Periodic flexoelectric domains (FDs) stimulated by both electric and optical fields were examined in a photosensitive chiral nematic liquid crystal. It was observed that the optical field is an excellent candidate to modulate the morphology and electric threshold voltage of FDs, by either an optically induced surface effect through photo-alignment or an optically induced bulk effect through the optical control of chirality. With preset spatial patterns of the optical field, proof-of-concept prototypes of ring-shaped and fan-shaped gratings were constructed, which can be used in encoders to measure the angle directly and precisely.

Light polarization rotations, created by applied optical field, are examined experimentally and theoretically in a photosensitive chiral nematic fluid. The polarization rotation of the transmitted beam is initiated by illuminating the sample with uniform UV light. The operation is tunable and reversible, depending on the UV intensity. It was revealed that the rotations can be ascribed to the optical-field-induced chirality effect, where the helical structure in chiral nematics changes in accordance with the UV intensity. The evolution of the helical structure as well as its effect on the light polarization upon illumination by uniform UV light have been monitored experimentally and compared by calculations based on the continuum theory. Our results proved that a polarization field with specific characteristics can be achieved using the remote and precise optical control.

Rotation of the light polarization of a diffracted beam, manipulated by imposed patterned UV optical field, is proposed and examined in a photosensitive chiral nematic liquid crystal. Such operations can be performed dynamically and reversibly, by varying the UV intensity. It was revealed that the rotations are ascribed to an optical-field-induced chirality effect, where the helical structure in chiral nematics changes periodically in space in accordance with the UV intensity profile. Using continuum elastic theory and the optical Jones matrix technique, we determined the dependence of polarization orientation on UV intensity. By changing the polarity and magnitude of chirality in chiral nematics, UV light can control the orientation of the polarization state sweeping across the entire horizontal plane, making it possible for polarization converter applications. Proof-of-concept prototypes capable of generating customized vortex and vector beams are demonstrated, proving that by virtue of remote and precise control of the optical field, a polarization field with specific spatial structures can be generated.

The electrical propertiesElectrical Properties of multiwalled carbon nanotubesMultiwalled Carbon Nanotube (MWCNT) based nanocompositesNanocomposites were experimentally investigated in the frequency range between 100 Hz and 1 MHz and temperature between 240 and 380 K. Two types of dielectricMatrices matricesEpoxy (Epoxy and Polyester) werePolyester used to produce two series of nanocompositesNanocomposites (Polyester-MWCNT and Epoxy-MWCNT) with different concentrations of MWCNTMultiwalled Carbon Nanotube. The obtained temperature dependence of electrical propertiesElectrical Properties of the various samples was compared and explained. Results show that the PolyesterPolyester-MWCNT nanocomposites present a lower percolation thresholdPercolation Threshold than the EpoxyEpoxy-MWCNT nanocomposites. An important thermoelectric phenomenon of transition was found in these two nanocompositesNanocomposites, above the percolation thresholdPercolation Threshold, which is the positive temperature coefficient in the resistivity effect. Moreover, the results showed that PolyesterPolyester-MWCNTMultiwalled Carbon Nanotube nanocompositesNanocomposites exhibit the maximum positive temperature coefficientPositive Temperature Coefficient intensity.

The response to applied electric field of a newly synthesized hockey-stick nematic 8-F-OH has been stud-ied. Unusual pattern forming scenarios were found, which show rich variety as a function of frequency, temperature and initial director alignment. The behavior has been compared with those, observed in calamitic and bent-core nematics. Experiments show that contrary to calamitic nematics, here the elas-tically strongly deformed layers due to Freedericksz transition play a decisive role in the formation of reg-ular patterns. This is attributed to the cybotactic nematic phase with polar smectic clusters, where flexoelectricity might also have influence. Static and dynamic behaviors of the patterns have been char-acterized and an application idea is formulated.(c) 2022 Published by Elsevier B.V.

