Electric Field-induced Deformation Research Articles

Electric field-induced deformation and breakup of water droplets in polymer-flooding W/O emulsions: A simulation study

Polymers have a significant impact on the electrostatic demulsification efficiency of water-in-oil (W/O) emulsions by altering the motion characteristics of water droplets. To uncover the underlying micro-mechanism behind electric field-induced deformation and breakup of polymer-containing water droplets, we employed the molecular dynamics (MD) method to investigate the effect of different polymer concentrations (cp) on droplet motion under a DC electric field. The simulation results indicated that polyacrylamide (PAM) molecules possess a prominent electrostatic potential (ESP), resulting in a strong electrostatic attraction between PAM and water. The interface film of the droplet was found to exhibits high strength due to the formation of hydrogen bonds with a lifetime of 1 ps between polymers. The addition of polymers increased the polarity of the droplets, thereby promoting their deformation and breakup under an electric field. Both polarized polymers and ions migrated by carrying water molecules, which ultimately led to droplet breakup. At cp = 0.0221 mol·L−1, the critical field strength (Ec) for droplet breakup was the lowest, at only 0.672 V·nm−1. As cp > 0.0221 mol·L−1, the steric hindrance of PAM molecules and the strong interfacial film impeded the breakup of the droplets. These findings provided a theoretical basis for enhancing the electrostatic demulsification efficiency of polymer-flooding crude oil.

Nematic twist-bend phase of a bent liquid crystal dimer: field-induced deformations of the helical structure and macroscopic polarization

The twist-bend nematic (Ntb) phase is a recent addition to the family of nematic (N) phases of liquid crystals (LCs). A net polar order in the Ntb phase under an external electric field is interesting and it was predicted in several recent theoretical studies. We investigated the field-induced polarization behaviour, dielectric, and electro-optic properties of a bent LC dimer CB7CB in the N and Ntb phases. A threshold-dependent polarization current response was obtained in both the phases under triangular and square-wave input electric fields, existing till frequencies as high as 150 Hz. The polarization switching times were found in ∼1 ms region, especially in the N phase. In the Ntb phase, electric field-induced deformation of the helical structure was observed, like ferroelectric LCs. Dielectric measurements revealed the presence of cybotactic clusters via collective relaxations. The dielectric anisotropy (Δϵ) is negative at the frequencies of polarization measurements. The net polarization resulted from field-induced reorientation of cybotactic clusters and additionally from the field-induced deformation of helical structures in the Ntb phase. We explored the possibility of ionic contributions to the net polarization by synthesizing TiO2 nanoparticles (NPs) dispersed CB7CB LC nanocomposite. Incorporation of the NPs resulted in reduction of the collective order, increase in the ionic impurity content and conductivity, but an extinction of the field-induced polarization response. Our results demonstrate that the net polarization has competing contributions from both ferroelectric-like and ionic origin (up to ∼10 Hz) in the LC phases, but it becomes dominantly ferroelectric-like at higher frequencies.

Spatially periodic deformations in hybrid aligned flexoelectric nematic layers

ABSTRACT Electric field-induced deformations in hybrid aligned layers of nematic liquid crystals possessing flexoelectric properties were investigated numerically. The thresholdless one-dimensional deformations occurred at low voltages whereas the spatially periodic deformations, taking the form of parallel stripes, were found above sufficiently high threshold voltage. The threshold voltage decreased when the flexoelectric properties get stronger. The stripes were parallel to the direction of planar alignment. The director distributions in the stripes induced by positive voltages differed qualitatively from those arising at negative voltage. The evolution of deformations during increase of bias voltage was investigated. The role of flexoelectric properties and of dielectric anisotropy was studied. The spatial period of deformations shortened with increasing voltage and with increasing flexoelectric coefficients.

Dynamics of Electric Field Induced Deformations in Homeotropic Bent-Core Nematic Layers

Dynamics of Electric Field Induced Deformations in Homeotropic Bent-Core Nematic Layers

Effect of ac electric field on the dynamics of a vesicle under shear flow in the small deformation regime.

