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Studies of sessile droplets and fluid bridges of a ferroelectric nematic liquid crystal in externally applied electric fields are presented. It is found that above a threshold, the interface of the fluid with air undergoes a fingering instability or ramification, resembling to Rayleigh-type instability observed in charged droplets in electric fields or circular drop-type instabilities observed in ferromagnetic liquids in magnetic field. The frequency dependence of the threshold voltage was determined in various geometries. The nematic director and ferroelectric polarization direction was found to point along the tip of the fingers that appear to repel each other, indicating that the ferroelectric polarization is essentially parallel to the director. The results are interpreted in connection to the Rayleigh and circular drop-type instabilities.

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. I...

A thermal gradient-induced circular motion of particles placed on ferroelectric nematic liquid crystal sessile drops is demonstrated and explained. Unlike hurricanes and tornadoes that are the prime examples for thermal motors and where turbulent flows are apparent, here the texture without tracer particles appears completely steady indicating laminar flow. We provide a simple model showing that the tangential arrangement of the ferroelectric polarization combined with the vertical thermal gradient and the pyroelectricity of the fluid drives the rotation of the tracer particles that become electrically charged in the fluid. These observations provide a fascinating example of the unique nature of fluid ferroelectric liquid crystals.

Using a photo-responsive dimer exhibiting the transition between nematic (N) and twist-bend nematic (NTB) phases, we prepared spherical cap-shaped droplets on solid substrates exposed to air. The internal director structures of these droplets vary depending on the phase and on the imposed boundary conditions. The structural switching between the N and NTB phases was successfully performed either by temperature control or by UV light-irradiation. The N phase is characterized by an extremely small bend elastic constant K3, and surprisingly, we found that the droplet-air interface induces a planar alignment, in contrast to that seen for typical calamitic liquid crystals. As a consequence, the director configuration was stabilized in a structure substantially different from that normally found in conventional nematic liquid crystalline droplets. In the twist-bend nematic droplets characteristic structures with macroscopic length scales were formed, and they were well controlled by the droplet size. These results indicated that a continuum theory is effective in describing the stabilization mechanism of the macroscopic structure even in the twist-bend nematic liquid crystal droplets exhibiting director modulations on a scale of several molecular lengths.

The authors study the behavior of liquid crystal lens-shape drops in magnetic fields and find that the spatial variation of director becomes very complex, involving a defect wall that moves in time. The paper proposes a theory to explain the experimental observations and shows how to suppress the defects

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 interactions of phospholipids with liquid crystals have formed the basis for attractive biosensor technologies, but many questions remain concerning the basic physics and chemistry of these interactions. Phospholipids such as 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC), at sufficiently high (∼1 μM) concentrations and/or sufficiently long times, turn the liquid crystal director perpendicular to the LC/water interface. If the other side of the LC film is in contact with a surface that prefers perpendicular alignment, the LC film appears completely dark between crossed polarizers. Recently, however, Popov et al. (J. Mater. Chem. B, 2017, 5, 5061) noted that at even higher (∼10 μM) DLPC concentrations, the liquid crystal texture brightens again between crossed polarizers. To explain this surprising observation, it was suggested that the LC interface might bend. In this paper we show by optical surface profiler measurements that indeed the interface of the LC film of 4-cyano-4'-octylbiphenyl (8CB) suspended in a transmission electron microscopy (TEM) grid with openings of ∼0.5 mm in diameter bends towards the lipid-coated interface. We demonstrate that where the bending occurs, the bent interface exhibits extreme sensitivity to air pressure variations, producing an optical response with acoustic stimulation. Finally, we suggest a physical mechanism for this astonishing result.

Utilization of the self-organizing nature of soft materials is promising for fabricating micro- and nano-structures, which can be applied for optics. Because of the high birefringence, liquid crystals are especially suitable for optoelectronic applications such as beam steering and polarization conversion. On the other hand, most self-organized patterns in liquid crystals are one-dimensional and there are only a few examples of two dimensional systems. Here we study the light diffraction from a micro-pixelated pattern of a nematic liquid crystal which is formed by self-organization of topological defects. We demonstrate that the system works as a tunable two dimensional optical grating, which splits the incident laser beam and changes the polarization property. The intensity can be controlled by electrical voltages, which cause extinction of the zeroth-order beam. The polarization properties depend on the location of spots. The numerical calculation and the theoretical analysis not only support the experimental results but also unveil the uniqueness of the pixelated structure.

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.