This review, dedicated to the memory of B. K. Sadashiva, concerns electrically driven patterns and defects in bent core nematics (BCNs) made of hard-core, soft-core or bent-rod type of molecules. It covers (a) spatially periodic patterns associated with electroconvection and flexoelectric deformation, (b) dislocations in these patterned states and (c) various types of defects induced essentially in homogeneous, unpatterned states of BCNs. While in general, the wealth of pattern morphologies is just as rich in BCNs as in rod-like nematics, certain features of electroconvection have so far been observed only in some BCN materials. Some attributes relating to the generation and behaviour of defects in electric field may also be ascribed to material properties specific to BCNs. It is also shown that the electric field-induced patterns as well as their modification by light irradiation may serve as optical gratings and thus establishes the background for designing new photonic devices.

In nanofluidNanofluid composites, competing interactions, interplay and proximity effects at the interface between the different constituents often lead to interesting physical propertiesChemico-Physical Properties, sometimes to novel effects and to new functionalities. In this paper, we focus our interest on the electrical and dielectric propertiesDielectric Properties of the graphene oxide (GO)/water nanofluidNanofluid composite and on their modeling. These properties are reported in the frequency range 1‒1 MHz and in the temperature range from 295 to 309 K. The temperature dependence of the DC electrical conductivity shows a typical negative temperature coefficient in resistivity (NTCR) effect of this material. The mechanism responsible for the change in resistivity is probably predominantly tunneling, wherein the GO particles are not in physical contact and the electrons tunnel through the water gap between them. The DC electrical conductivity obeys an Arrhenius law below and above a critical temperature; that allows us to calculate both activation energies. Moreover, the dielectric responseDielectric Response was analyzed using complex permittivityComplex Permittivity formalism. A relaxation phenomenonRelaxation Phenomenon is induced in the nanofluidNanofluid suggesting that the presence of the GO particles greatly affects the dielectric propertiesDielectric Properties of the water due to the polarization phenomenon created by them. The Havriliak–Negami model was used to fit the experimental results.

While morphological gratings made of solid structural materials have permanent periodicity, gratings induced by a periodic flow provide wide tunability utilizing the pattern formation mechanism and its dependence on the material properties. As a paradigm of such flow, here electric-field induced patterns, manifestations of electroconvection, are reported in a hybrid aligned cholesteric liquid crystal containing a chiral photosensitive dopant. The irradiation by UV light rotates the pattern via alteration of the helical pitch, where the magnitude and direction of the rotation angle depends on the UV light intensity, the concentration and type of the chiral dopant. A prototype grating capable of two-dimensional beam steering is demonstrated. Such tunability will guide chemists to search for smart molecular structures so as to control complex fluids in developing photonic devices with desired parameters.

Large‐scale patterning of topological defects is vital and challenging from both fundamental and technological points of view in anisotropic fluids. However, this is usually difficult because of their unfavorably high‐energy states. Here, a simple but general pathway for topology engineering is presented: processing topological defects and shape large‐scale patterns in materials with liquid crystalline nature. Dragging field is created through flowing materials at liquid–liquid crystal phase transition temperature or designing electric‐field driven temperature gradient. The dragging fields coupled to a dense colony of topological defects with random spatial distribution form nontrivial periodic ordered topological patterns that are energetically unfavorable compared to the uniform ground state, but are stable in the stationary state. Topological polymeric films based on the strategy are also fabricated. The dragging speed and surface interactions are found to be dominant factors in generating and stabilizing the patterns. This strategy endows fluids with regular and large‐scale topological patterns, paving a new way for the development of fluids and gels with spatially modulated topological nature.

A new photoresponsive bent-core nematic (BCN) material, which exhibits flexoelectric domains (FDs) driven by electric field, is reported. Unexpectedly, it is found that the morphologies of FDs can be controlled by irradiation with light fields. This light tunability is ascribed to the photoisomerization effect of the azo moiety within the BCN molecules, where the ratio of trans and cis isomers changes according to the parameters of the light field, resulting in adjustable electric threshold and periodicity of FDs. Based on this principle, a prototype of controllable optical grating is assembeled, whose operation can be manipulated by the wavelength or intensity of light. Due to the easy, instant, and remote operation by light, this optical, contactless tunability has a great advantage over traditional electric control in tunable photonic devices.

The dynamic magnetic susceptibility of ferronematics (nematic liquid crystals doped with magnetic nanoparticles) has been measured for high concentrations of nanoparticles. For a given composition of the ferronematic (the same nanoparticles, and the same liquid crystal matrix) we have found the optimal volume concentration of the magnetic particles, for which the largest relative enhancement of the dynamic magnetic susceptibility is obtained upon the application of a bias magnetic field in the isotropic phase.