Vesicles or biological cells under simultaneous shear and electric field can be encountered in dielectrophoretic devices or designs used for continuous flow electrofusion or electroporation. In this work, the dynamics of a vesicle subjected to simultaneous shear and uniform alternating current (ac) electric field is investigated in the small deformation limit. The coupled equations for vesicle orientation and shape evolution are derived theoretically, and the resulting nonlinear equations are handled numerically to generate relevant phase diagrams that demonstrate the effect of electrical parameters on the different dynamical regimes such as tank treading (TT), vacillating breathing (VB) [called trembling (TR) in this work], and tumbling (TU). It is found that while the electric Mason number (Mn), which represents the relative strength of the electrical forces to the shear forces, promotes the TT regime, the response itself is found to be sensitive to the applied frequency as well as the conductivity ratio. While higher outer conductivity promotes orientation along the flow axis, orientation along the electric field is favored when the inner conductivity is higher. Similarly a switch of orientation from the direction of the electric field to the direction of flow is possible by a mere change of frequency when the outer conductivity is higher. Interestingly, in some cases, a coupling between electric field-induced deformation and shear can result in the system admitting an intermediate TU regime while attaining the TT regime at high Mn. The results could enable designing better dielectrophoretic devices wherein the residence time as well as the dynamical states of the vesicular suspension can be controlled as per the application.

Controlling nanowire growth through electric field-induced deformation of the catalyst droplet.

Semiconductor nanowires with precisely controlled structure, and hence well-defined electronic and optical properties, can be grown by self-assembly using the vapour–liquid–solid process. The structure and chemical composition of the growing nanowire is typically determined by global parameters such as source gas pressure, gas composition and growth temperature. Here we describe a more local approach to the control of nanowire structure. We apply an electric field during growth to control nanowire diameter and growth direction. Growth experiments carried out while imaging within an in situ transmission electron microscope show that the electric field modifies growth by changing the shape, position and contact angle of the catalytic droplet. This droplet engineering can be used to modify nanowires into three dimensional structures, relevant to a range of applications, and also to measure the droplet surface tension, important for quantitative development of strategies to control nanowire growth.

Giant Electric-Field-Induced Strain in PVDF-Based Battery Separator Membranes Probed by Electrochemical Strain Microscopy.

Efficiency of lithium-ion batteries largely relies on the performance of battery separator membrane as it controls the mobility and concentration of Li-ions between the anode and cathode electrodes. Recent advances in electrochemical strain microscopy (ESM) prompted the study of Li diffusion and transport at the nanoscale via electromechanical strain developed under an application of inhomogeneous electric field applied via the sharp ESM tip. In this work, we observed unexpectedly high electromechanical strain developed in polymer membranes based on porous poly(vinylidene fluoride) (PVDF) and poly(vinylidene fluoride-co-chlorotrifluoroethylene) (PVDF-CTFE) and, using it, could study a dynamics of electroosmotic flow of electrolyte inside the pores. We show that, independently of the separator membrane, electric field-induced deformation observed by ESM on wetted membrane surfaces can reach up to 10 nm under a moderate bias of 1 V (i.e., more than an order of magnitude higher than that in best piezoceramics). Such a high strain is explained by the electroosmotic flow in a porous media composed of PVDF. It is shown that the strain-based ESM method can be used to extract valuable information such as average pore size, porosity, elasticity of membrane in electrolyte solvent, and membrane-electrolyte affinity expressed in terms of zeta potential. Besides, such systems can, in principle, serve as actuators even in the absence of apparent piezoelectricity in amorphous PVDF.

On Dual-Grid Level-Set Method for Computational-Electro-Multifluid-Dynamics Simulation

An axisymmetric Dual-Grid Level-Set Method (DGLSM) is proposed for transient simulation of two-fluid electrohydrodynamic flow. Level-set and electric field equations are solved on a uniform grid which is twice that for Navier-Stokes equations, for accurate computation of interface and forces due to surface tension and electric field. Code validation and performance study is done for two problems: coalescence and electric field-induced deformation of drops (both fluids leaky and perfect dielectrics). The DGLSM gives results of almost the same accuracy as a fine-grid-based traditional level-set method (LSM) in a computational time slightly more than that for a coarse-grid-based LSM.

Backflow in flexoelectric nematic layers deformed by electric field

Coupling between director reorientation and flow appearing in nematic layers during arising and decay of electric field-induced deformations, known as backflow, was analysed numerically. Nematics possessing flexoelectric properties, in which the converse flexoelectric effect occurred, were considered. The velocity of the nematic liquid, as well as the director orientation angle, was calculated as functions of time and coordinate in the layer after applying a voltage step, as well as by switching off the voltage. Homeotropic and planar layers were taken into account. Several distinct forms of the backflow were found which differ from those observed for the non-flexoelectric nematic. Correlations between the flow patterns and the director distributions were determined.