Temperature triggered anchoring transitions in nematic liquid crystals have been investigated at solid, as well as at air interfaces. We established a searching algorithm based on sessile droplets allowing an efficient discovery of compounds that show orientational transitions. We have identified two types of the transition, one on the coated substrate and another at the free surface bounded by air. As a consequence of the two types of anchoring transitions, we revealed two distinct textures involving topological defects in spherical caps of nematics governed by the reorganisation of the director field. In order to understand this surface induced restructuring of the director field, we performed optical simulations of the sessile droplets. Apart from the known discontinuous interfacial orientational transition we have also identified continuous director restructuring. Moreover we have also detected a double transition, involving a re-entrant behaviour.

Experimental studies of the structure of plano-convex lens shaped liquid crystal droplets subjected to electric fields are presented. Similar to previous observations by Salamon et al. (2020) in DC magnetic fields, we found the formation of an inversion wall normal to AC electric fields. While at low frequencies the direction of the wall is stationary, at higher frequencies it turns toward the external electric field. In both cases, the defect wall is also swept toward the periphery of the drop, where it eventually disappears. The linear displacement of the electric field-induced defect wall could be described by an exponential time dependence without any fitting parameter. This combined with threshold for director deformation enables to determine both the bend elastic constant and the rotational viscosity using much less substance than existing techniques. The rotation of the defect wall at high frequencies is a result of the antiparallel orientation of the effective moment vector and the electric field due to the lower dielectric constant and higher electric conductivity of the defect wall than of those of the rest of the liquid crystal droplet. Uniform electric field-induced generation, rotation and linear movement of defect walls is a unique phenomenon in soft matters.

Smart viscoelastic materials that respond to specific stimuli are one of the most attractive classes of materials important to future technologies, such as on-demand switchable adhesion technologies, actuators, molecular clutches, and nano-/microscopic mass transporters. Recently it was found that through a special solid-liquid transition, rheological properties can exhibit significant changes, thus providing suitable smart viscoelastic materials. However, designing materials with such a property is complex, and forward and backward switching times are usually long. Therefore, it is important to explore new working mechanisms to realize solid-liquid transitions, shorten the switching time, and enhance the contrast of rheological properties during switching. Here, a light-induced crystal-liquid phase transition is observed, which is characterized by means of polarizing light microscopy (POM), photorheometry, photo-differential scanning calorimetry (photo-DSC), and X-ray diffraction (XRD). The light-induced crystal-liquid phase transition presents key features such as (1) fast switching of crystal-liquid phases for both forward and backward reactions and (2) a high contrast ratio of viscoelasticity. In the characterization, POM is advantageous in offering information on the spatial distribution of LC molecule orientations, determining the type of liquid crystalline phases appearing in the material, and studying the orientation of LCs. Photorheometry allows measurement of a material's rheological properties under light stimuli and can reveal the photorheological switching properties of materials. Photo-DSC is a technique to investigate thermodynamic information of materials in darkness and under light irradiation. Lastly, XRD allows studying of microscopic structures of materials. The goal of this article is to clearly present how to prepare and measure the discussed properties of a photorheological material.

Electric field-induced patterns in liquid crystals have been observed and studied for about 50 years. During this time, a great variety of structures, detected under different conditions, have been described; theoretical descriptions were also developed parallel with the experiments and a huge number of papers have been published. The non-vanishing interest in the topic is due to several factors. First, most experimentalists working with new (or even well-known) liquid crystals apply sooner or later an electric field for different purposes and, as a response, often (maybe undesirably or unexpectedly) have to face with emergence of patterns. Second, understanding the complexity of the formation mechanism of regular patterns in a viscous, anisotropic fluid is an extremely challenging theoretical task. Third, specialists in display fabrication or in other applications are also interested in the results; either to make use of them or in order to avoid field-induced patterns. In this review, we attempt...

$O\phantom{\rule{0}{0ex}}p\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}l$ $v\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}s$ (corkscrews of light spinning about zeros in the electric field) offer many applications in $e.g.$ optical communication, superresolution microscopy, and astronomical imaging, so creating them controllably is of considerable interest. The authors reveal that an electric-field-induced, self-assembled grid of topological defects can generate optical vortices at different length scales, via two mechanisms: directly by transmission through an individual defect, and by diffraction from a dislocation in the grid pattern. The efficiency of generating optical vortices here can approach 100%, even for different wavelengths of light, and can be tuned by an applied voltage.