We investigate flexodomains, which are observed in planar layers of certain nematic liquid crystals, when a dc voltage U above a critical value U-c is applied across the layer. They are characterized by stationary stripelike spatial variations of the director in the layer plane with a wave number p(U). Our experiments for different nematics demonstrate that p(U) varies almost linearly with U for U > U-c. That is confirmed by a numerical analysis of the full nonlinear equations for the director field and the induced electric potential. Beyond this numerical study, we demonstrate that the linearity of p(U) follows even analytically, when considering a special parameter set first used by Terent' ev and Pikin [Soy. Phys. JETP 56, 587 (1982)]. Their theoretical paper serves until now as the standard reference on the nonlinear analysis of flexodomains, since it has arrived at a linear variation of p(U) for large U >> U-c. Unfortunately, the corresponding analysis suffers from mistakes, which in a combination led to that result.

The influence of UV light-induced pitch contraction and dilation on the electroconvection patterns (ECPs) of a chiral nematic liquid crystal containing a photoresponsive chiral dopant is investigated in planar-aligned cells. It is observed that the helical twisting power of the dopant changes (even undergoes handedness inversion) under UV irradiation; consequently, the pitch and the direction of the convection rolls in ECPs (being either parallel with or perpendicular to the surface alignment) could be controlled by the UV intensity and the ac voltage. In contrast to traditional methods, where the pitch is modulated by electric or thermal fields, our method of applying a light field allows a remote and contactless manipulation of the pitch, which is easily detectable via the morphological changes of ECPs. The ability to control the orientation of ECPs by tuning the light intensity can conveniently be utilized as an optical grating, allowing switchable, dual-mode operation.

A stable lattice of topological defects occurring at the electric reorientation of a nematic fluid is studied by quantitative polarimetry and laser diffraction. Generation of optical vortices by topological defects is demonstrated in the case of two distinct mechanisms. First, individual defects convert circularly polarized light partially into a vortex beam with opposite handedness, while beams diffracted on the defect lattice do not carry vorticity. Second, dislocation of the lattice structure is a topological defect on a larger length scale; then beams diffracted on a single dislocation possess optical vortex character. The vortex-generation efficiency is tunable by the applied voltage.

The possibilities to induce regular, stationary stripe patterns with easily controllable wavenumber have been investigated with the aim to apply them in optical devices as gratings. Several, electric field induced phenomena have been studied on three members of a homologous series of a bent core nematic. Pattern morphologies, threshold voltages and wave numbers have been determined. Temporal evolution and switching dynamics have been analyzed.

The influence of magnetic field on the isotropic-to-nematic phase transition temperature is investigated in neat bent-core and calamitic liquid crystals, in their mixture, and in samples doped with spherical magnetic nanoparticles for two different orientations of the magnetic field. A magnetic-field-induced negative or positive shift of the transition temperature was detected depending on the magnetic field orientation with respect to the initial orientation of the nematic phase, and on the type of liquid crystal matrix.

The influence of UV light-induced pitch contraction and dilation on the electroconvection patterns (ECPs) of a chiral nematic liquid crystal containing a photoresponsive chiral dopant is investigated in planar-aligned cells. It is observed that the helical twisting power of the dopant changes (even undergoes handedness inversion) under UV irradiation; consequently, the pitch and the direction of the convection rolls in ECPs (being either parallel with or perpendicular to the surface alignment) could be controlled by the UV intensity and the ac voltage. In contrast to traditional methods, where the pitch is modulated by electric or thermal fields, our method of applying a light field allows a remote and contactless manipulation of the pitch, which is easily detectable via the morphological changes of ECPs. The ability to control the orientation of ECPs by tuning the light intensity can conveniently be utilized as an optical grating, allowing switchable, dual-mode operation.

Nematic liquid crystals exhibit a great variety of pattern forming instabilities. Besides the electric field-induced Fréedericksz transition and flexodomains we introduce the main types of electroconvection patterns. We also address thermally induced mechanisms, as pattern formation at phase transitions or buoyancy driven Rayleigh-Bènard convection. Finally we discuss shear induced phenomena and viscous fingering excited by pressure gradient.