Electromechanical Deformation Research Articles

Plasmonic imaging of subcellular electromechanical deformation in mammalian cells.

.A membrane potential change in cells is accompanied with mechanical deformation. This electromechanical response can play a significant role in regulating action potential in neurons and in controlling voltage-gated ion channels. However, measuring this subtle deformation in mammalian cells has been a difficult task. We show a plasmonic imaging method to image mechanical deformation in single cells upon a change in the membrane potential. Using this method, we have studied the electromechanical response in mammalian cells and have observed the local deformation within the cells that are associated with cell–substrate interactions. By analyzing frequency dependence of the response, we have further examined the electromechanical deformation in terms of mechanical properties of cytoplasm and cytoskeleton. We demonstrate a plasmonic imaging approach to quantify the electromechanical responses of single mammalian cells and determine local variability related to cell–substrate interactions.

Tuning the Surface Chemistry of Graphene Oxide for Enhanced Dielectric and Actuated Performance of Silicone Rubber Composites

The influence of reduction temperature on the electromechanical properties and actuation behavior of polydimethylsiloxane (PDMS) dielectric elastomer containing the thermally reduced graphene oxide (rGO) with different surface chemistry has been systematically investigated. A set of rGO nanosheets was prepared by thermal reduction of graphene oxide (GO) at four temperatures (150, 200, 300, and 400 °C). The dielectric permittivity, dielectric loss, and elastic modulus of PDMS composites were increased, while the electrical breakdown strength of composites was decreased with an increase of the reduction temperature of GO. A thermodynamic model applied for studying the electromechanical deformation and stability of PDMS/GO(rGO-x) dielectric elastomer composites showed that the optimum value of the break-point was observed in PDMS/rGO-300. It is shown for the first time that the variation of electromechanical instability and recovery behavior are attributed to the surface chemistry of rGOs. A critical reducti...

Critical curvature localization in graphene. II. Non-local flexoelectricity-dielectricity coupling.

As a sequel of part I (Kothari et al. 2018 Proc. R. Soc. A 474, 20180054), we present a general thermodynamic framework of flexoelectric constitutive laws for multi-layered graphene (MLG), and apply these laws to explain the role of crinkles in peculiar molecular adsorption characteristics of highly oriented pyrolytic graphite (HOPG) surfaces. The thermodynamically consistent constitutive laws lead to a non-local interaction model of polarization induced by electromechanical deformation with flexoelectricity-dielectricity coupling. The non-local model predicts curvature and polarization localization along crinkle valleys and ridges very close to those calculated by density functional theory (DFT). Our analysis reveals that the non-local model can be reduced to a simplified uc-local or e-local model (Kothari et al. 2018 Proc. R. Soc. A 474, 20180054) only when the curvature distribution is uniform or highly localized. For the non-local model, we calibrated and formulated the layer-number-dependent dielectric and intrinsic flexoelectric coefficients of MLGs. In addition, we also obtained layer-number dependent flexoelectric coefficients for uc-local and e-local models. Our DFT analysis shows that polarization-induced adsorption of neutral molecules at crinkle ridges depends on the molecular weight of the molecule. Furthermore, our detailed study of polarization localization in graphene crinkles enables us to understand previously unexplained self-organized adsorption of C60 buckyballs in a linear array on an HOPG surface.

A microstructure-based approach to modeling electrostriction that accounts for variability in spatial locations of domains