In this paper, we present a new photoswitchable viscoelastic material with liquid crystalline natures, which exhibits two types of nematic liquid crystalline phases and a crystal phase beneath the isotropic liquid. We present its light-triggered phase behaviors and photo-switchability of the viscoelastic properties.

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 $Ug{U}_{c}$. 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 [Sov. 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 $U\ensuremath{\gg}{U}_{c}$. Unfortunately, the corresponding analysis suffers from mistakes, which in a combination led to that result.

In attempting to extend the utility of liquid crystals (LCs) in optics, the authors show that the orientation of the electric-field-induced convection patterns of a cholesteric LC can be modified via ultraviolet (UV) irradiation. This tunability is due to a photosensitive chiral dopant, which can change its molecular shape upon illumination; this makes the pitch of the cholesteric helical structure dependent on the intensity of the UV light, while the pattern's orientation is governed by the thickness-to-pitch ratio. This phenomenon allows the design of innovative photonic devices, such as light deflectors switchable by UV pulses.

Liquid-crystal display technology is ubiquitous, but these materials can also be used to make electrically tunable components for optics and photonics. The authors demonstrate voltage control of the wavelength of flexoelectric domains in a transmissive diffraction grating, which enables dynamic light steering. They prove that the mechanism of pattern onset differs from that of switching between flexodomain states, and they explain the surprising finding that the response to increasing voltage is much slower than that to decreasing voltage.

The formation of flexoelectric stripe patterns (flexodomains) was studied under the influence of external electric and magnetic fields in a nematic liquid crystal. The critical voltage and wavevector of flexodomains were investigated in different geometries by both experiments and simulations. It is demonstrated that upon altering the orientation of the magnetic field with respect to the director, the critical voltage and wavenumber behave substantially differently. In the geometry of the twist Freedericksz transition, a non-monotonic behavior as a function of the magnetic field was found.

The effect of superposed dc and ac applied voltages on two types of spatially periodic instabilities in nematic liquid crystals, flexoelectric domains (FD) and electroconvection (EC), was studied. The onset characteristics, threshold voltages and critical wave vectors, were determined. We found that in general the superposition of driving with different time symmetries inhibits the pattern forming mechanisms for FD and EC as well. As a consequence the onset extends to much higher voltages than the individual dc or ac thresholds. A dc bias induced reduction of the crossover frequency from the conductive to the dielectric EC regimes and a peculiar transition between two types of flexodomains with different wavelengths were detected. Direct measurements of the change of the electrical conductivity and its anisotropy, induced by the applied dc voltage component, showed that the dc bias substantially affects both parameters. Taking into account the experimentally detected variations of the conductivity in the linear stability analysis of the underlying nemato-hydrodynamic equations, a qualitative agreement with the experimental findings on the onset behavior of spatially periodic instabilities was obtained., 16 pages, 18 figures

This article summarizes the results obtained by various experimental methods on the physical properties of a bent-core nematic liquid crystal 4-chloro-1,3-phenylene bis-4-[4′-(9-decenyloxy) benzoyloxy] benzoate (ClPbis10BB). The material exhibits unusual properties in all aspects tested. Its bend flexoelectric coefficient is 1000 times larger than in calamitics; it is viscoelastic with a large, shear-rate-dependent viscosity. Its bend and twist elastic constants are abnormally low; thus the nematic phase can be rendered to be a blue fog phase with a small amount of chiral dopant. It shows very high flow birefringence and unusually small leading Landau coefficient. It has two types of isotropic phases; at lower temperature it is probably tetrahedratic that can be transferred into the nematic phase with magnetic field. ClPbis10BB has a frequency-dependent conductivity anisotropy which is characterized by a double sign inversion. It exhibits various electroconvection (EC) patterns which are currently not und...

A systematic overview of various electric-field induced pattern forming instabilities in nematic liquid crystals is given. The standard hydrodynamic description of nematics predicts the occurrence of striped patterns (rolls) in various wavenumber ranges, which depend on the anisotropy of the dielectric permittivity and that of the electrical conductivity as well as on the initial director orientation (planar or homeotropic). We discuss in detail three basic configurations with emphasis on the characterization of the threshold voltage and the critical wavenumber of the resulting patterns. Some features of the weakly nonlinear behavior are also addressed. Experimental exploration of additional pattern types, not captured by the standard model of electroconvection, is also presented. The impact of flexoelectricity as a possible explanation of the nonstandard EC patterns is discussed.

