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Dielectric elastomer (DE) is a type of soft material that possesses voltage-induced deformation. Irregular vibrations in a circular DE membrane with damping under periodic loading are investigated. The modified Lagrange equation is used to develop the dynamical model for the membrane system using Gent strain energy density function. The resulting governing equation describes axisymmetric motion. Under various loading conditions, rich dynamical properties accompanying with the membrane system with stiffening, such as periodic, quasiperiodic, and chaotic motions, are investigated. First, the numerical results indicated that the membrane system has an asymmetric homoclinic orbit for a given set of normalized parameters under constant load. Furthermore, it is revealed that for different damping values, the basin of attraction is quite different, implying a change in the nonlinear dynamics of the membrane system. The bifurcation diagram reveals the emergence of chaotic oscillation, which manifests for time-varying loads. Poincare map, power spectral density, and Lyapunov exponent are used to identify the chaotic vibration. Besides, the system is more susceptible to chaos in the absence of damping. The phase plane and strange attractor analysis are used to demonstrate the effect of damping. An adaptive integral backstepping sliding mode control (AIBSMC) theory-based chaos control technique is proposed to prevent chaos. The system is also guided along a predetermined path during actuation using this control strategy. The current investigation will assist in dealing with complex irregular dynamics of the DE-based energy harvester and actuator operating in a time-varying loading environment. • Examines irregular vibrations under periodic loading in a circular DE membrane with damping. • Identifies several dynamical behaviors in the membrane system, such as periodic, quasiperiodic, and chaotic motions. • Displays distinct basin of attraction behaviors for different damping settings. • Proposes an AIBSMC technique to avoid chaos and direct the system during actuation. • This study is critical for DE-based energy harvesters and actuators. [ABSTRACT FROM AUTHOR]

The gelation procedure and the gelation time of gelatin gels may lead to apparently similar materials, however, with different rheological fingerprints under small and large oscillatory shear deformations. Here, in the first paper of this series, gelation of 3 and 5% w/w gelatin solutions at 5 °C was performed in situ on the rheometer plate and in custom-made casting modules to obtain individual gel layers. Large amplitude oscillatory shear (LAOS) tests were performed. The nonlinear deformation regime was qualitatively analyzed using normalized Lissajous-Bowditch curves. The MITlaos software was employed to decompose the total intracycle stress response and to calculate the Chebyshev coefficients ratios. The dynamic moduli of the fresh gels were measured directly after the in situ preparation and within a time frame until 1.5 h. In the strain sweeps, we observed intense stiffening followed by yielding above 200% strain. However, the individual gel layers aged for 24 and 48 h show different LAOS fingerprints. The extensive loops in the viscous Lissajous-Bowditch curves indicate an elastoplastic material response. Based on the overall nonlinear rheological response, we propose a structural formation that describes the behavior of the gels for the conditions studied here. In the second paper of this series, we give the impact of hard micro-fillers (glass beads) and we describe the nonlinear characteristics of the filled gels. [ABSTRACT FROM AUTHOR]

Abstract: Inherited mutations in the gene coding for the intermediate filament protein desmin have been demonstrated to cause severe skeletal and cardiac myopathies. Unexpectedly, some of the mutated desmins, in particular those carrying single amino acid alterations in the non-α-helical carboxy-terminal domain (“tail”), have been demonstrated to form apparently normal filaments both in vitro and in transfected cells. Thus, it is not clear if filament properties are affected by these mutations at all. For this reason, we performed oscillatory shear experiments with six different desmin “tail” mutants in order to characterize the mesh size of filament networks and their strain stiffening properties. Moreover, we have carried out high-frequency oscillatory squeeze flow measurements to determine the bending stiffness of the respective filaments, characterized by the persistence length l p. Interestingly, mesh size was not altered for the mutant filament networks, except for the mutant DesR454W, which apparently did not form proper filament networks. Also, the values for bending stiffness were in the same range for both the “tail” mutants (l p =1.0–2.0 μm) and the wild-type desmin (l p =1.1±0.5 μm). However, most investigated desmin mutants exhibited a distinct reduction in strain stiffening compared to wild-type desmin and promoted nonaffine network deformation. Therefore, we conclude that the mutated amino acids affect intrafilamentous architecture and colloidal interactions along the filament in such a way that the response to applied strain is significantly altered. In order to explore the importance of the “tail” domain as such for filament network properties, we employed a “tail”-truncated desmin. Under standard conditions, it formed extended regular filaments, but failed to generate strain stiffening. Hence, these data strongly indicate that the “tail” domain is responsible for attractive filament–filament interactions. Moreover, these types of interactions may also be relevant to the network properties of the desmin cytoskeleton in patient muscle. [Copyright &y& Elsevier]

Abstract: We have investigated the viscoelastic properties of the cytoplasmic intermediate filament (IF) proteins desmin and vimentin. Mechanical measurements were supported by time-dependent electron microscopy studies of the assembly process under similar conditions. Network formation starts within 2 min, but it takes more than 30 min until equilibrium mechanical network strength is reached. Filament bundling is more pronounced for desmin than for vimentin. Desmin filaments (persistence length l p ≈900 nm) are stiffer than vimentin filaments (l p ≈400 nm), but both IFs are much more flexible than microfilaments. The concentration dependence of the plateau modulus G 0 ∼ c α is much weaker than predicted theoretically for networks of semiflexible filaments. This is more pronounced for vimentin (α=0.47) than for desmin (α=0.70). Both networks exhibit strain stiffening at large shear deformations. At the transition from linear to nonlinear viscoelastic response, only desmin shows characteristics of nonaffine network deformation. Strain stiffening and the maximum modulus occur at strain amplitudes about an order of magnitude larger than those for microfilaments. This is probably attributable to axial slippage within the tetramer building blocks of the IFs. Network deformation beyond a critical strain γmax results in irreversible damage. Strain stiffening sets in at lower concentrations, is more pronounced, and is less sensitive to ionic strength for desmin than for vimentin. Hence, desmin exhibits strain stiffening even at low-salt concentrations, which is not observed for vimentin, and we conclude that the strength of electrostatic repulsion compared to the strength of attractive interactions forming the network junctions is significantly weaker for desmin than for vimentin filaments. These findings indicate that both IFs exhibit distinct mechanical properties that are adapted to their respective cellular surroundings [i.e., myocytes (desmin) and fibroblasts (vimentin)]. [Copyright &y& Elsevier]