Electric field-induced deformation of polydimethylsiloxane polymers

The deformation of polydimethylsiloxane (PDMS) spheres under uniform external electric field was studied experimentally and analytically. In the experiments, ∼1 mm diameter PDMS spheres with base-to-curing-agent mixing ratios of 10:1 and 60:1 were exposed to uniform external electric field with varying magnitudes and poling durations. The spheres elongate along the electric field direction. For a given electric field strength, the sphere deformation is initially a time function but reaches a terminal strain value over a certain time period. This terminal strain value is larger for stronger external electric fields and larger PDMS mixing ratio spheres. At this state, the material is no longer poled and the surface charge distribution remains constant. In the analysis, an expression for the sphere deformation is obtained by modeling the PDMS as a linear elastic solid and solving the Navier equation along with Maxwell’s equations for boundary conditions. The analysis takes into account the surface charge distribution and predicts well the experimental trends.

Converse flexoelectric effect in homeotropic nematic layers with asymmetric surface anchoring

The dc electric field-induced deformations of conducting flexoelectric nematic layers were studied numerically. Asymmetric boundary conditions expressed by different anchoring strengths on the limiting surfaces were assumed. Nematic material was characterised by negative dielectric anisotropy. Both signs of the sum of flexoelectric coefficients were taken into account. The electric properties of the layer were described in terms of a weak electrolyte model. Mobility of cations was assumed to be one order of magnitude lower than that of anions. Quasi-blocking electrode contacts were assumed. The threshold voltages for the deformations and the director distributions in the deformed layers were calculated for low, moderate and high ion concentrations. The director distributions were also determined. The results show that asymmetry caused by difference between the anchoring strengths and by difference between mobilities of anions and cations lead to two threshold values for a given layer corresponding to two polarities of the bias voltage. Additionally, the values of both thresholds depend on the sign of the flexoelectric parameter. In every case under consideration, the threshold is significantly lowered when the ion concentration is high.

Electric field-induced deformation behavior in mixed Bi0.5Na0.5TiO3 and Bi0.5(Na0.75K0.25)0.5TiO3-BiAlO3

In this study, a method in which an electric field-induced phase transition in bismuth-based ceramics is used at a relatively low field of 40 kV/cm is proposed. This study investigated the electrical properties and electric field-induced strain of Bi0.5Na0.5TiO3 (BNT) single particle-added Bi0.5(Na0.75K0.25)0.5TiO3-BiAlO3 (BNKT-BA) ceramics. Strains of 0.27% and 0.29%, for 0.94BNKT-0.06BA with 10% BNT and 0.93BNKT-0.07BA with 20% BNT, were observed, respectively. These strains were attributed to the transition from non-polar to ferroelectric phases in BNKT-BA small grains with ferroelectric BNT large grains during external electric field execution.

Geometrical deformation and failure behavior of C60 fullerene dimer under applied external electric field

By the quantum-molecular dynamics (QMD) technique based on the Roothaan–Hall equation and the Newton motion law, geometrical deformation and failure behavior of C60 fullerene dimer (2C60) as well as single C60 fullerene under applied external electric field are simulated. Further, the effects of the electric field direction on the electric field-induced deformation, polarization-charge distribution and dipole moment of the fullerene molecules are discussed systemically. It is found that the geometrical configuration and failure behavior of the 2C60 molecule are sensitive to the electric field direction, that when the electric field direction is parallel to the bridging C–C bonds of the 2C60 molecule the 2C60 fails easily, and that when the electric field direction is perpendicular to the 2C60 fails difficultly and has the same polarization and failure mechanism as the single C60.

Local electromechanical properties of ferroelectric materials for piezoelectric applications

ABSTRACTLocal electromechanical characterization is becoming prerequisite for the development of ferroelectric-based piezoelectric devices including multilayer actuators, micromotors, piezoelectric filters and, especially, microelectromechanical systems (MEMS), which combine piezoelectric elements and control electronics on the same chip. In this work, we present the results of local electromechanical characterization of several important ferroelectric materials including Pb(Zr, Ti)O3 (PZT) and (Pb, La)(Zr, Ti)O3 (PLZT) in both thin film and ceramic form. Local piezoelectric hysteresis measurements are performed by the piezoelectric force microscopy (PFM) that detects small electric field-induced deformation on the nanoscale e. g., within the single grain of a polycrystalline material. A number of novel phenomena is observed with increasing dc bias voltage including the jump of ferroelectric domain wall to the grain boundary, the “fingerlike” instability of domain wall, and the local phase transition into ferroelectric phase.