A regular domain structure consisting of parallel stripes – flexodomains – have been induced by low frequency (subHz) electric voltage in a bent core nematic liquid crystal. The wavelength of the pattern is in the range of 1–10 micrometers and thus can conveniently be observed in a polarizing microscope. It also serves as an optical grating and produces a regular system of laser diffraction spots. The pattern was found to emerge and disappear consecutively in each half period of the driving, with the wavelength of the flexodomains changing periodically as the ac voltage oscillates. Analyzing the polarization characteristics of the diffracted light, the polarization of the first order spot was found perpendicular to that of the incident light, in accordance with a recent theoretical calculation.

Electroconvection is a classical example of pattern-forming phenomena in liquid crystals, typically observed in nematics with negative dielectric and positive conductivity anisotropies. This article focuses on how electroconvection in the homeotropic geometry differs from that in planar alignment. The influence of an additional magnetic field on the pattern characteristics and on secondary instabilities (the normal roll–abnormal roll transition) is discussed. The homeotropic alignment offers unique possibilities also for studying defect motion. Basic characteristics of some patterns of large wavelength are presented and compared with those of the classical Carr–Helfrich structures. Finally, electroconvection in substances with negative conductivity anisotropy is addressed.

In homeotropically aligned nematics with negative dielectric anisotropy the electrohydrodynamic instability occurs above a bend Freedericksz transition. In the presence of a magnetic field parallel to the liquid crystal slab, ordered roll patterns with a well-defined uniform wave vector id appear above the onset of convection. By rotating the cell around an axis perpendicular to the slab by a small angle α, one can manipulate the system into a state with wave vector = id + Δ, where Δ is roughly perpendicular to id. We have studied experimentally the motion of defects, which then move essentially perpendicular to the rolls. The direction as well as the magnitude of the velocity as a function of Δ agrees with predictions of the weakly nonlinear theory. In particular, we obtain evidence for the nonanalyticity for Δ → 0.

The effect of superimposed ac and dc electric fields on the formation of electroconvection and flexoelectric patterns in nematic liquid crystals was studied. For selected ac frequencies, an extended standard model of the electrohydrodynamic instabilities was used to characterize the onset of pattern formation in the two-dimensional parameter space of the magnitudes of the ac and dc electric field components. Numerical as well as approximate analytical calculations demonstrate that depending on the type of patterns and on the ac frequency, the combined action of ac and dc fields may either enhance or suppress the formation of patterns. The theoretical predictions are qualitatively confirmed by experiments in most cases. Some discrepancies, however, seem to indicate the need to extend the theoretical description.

Two kinds of electroconvection (EC) patterns in an ether-bridged bent-core nematic liquid crystal material (BCN), which appear in different frequency ranges, are examined and compared in this paper. One is a longitudinal pattern with the stripes parallel to the orientation of the BCN and with a periodicity of approximately the cell thickness, occurring in the high frequency range of several hundreds Hz; the other one is oblique stripes, which results in a zig-zag pattern, and appears in the low frequency range of several tens Hz. In addition, within an intermediate frequency range, transformations from oblique to longitudinal and then to normal stripes occur at increased ac voltages. In particular, we investigated the temperature behavior of longitudinal and oblique stripes: when the temperature T increases and approaches the clearing temperature Tc, the contrast of the domains is enhanced and the frequency range of existence becomes wider, while the onset voltages increase only moderately instead of diverging, thus suggesting an isotropic mechanism of pattern formation.

A periodic structure (the prewavy pattern) found below the onset of electroconvection in a homeotropically aligned nematic liquid crystal is reported. It is characterized by a periodic modulation of the director in the xy-plane, which can be distinguished from electroconvection patterns with modulation in the xz-plane. The prewavy instability is investigated in detail; a phase diagram in the frequency-voltage plane, the voltage dependence of the azimuthal rotation angle, the voltage and frequency dependence of the wavelength of the prewavy pattern, and the director field in the prewavy pattern are provided. This prewavy pattern always evolves into the so-called chevrons above onset of convection. The wavelength of the chevrons and the orientation of their alternating zig and zag rolls depend on the director structure of the prewavy pattern.

