr. and Electronics Engineers}, address = {Piscataway}, issn = {1083-4435}, doi = {10.1109/TMECH.2014.2301633}, pages = {975 -- 977}, year = {2015}, abstract = {This paper reports a new way to produce a rotation motion actuated by dielectric elastomer actuators. Two specific electrode designs have been developed and the rotation of the actuator centers has been demonstrated and measured. At low strains, the rotation shows a nearly quadratic dependence with the voltage. This behavior was used to compare the performances between the two proposed designs. Among the tested configurations, a maximal rotation of 10 degrees was achieved.}, language = {en} }