Electric field-induced layer deformations in the subphases of an antiferroelectric liquid crystal device

The layer structure in the antiferroelectric, ferrielectric, and ferroelectric phases of a liquid crystal device is reported, together with its electric field-induced deformation. The field-free chevron angle is comparable to the steric tilt angle, but differs significantly from the optical tilt angle. A sharp field threshold is observed for the chevron to bookshelf transition in the antiferroelectric phase at 1.3 V/μm, while layer deformations occur at much lower fields (0.3 V/μm) in the other subphases. Models are proposed for the layer deformations.

Polygonal Deformation of Toroidal Smectic-A Domains in Isotropic Phase Induced by Electric-Field Application

Electric field-induced polygonal deformation of smectic-A toroidal domains in isotropic phase has been observed in the binary mixtures of octyloxycyanobiphenyl with dodecyl alcohol. It is found that the number of sides of polygonally deformed smectic-A domains ( m ) is increased with increasing electric field, and that at a constant electric field larger smectic-A domains possess higher values of m . These experimental findings can successfully be explained in terms of numerical solutions to the variation problem for equilibrium shapes of smectic-A phase in isotropic phase [H. Naito, M. Okuda and Z. Ou-Yang: Phys. Rev. Lett. 70 (1993) 2912].

Boundary condition effects on field-induced deformation modes in polymer dispersed liquid crystals

Electric field-induced deformation modes, which are considered one of the causes of the hysteresis in the V-T response of polymer dispersed liquid crystal (PDLC), were investigated. We constructed model PDLC cells with varied alignment treatments at the interface of the liquid crystal droplets. The electric field-induced deformation modes of the liquid crystals in the model cells were analyzed by polarized microscope observations. Our results suggested a possible way to hysteresis-free PDLC devices.

Deformations of a Planar Nematic-Isotropic Interface in Uniform and Nonuniform Electric Fields

The electric field-induced deformations of a planar nematic(N)-isotropic(I) interface were studied, using a surface-nucleated N film prepared in a usual sandwich-type cell. When the applied AC voltage (10 Hz-100 kHz) was small, a uniform elastic distortion of the director was observed. But. at higher voltages, the planar interface became unstable and a striped pattern. having periodicity of a few hundred micrometers. appeared mostly along the rubbing direction of the substrate. The striped pattern was essentially static and was found to correspond to the undulation of the interface. The instability is interpreted in terms of the hydrostatics of nematics, and coupling between the N-I interface and the director is shown to be large responsible. The deformation of the interface in an intentionally nonuniform electric field was also observed. Based on a simple estimate of the interface profile, the N-I interfacial tension was evaluated to be 2.1 × 10-2 erg/cm2.

Measurement of Director Orientation at the Nematic—Isotropic Interface Using a Substrate-Nucleated Nematic Film

Abstract A flat and stable interface between the nematic and isotropic phases was prepared in a sandwich-type cell under thermal equilibrium conditions to determine the director orientation at the nematic-isotropic interface. The preparation method is based on the fact that a rubbed polyvinylalcohol film nucleates a surface nematic film when the isotropic liquid of a nematogen containing a small amount of impurity is cooled down passing through the clearing point. The impurity causes the nematic and isotropic phases to coexist over a finite temperature range in which the nucleated nematic film keeps its interface between the isotropic phase parallel to the substrate. Using the nematic-isotropic interface obtained this way, we determined the preferred orientation at the interface for a nematogen, 4-cyano-4′-n-pentylbiphenyl, mixed with a small amount of hexamethylbenzene, by observing the electric field-induced deformations in the nematic layer. For this purpose, a simple procedure which requires little kn...

Field-induced deformation of hybrid-aligned nematic liquid crystals: New multicolor liquid crystal display

Electric field-induced deformation is studied in a unique hybrid-aligned nematic (HAN) cell, in which the director of a nematic liquid crystal (LC) with either positive or negative dielectric anisotropy is varied continuously, lying perpendicular to one substrate and parallel to the other. The experimental results show the absence of threshold field in the birefringence–vs–applied voltage relationship as well as the slow variation of birefringence with voltage; these phenomena are analyzed in terms of the Frank-Oseen continuum theory. Also, the above characteristics of HAN are demonstrated to have such advantages in multicolor LC display applications as low operating voltage and fairly good color separation together with uniform and bright color generation.