The discovery of polyvinylidene fluoride (PVDF) based family of relaxor ferroelectric (RFE) polymers has attracted attention due to their high electrostrictive strain and relatively low hysteresis loss. These RFE polymers exhibit complex microstructures containing both crystalline domains and amorphous domains; the interactions of the crystalline domains drive the electrostrictive behavior of these EAPs, while the amorphous domains dictate the mechanical behavior of the materials. Furthermore, the crystalline domains are spatially and orientationally distributed across the amorphous medium, further complicating the morphology of RFE polymers. Although a number of studies have focused on experimental and computational investigation of the interaction among different crystalline phases of this family of RFE, electrostriction models that represent the variabilities in the microstructure of biphasic RFE polymers are lacking. The proposed model aims to link the semicrystalline microstructure to the observed electromechanical coupling. An energy density function is constructed for a representative volume element (RVE) of the EAP, including a term for each phase, crystalline and amorphous. The interaction of the crystalline domains is based on the Coulomb interaction energy between a pair of dipoles. The responses of the amorphous domains are predicted by a modified hyperelastic stress–stretch eight-chain model. The total free energy is then analyzed under constitutive laws for an isothermal electromechanical deformation to determine the stresses generated in the RVE. The strain versus electric field, i.e. the electrostriction, relationship is calculated from a self-equilibrium condition of the Cauchy stress. The microstructure of the material is taken into account by applying the dipolar energy to a semicrystalline network model, in which a dipole that represents a crystalline domain is surrounded by an amorphous medium. The semicrystalline RVE experiences interactions with neighboring crystallites, which drives the electrostriction of the material. Two basic cases of the semicrystalline network model are explored to study the effects of spatial variation of crystalline domain locations relative to each other. Furthermore, higher fidelity descriptions of spatial location are introduced through the addition of a probability density function (PDF) of dipoles around a central dipole. Comparing the model to experimental results from the literature allows best-fit determination of the model parameters describing the PDF, e.g. aspects of microstructure, itself. These results, which agree well with experimental data, imply that the model has an ability to infer key information about the microstructure of the material by fitting the distribution of dipoles with a single adjustable parameter. The model is unique in that its descriptions of the crystalline domains is amenable to direct measurement by spectroscopic scattering techniques. Consequently, adjustable parameters in the model are linked to physical characteristics that are quantifiable, such as magnitudes of dipole moments and spatial distribution parameters.The model may also be used to elucidate aspects of network morphology using best fit of these physically meaningful adjustable parameters to experimental data, possibly providing a link between processing-structure-property relationships for future researchers.

Time‐dependent deformation of structurally chiral polymer networks in stabilized cholesteric liquid crystals

ABSTRACTPolymerization of crosslinkable liquid crystal monomers in chiral liquid crystalline media stabilizes the phase and enables distinct electro‐optic properties relative to small‐molecule analogs. Particularly interesting are cases where the polymerization forms a crosslinked polymer network that maintains a “structural” chirality. Recent reports have employed this methodology to realize a diverse set of electro‐optic responses in polymer stabilized cholesteric liquid crystals (PSCLCs) including reflection bandwidth broadening, reflection wavelength tuning, and dynamic scattering modes. It has been proposed that the mechanism at the root of these electro‐optic responses is an ion‐mediated, electromechanical deformation of the stabilizing and structurally chiral polymer network. In an effort to better understand the nature of these deformations, here we have characterized the electro‐optic response of PSCLCs with different polymer concentrations and crosslink densities. The dynamic response of PSCLCs to electric fields exhibits a time‐dependent behavior reminiscent of the creep of polymeric materials to mechanical deformations. The electro‐optic response can be described as the superposition of two contributions: the fast deformation of a relatively soft component of the polymer network (1–2 s) and the slower (10–20 s) deformation of a harder component. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 1087–1093

Electroresponsive Homogeneous Polyelectrolyte Complex Hydrogels from Naturally Derived Polysaccharides

Homogeneous polyelectrolyte complex (PEC) hydrogels made from chitosan and carboxymethylcellulose were prepared in the LiOH/KOH/urea aqueous system through a freeze–thawing method. Following the treatments of sequential chemical and physical cross-linking, the resulting hydrogels with supertough mechanical strength can operate as fast response actuators under electrical stimulus in salt aqueous solutions. The electromechanical behaviors of the hydrogels are strongly dependent on experimental parameters such as electric voltage, solvent constituents, pH, and ionic strength. It is proposed that the electromechanical deformation of hydrogel originates from a dynamic osmotic equilibrium effect taking place at the interface between the hydrogel and the surrounding medium, which is induced by the migration of ions throughout the gel network. In addition, programmable 3D shape transformations were obtained by using the PEC hydrogel with designed 2D geometric patterns. Moreover, the bending actuation behavior of ...