The dynamic response of dendritic solidification to spatially homogeneous time-periodic forcing has been studied. Phase-field calculations performed in two dimensions (2D) and experiments on thin (quasi 2D) liquid crystal layers show that the frequency of dendritic side-branching can be tuned by oscillatory pressure or heating. The sensitivity of this phenomenon to the relevant parameters, the frequency and amplitude of the modulation, the initial undercooling and the anisotropies of the interfacial free energy and molecule attachment kinetics, has been explored. It has been demonstrated that besides the side-branching mode synchronous with external forcing as emerging from the linear Wentzel-Kramers-Brillouin analysis, modes that oscillate with higher harmonic frequencies are also present with perceptible amplitudes., 15 pages, 23 figures, Submitted to Phys. Rev. E

Unusual electroconvection scenarios have been found in a homeotropic nematic liquid crystal subjected to a small magnetic field: two Lifshitz frequencies, oscillations between pattern free state and chaos, and CRAZY rolls. The variation of the azimuthal angle of the director has been measured around the normal roll—abnormal roll transition at various frequencies. Experimental results agree well with theoretical expectations. A model is proposed to describe the structure of CRAZY rolls.

This contribution deals with three types of orientational phenomena generated by oscillatory #ow in nematic liquid crystals: (1) Uniform orientational instabilities in the geometry where the director is initially oriented within the #ow plane. The instability occurs under rectilinear Poiseuille #ow and not under Couette #ow. (2) The "rst instability under rectilinear Couette #ow is a bifurcation to a stationary roll pattern with the roll axis perpendicular to the #ow direction. The threshold amplitude has been measured for homeotropic alignment and calculated by linear stability analysis. (3) An interesting slow director precession and nonlinear waves which arise in homeotropically aligned nematics exposed to an elliptical shear #ow in the bend FreH edericksz distorted state. The experimental data are compared with a theoretical analysis. The precession can also be generated by oscillatory compression. ( 2000 Elsevier Science B.V. All rights reserved.

Recent experimental results on electroconvection in homeotropic nematic liquid crystals are summarized. Static and dynamic properties of the patterns at and above threshold are discussed. The azimuthal angle of the director is determined for various pattern morphologies (normal, abnormal and CRAZY rolls). Preliminary results on controlled defect motion are presented.

Free growth properties of the smectic B liquid crystalline phase into the supercooled nematic have been investigated in quasi-twodimensional geometry. Different orientation combinations of the two phases have been achieved experimentally and the interfacial patterns have been studied and analysed as a function of undercooling. The angular dependence of the surface tension has been deduced from the shape of the interface in thermal equilibrium. The experimentally determined surface tension anisotropy has been incorporated into computer simulations based on the phase-field model. The simulations have reproduced qualitatively the rich variety of morphologies observed in the experiments for a given set of undercoolings in three geometries. Anisotropic heat diffusion on the nematic side, relevant to our experimental system has also been introduced. Both in the experiments and in the simulations we find that the growth is faster in the lower heat diffusion direction.

One of the most spectacular and practically important growth modes observed in nature is dendritic solidification [1]. A dominant part of the present knowledge on dendrites originates from studies on transparent organic model systems [1,2]. The nonlinearity of the problem implies that periodic external perturbations may lead to specific resonance patterns that could be used for regulating the growth morphology. The understanding of dynamic response to such perturbations could open a novel route to materials of application tailored properties. A spatially inhomogeneous controlling of dendritic sidebranching has already been achieved by periodic local heating of the dendrite tip by laser beam [3], or by oscillatory flow field [4]. In this work we explore the possibility of controlling dendritic growth homogeneously throughout the sample via pressure oscillations that influence the melting point, and uniform periodic heating in the volume. We use the phase-field model, an approach that is able to handle all stages of morphology evolution including dendritic growth [1,5,6], to identify the most important process parameters and resonance conditions in two dimensions (2D). The predictions are tested by experiments on quasi-2D liquid crystal layers, known as suitable model materials [7]. The phase-field model represents a dynamic extension of the Cahn-Hilliard theory of first-order phase transformations that couples the evolution of the order-parameter distribution [fr, t phase field] to thermal transport [1,5,6]. The respective equations that incorporate the anisotropy of both the interfacial free energy su so ˜ su and kinetic coefficient bu bo ˜

Although the alkyl-bicyclohexyl-carbonitrile compounds supposed to be chemically stable, an aging behaviour was observed which strongly influences the growth of smectic B germs in the undercooled nematic phase. The aging also results in a broadening of the phase coexistence temperature ranges and in a saturating shift of phase transition temperatures with a temperature dependent dynamics. The phenomenon has been characterized by polarizing microscopy and DSC, and an experimental evidence has been given that the air atmosphere is responsible for the thermally induced ageing.