•Nonlinear electro-opto-mechanical coupling in dielectric nematic elastomers is simulated.•Governing and constitutive equations are derived based on the variational approach.•A modification of the ...semi-soft elastic energy of nematic elastomers is proposed.•The subcritical character of the solid Fréedericksz transition is changed to supercritical.•Analytical and finite element solutions for the solid Fréedericksz transition are obtained.
Based on the variational principle, we derive the balance equation for momentum, Maxwell equation, and the equation of director fields for nonlinear electro-opto-mechanical coupling in dielectric liquid crystal elastomers (LCEs). Further, we establish a simple constitutive model to study the electric-field-induced director reorientation with the deformation of monodomain nematic LCEs, which is called the solid Fréedericksz transition (SFT). Semi-analytical method is utilized to obtain the solutions of stress-free homogeneous SFT. The results indicate that the semi-soft elasticity of LCEs is insufficient to simulate the supercritical SFT of samples, as observed in earlier reported experiments. A modification of the semi-soft elastic energy of LCEs is proposed to change the subcritical character of the SFT to supercritical. In addition to the semi-analytical methods, finite element method is employed to simulate the experiments in which samples are immersed in dielectric liquids. The simulation results show good agreement with the experimental data from literature.
Multi-walled carbon nanotubes (MWCNTs) have been shown to self-organise, and when added as a guest to form a nanocomposite, their director couples with an organic liquid crystal (LC) host. Here, ...effects of MWCNTs on the low-frequency dielectric properties and Fréedericksz transition of the LC 4-cyano-4ʹ-pentylbiphenyl (5CB) are studied. Anti-parallel electro-optic cells were filled with nanocomposites at weight percent concentrations of MWCNT to 5CB of: 0 (neat), 0.01, 0.10, 0.20, and 0.50. Low concentration was chosen to minimise Van der Waals attraction normally responsible for aggregation of MWCNTs. Dielectric relaxation spectroscopy was used to study interactions between MWCNTs and 5CB at frequencies from 20 Hz to 1 MHz. We propose a mechanism based upon measurements of the complex dielectric function which suggests that MWCNTs act as a slow-moving boundary within the sample cell at low frequencies and low applied electric fields, where the MWCNTs reorient along with the 5CB LC molecules. At higher frequencies and larger applied electric fields, the 5CB molecules rotate about their long axes while motion of the MWCNTs is frozen out.
In this paper, results acquired from capacitance measurements performed on composites based on nematic liquid crystal 4-cyano-4′-hexylbiphenyl (6CB) and spherical iron oxide nanoparticles of various ...sizes are presented. Electric and magnetic Fréedericksz transitions, as well as structural transitions in combined electric and magnetic fields, were investigated. The obtained results showed the lowering of the threshold magnetic field with an increase in the volume concentration of nanoparticles. Estimations based on results obtained from measurements suggest soft anchoring between liquid crystal director and nanoparticles magnetization vector.
•A novel composite magnetite-polymer particle/lyotropic liquid crystal was obtained.•The experimental magnetic Fréedericksz transition using a laser beam was studied.•The estimated critical field was ...compared to our new theoretical model.
We obtained a new highly ordered colloidal composite using specially manufactured magnetite-polymer nanoparticles and lyotropic liquid crystal. A good compatibility between the components was ensured by the functionalization of the particles during their synthesis. We studied the laser light transmission for the mixtures filled in sandwich-glass cells with homeotropic and planar treatment of the surfaces under external magnetic field. The Fréedericksz transition critical field was estimated, and its’ behavior was compared to our new theoretical model based on the Brochard-de Gennes one.
•Character of easy axis - electric field coupling affects the character of orientation instability of nematic cell.•Electro-sensitive easy axis noticeably changes the director field deformation in ...nematic cell in applied electric field.•Electro-optical response of nematic cell depends on character of easy axis - electric field coupling.
The orientational instability of a director in a planar flexoelectric nematic liquid crystal cell in a constant electric field oriented perpendicular to the cell substrates is investigated. The easy axis on the surface of one of the polymer substrates can change its orientation, which is due to the impact of the initial surface orientation, liquid crystal, and electric field. The influence of the latter leads to the reorientation of the elastic parts of the polymer molecules of the substrate, which is a consequence of the interaction of intrinsic or induced dipole moments with the electric field. The corresponding contribution of such an interaction to the surface free energy of the nematic is considered to be linear or quadratic in terms of the electric field strength E. It is established that the orientational instability of the director has a threshold in the case of quadratic effect of the electric field and is thresholdless if the effect is linear. The temporal behavior of the director after the application of voltage with the subsequent transition of the system to a stationary state and its return to the initial homogeneous state after switching off the voltage is studied. The characteristic turn-on/off times of the system and the time of reaching the stationary state are calculated and their dependence on the system parameters is investigated. The transmittance of the nematic liquid crystal cell is calculated for a normally incident light.