Unexpectedly high piezoelectricity of Sm-doped lead zirconate titanate in the Curie point region

Large piezoelectric coefficients in polycrystalline lead zirconate titanate (PZT) are traditionally achieved through compositional design using a combination of chemical substitution with a donor dopant and adjustment of the zirconium to titanium compositional ratio to meet the morphotropic phase boundary (MPB). In this work, a different route to large piezoelectricity is demonstrated. Results reveal unexpectedly high piezoelectric coefficients at elevated temperatures and compositions far from the MPB. At temperatures near the Curie point, doping with 2 at% Sm results in exceptionally large piezoelectric coefficients of up to 915 pm/V. This value is approximately twice those of other donor dopants (e.g., 477 pm/V for Nb and 435 pm/V for La). Structural changes during the phase transitions of Sm-doped PZT show a pseudo-cubic phase forming ≈50 °C below the Curie temperature. Possible origins of these effects are discussed and the high piezoelectricity is posited to be due to extrinsic effects. The enhancement of the mechanism at elevated temperatures is attributed to the coexistence of tetragonal and pseudo-cubic phases, which enables strain accommodation during electromechanical deformation and interphase boundary motion. This work provides insight into possible routes for designing high performance piezoelectrics which are alternatives to traditional methods relying on MPB compositions.

Dynamic Electromechanical Response of a Viscoelastic Dielectric Elastomer under Cycle Electric Loads

Dielectric elastomer (DE) is able to produce large electromechanical deformation which is time-dependent due to the viscoelasticity. In the current study, a thermodynamic model is set up to characterize the influence of viscoelasticity on the electromechanical and dynamic response of a viscoelastic DE. The time-dependent dynamic deformation, the hysteresis, and the dynamic stability undergoing viscoelastic dissipative processes are investigated. The results show that the electromechanical stability has strong frequency dependence; the viscoelastic DE can attain a larger stretch in the dynamic response than the quasistatic actuation. Furthermore, with the decreasing frequency of the applied electric load, the viscoelastic DE system will present dynamic stability evolution from an aperiodic motion to the quasiperiodic motion. The DE system may also experience a stability evolution from a single cycle motion to multicycle motion with the increasing relaxation times. The value and variation trend of the amplitude of the stretch are highly dependent on the excitation frequency and the relaxation time.

Electrical Control of Shape in Voxelated Liquid Crystalline Polymer Nanocomposites.

Liquid crystal elastomers (LCEs) exhibit anisotropic mechanical, thermal, and optical properties. The director orientation within an LCE can be spatially localized into voxels [three-dimensional (3-D) volume elements] via photoalignment surfaces. Here, we prepare nanocomposites in which both the orientation of the LCE and single-walled carbon nanotube (SWNT) are locally and arbitrarily oriented in discrete voxels. The addition of SWNTs increases the stiffness of the LCE in the orientation direction, yielding a material with a 5:1 directional modulus contrast. The inclusion of SWNT modifies the thermomechanical response and, most notably, is shown to enable distinctive electromechanical deformation of the nanocomposite. Specifically, the incorporation of SWNTs sensitizes the LCE to a dc field, enabling uniaxial electrostriction along the orientation direction. We demonstrate that localized orientation of the LCE and SWNT allows complex 3-D shape transformations to be electrically triggered. Initial experiments indicate that the SWNT-polymer interfaces play a crucial role in enabling the electrostriction reported herein.

Effect of constrained fibers on electromechanical actuation of charge-controlled dielectric elastomers

As is known, electromechanical deformation of voltage-controlled dielectric elastomers (DEs) is significant but is susceptible to pull-in and snap-through instabilities, while the large stable deformation can be achieved by spraying charge on DE surfaces, i.e., charge-controlled DEs. In this article, we investigate the effect of constrained fibers on electromechanical actuation of charge-controlled DEs by involving two special deformation modes: uniaxial tension and pure shear state. The coupled effects between the geometrical sizes and the mechanical tensile force of the charge-controlled DEs are also explored. The research results indicate that, different from voltage-controlled DEs, the electromechanical stretch of charge-controlled DEs with constrained fibers does not always show a beneficial merit with respect to that of the charge-controlled DEs without constrained fibers.

Instrumentation System for Synchronous Recording of EME and Mechanical Deformation Parameters in Lab-Scale Tests of Rock Specimens

The instrumentation system designed for the synchronous recording of electromechanical emission and mechanical deformation parameters in lab-scale tests of rock specimens is presented. Based on the data of implemented tests, the change in the structure and spectrum of EME signals at the prediscontinuity stage in rocks is determined, and the failure prediction criteria are proposed.

Loss of tension in electromechanical actuation of fiber-constrained viscoelastic dielectric elastomers

Dielectric elastomers (DEs) under pure shear state can generate the giant deformation without the pull-in and snap-through instabilities. The pure shear state is usually achieved by using fibers to constrain the deformation in an in-plane direction. Since the DEs cannot support the compressed stress, the instability of loss of tension (LT) may occur in the fiber-constrained direction as the voltage-induced Maxwell stress increases. In this article, by incorporating the effects of different values of tensile force, applied voltage (including both step and ramp voltages), prescribed constrained deformation and viscoelasticity intensity, a viscoelastic model is employed to explore the instability of LT of fiber-constrained DEs with analysis of the electromechanical deformation and stress evolution.

Electromechanical deformation sensors based on polyurethane/polyaniline electrospinning nanofibrous mats

Conducting electrospun nanofibrous mats elicit great interest in a wide range of applications. In this paper, polyurethane/polyaniline (PU/PANi) mats were prepared via the elecrospinning route with a polyurethane/dimethyl formamide spinning solution, and then in situ chemical polymerization of aniline monomers on the surface of PU nanofibers to deposit conductive PANi layer. In details, the morphology and structure of the PU/PANi mats were characterized by SEM, FTIR and the results indicated that the polyaniline stably deposited on the surface of polyurethane nanofiber. The electrical conductivity measurements indicated PU/PANi mats expressed good electrical conductivity (0.43S/cm) at the optimal polymerization time as 120min. As expected, PU/PANi mats showed strain sensitivity of (R-R0)/R0=0.75 (curvature=0.4cm−1) and the average strain gauge factor could reach 17.15 with applied stretching deformation (0∼110%). Therefore it could be deemed as the recommended nano-materials for flexible wearable device.

Computational modeling of electro-elasto-capillary phenomena in dielectric elastomers

We present a new finite deformation, dynamic finite element model that incorporates surface tension to capture elastocapillary effects on the electromechanical deformation of dielectric elastomers. We demonstrate the significant effect that surface tension can have on the deformation of dielectric elastomers through three numerical examples: (1) surface tension effects on the deformation of single finite elements with homogeneous and inhomogeneous boundary conditions; (2) surface tension effects on instabilities in constrained dielectric elastomer films, and (3) surface tension effects on bursting drops in solid dielectrics. Generally, we find that surface tension creates a barrier to instability nucleation. Specifically, we find in agreement with recent experimental studies of constrained dielectric elastomer films a transition in the surface instability mechanism depending on the elastocapillary length. The present results indicate that the proposed methodology may be beneficial in studying the electromechanical deformation and instabilities for dielectric elastomers in the presence of surface tension.

Photosensitivity of reflection notch tuning and broadening in polymer stabilized cholesteric liquid crystals.

The position or bandwidth of the selective reflection of polymer stabilized cholesteric liquid crystals (PSCLCs) prepared from negative dielectric anisotropy ("-Δε") liquid crystalline hosts can be shifted by applying a DC voltage. The underlying mechanism of the tuning or broadening of the reflection of PSCLCs detailed in these recent efforts is ion-facilitated, electromechanical deformation of the structurally chiral, polymer stabilizing network in the presence of a DC bias. Here, we show that these electro-optic responses can also be photosensitive. The photosensitivity is most directly related to the presence of photoinitiator, which is a known ionic contaminant to liquid crystal devices. Measurement of the ion density of a series of control compositions before, during, and after irradiation with UV light confirms that the ion density in compositions that exhibit photosensitivity is increased by irradiation and correlates to not only the concentration of the photoinitiator but also the type. Thus, the magnitude of the electrically tuned or broadened reflection of PSCLC of certain compositions when subjected to DC field is further increased in the presence of UV light. While interesting and potentially useful in applications such as architectural windows, the effect may be deleterious to some device implementations. Accordingly, compositions in which photosensitivity is not observed are identified.