In smectic-
A
(SmA) liquid crystals, rod-like molecules are organized into layers with their optical axis oriented along the normal to these layers. When subject to an external field and for a ...certain critical threshold, these layers can buckle into a new different configuration. Here, we consider SmA liquid crystals which are confined between two parallel plates aligned in the so-called
bookshelf
geometry and subject to an external magnetic field. By assuming that instability is induced by a uniform field, we investigate the influence of the anchoring strength on the critical threshold field and on the layers shape by a perturbative analysis to the equilibrium equations. Main differences with respect to the standard Fréedericksz transition of nematics are highlighted. The behavior of this threshold effect suggests a new way to measure geometrical and constitutive parameters of a SmA sample.
The twist-bend nematic phase (NTB) exhibits a complicated hierarchy of structures responsible for several intriguing properties presented here. These are: the observation of a fast electrooptic ...response, the exhibition of a large electroclinic effect, and the observation of an unusual pattern of the temperature dependence of birefringence of bent-shaped bimesogens in parallel-rubbed planar-aligned cells. These unusual effects inspired the use of highly sophisticated techniques that led to the discovery of the twist-bend nematic phase. Results of the optical retardation of a parallel-rubbed planar-aligned cell show that the ‘heliconical angle’ (the angle the local director makes with the optical axis) starts increasing in the high temperature N phase, it exhibits a jump at the N–NTB transition temperature and continues to increase in magnitude with a further reduction in temperature. The liquid crystalline parallel-rubbed planar-aligned and twist-aligned cells in this phase exhibit fascinating phenomena such as a demonstration of the beautiful stripes and dependence of their periodicity on temperature. The Fréedericksz transition in the NTB phase is found to be of the first order both in rubbed planar and homeotropic-aligned cells, in contrast to the second order transition exhibited by a conventional nematic phase. This transition shows a significant hysteresis as well as an abrupt change in the orientation of the director as a function of the applied electric field. Hierarchical structures are revealed using the technique of polymer templating the structure of the liquid crystalline phase of interest, and imaging of the resulting structure by scanning electron microscopy.
In this article we derive a Ginzburg–Landau energy functional for a nematic inhomogeneous superfluid in presence of an electric field. The molecules occupy an infinite cylinder Ω with cross section ...D. We suppose vacuum in R3∖Ω, with the possibility that an external electric field can be applied parallel to D. The Helmholtz free energy is obtained by taking the London limit of a Ginzburg–Landau nematic superconducting model in absence of magnetic fields, and by including an appropriate contribution of the electric potential energy. We show that the critical parameter inside Ω, which defines the Fréedericksz transition on the molecular alignment, is not only influenced by the effects of the electric field in the sample, but also by the additional contribution of the superfluid current. We take a particular solution for the Ginzburg–Landau equations, where the superfluid phase does not have circulation. Then, we demonstrate that the corresponding Fréedericksz threshold can be calculated, on an arbitrary domain, by using the notion of the first positive eigenvalue of the Laplacian. This eigenvalue depends on the chosen geometry and the boundary conditions on the nematic phase in the sample. Next, we apply our results in an infinite slab and in an infinite cylinder with circular cross section, where the nematic superfluid system is subjected to Dirichlet or Neumann boundary conditions in each case. We deduce a modified Fréedericksz threshold, for each configuration mentioned before, in a uniform electric field. In these instances we notice the remarkable fact that, for specific values and regimes of the intrinsic parameters, the critical fields are different than the ones obtained in the pure nematic case. Finally, we also study a Fréedericksz type threshold in a long hollow cylinder with uniform charge density, where molecules are reoriented by the electric field produced only by the internal charges of the sample. This setting suggests that, if molecules are oriented radially at the boundary of the region, a Fréedericksz type threshold appears in order to maintain the radial molecular distribution, which varies with the typical radii of the domain.
•Response of superfluid nematic liquid crystal under the action of an electric field.•Phase transition under the action of the electric field.•Calculus of the threshold for this transition in different geometries.
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