Effect of temperature on the electromechanical actuation of viscoelastic dielectric elastomers

The electromechanical deformation of viscoelastic dielectric elastomers (DEs) is primarily governed by three material parameters: permittivity, Young's modulus, and relaxation time. All three parameters are functions of temperature, so a complete description of the electromechanical behaviour of a DE must take thermal effects into account. In this paper, we have established a physical model for viscoelastic DEs that takes temperature effects into consideration. The actuation of a DE was measured under different temperatures to verify the model. A peak actuation stretch was obtained at around 363 K both experimentally and theoretically. Moreover, we also demonstrate the contribution of strain-stiffening induced by greater pre-stretching to the improvement of thermostability.

Modeling of the dynamic characteristic of viscoelastic dielectric elastomer actuators subject to different conditions of mechanical load

Subject to a mechanical load or a voltage, a membrane of a dielectric elastomer deforms. As for the deformation mode, the dynamic performance and stability are strongly affected by how mechanical forces are applied. In the current study, by using the Euler-Lagrange equation, an analytical model is developed to characterize the dynamic performance of a homogeneously deformed viscoelastic dielectric elastomer under the conditions of equal-biaxial force, uniaxial force, and pure shear state, respectively. Numerical results are shown to describe the electromechanical deformation and stability. It is observed that the resonant frequency (where the amplitude-frequency curve peaks) has dependencies on the deformation mode, the level of mechanical load, and the applied electric field. When a dielectric elastomer membrane is subject to equal-biaxial force or pure shear state, it undergoes a nonlinear quasi-periodic vibration. An aperiodic motion of the dielectric elastomer system is induced by the boundary condition of a uniaxial force.

Highly improved electro-actuation of dielectric elastomers by molecular grafting of azobenzenes to silicon rubber

Electromechanical deformation of silicone rubbers was efficiently improved by chemically grafting with strong polar azobenzenes.

Temperature Influence on the Viscoelastic Electromechanical Deformation of Dielectric Elastomer

Temperature can significantly affect the performance of a viscoelastic dielectric elastomer (DE). In the current study, we use a thermodynamic model to characterize the influence of temperature on the viscoelastic electromechanical response undergoing a constant electric load by taking into account the temperature dependent elastic modus and dielectric constant. Due to the significant viscoelasticity in the dielectric elastomer, DE membrane creeps in time and the inelastic stretch of DE is smaller than that of the total stretch. The results show that the total stretch of the viscoelastic electromechanical deformation increases with the increasing temperature until suffering electromechanical instability at a high temperature; the actuation performance is dominated by the moduli of the elastomer. This may be used to guide the design of dielectric elastomer actuators undergoing temperature variation.

Factors Affecting Electromechanical Properties of Ionic Polymer Actuators Based on Ionic Liquid-Containing Sulfonated Block Copolymers

We investigate the electromechanical properties of ionic polymer actuators based on sulfonated block copolymers and imidazolium ionic liquids (ILs) to unveil key factors affecting the actuation performance. First, the extent of electromechanical deformation of the actuators was proven to be largely affected by the type of anion in the ILs, as understood by the anion-dependent ionic conductivity, charging time, and Young’s modulus of IL-containing polymers. In particular, upon switching the anion from hexafluorophosphate to bis(trifluoromethane)sulfonimide, more than a 2-fold enhancement in the bending strain was observed, which has not been previously reported. Second, we show that the bending strain and durability of the actuators are tunable in a straightforward manner by controlling the block architecture and molecular weight of the polymer, where the use of triblock copolymers was found to be beneficial in enhancing the actuation performance. The high mechanical strength of IL-containing triblock copo...