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We report small angle x-ray scattering (SAXS) studies of isotropic, nematic, and smectic mesophases formed by binary mixtures of bent-core (BC) and rod-shaped (RS) molecules. While optical studies indicate that the components are fully miscible, SAXS reveals fascinating structures that are consistent with segregation on a nanoscopic scale. We find that tilted smectic clusters, which have been previously reported in both the nematic and isotropic states of the pure BC materials, are also present in mixtures with up to 50 wt% of the RS compound; this is consistent with previous dielectric and flexoelectric studies on such mixtures. Unexpectedly in this concentration range the clusters are present in the isotropic and in the induced smectic phase range, as well as throughout the nematic phase. The results in the smectic phase also reveal complex layering phenomena, providing important insight into the interaction between bent and rod-shaped molecules. These studies will be crucial in the design of promising new functional nanomaterials. [ABSTRACT FROM AUTHOR]

Angle-resolved, second-harmonic-light scattering (SHLS) measurements are reported for three different classes of thermotropic nematic liquid crystals (NLCs): polar and nonpolar rodlike compounds and a bent-core compound. Results revealing well-defined scattering peaks are interpreted in terms of the electric polarization induced by distortions of the nematic orientational field ("flexopolarity") associated with inversion wall defects, nonsingular disclinations, analogous to Neel walls in ferromagnets, that often exhibit a closed loop morphology in NLCs. Analysis of the SHLS patterns based on this model provides a "proof-of-concept" for a potentially useful method to probe the flexopolar properties of NLCs. [ABSTRACT FROM AUTHOR]

We present magneto-optic measurements on two materials that form the recently discovered twist-bend nematic Mb) phase. This intriguing state of matter represents a fluid phase that is orientationally anisotropic in three directions and also exhibits translational order with periodicity several times larger than the molecular size. Ntb materials may also spontaneously form a visible, macroscopic stripe texture. We show that the optical stripe texture can be persistently inhibited by a magnetic field, and a 25 T external magnetic field depresses the N-Ntb phase transition temperature by almost 1°C. We propose a quantitative mechanism to account for this shift and suggest a Helfrich-Hurault-type mechanism for the optical stripe formation. [ABSTRACT FROM AUTHOR]

We present magneto-optical measurements on two liquid crystals that exhibit a wide temperature-range amorphous blue phase (BPIII). Magnetic fields up to 25 T are found to suppress the onset of BPIII in both materials by almost 1 °C. This effect appears to increase nonlinearly with the field strength. The effect of high fields on established BPIIIs is also reported, in which we find significant hysteresis and very slow dynamics. Possible explanations of these results are discussed. [ABSTRACT FROM AUTHOR]

We demonstrate subnanometer resolution cryo-TEM imaging of smectic layers in the smectic and nematic phases of two bent-core liquid crystals. Our results show perfect periodicity over several hundred layers in the smectic phase and also provide the first direct evidence of smectic clusters on length scales of 30-50 nm in a nematic liquid crystal. The results are corroborated with small angle x-ray scattering measurements. The observation of smectic clusters in the nematic phase is of special interest in bent-core liquid crystals, where the smectic clusters are stable over wide temperature ranges, in contrast to the well-known pretransitional "cybotactic" clusters that appear only in the vicinity of a bulk smectic phase. The means to characterize and manipulate this nanoscale molecular order could open up completely new liquid crystal-based technologies. [ABSTRACT FROM AUTHOR]

We report on a series of measurements on the microscopic structure and the magneto-optical properties of a calamitic liquid crystalline compound in its nematic phase. Structural studies show the existence of short-range, tilted smectic order consistent with pretransitional effects above an underlying smectic phase. Concomitantly, magneto-optical results exhibit the existence of an optic axis not collinear with the uniaxial director. This apparent biaxial nature is discussed within the context of coupling between the tensor nematic and the smectic order parameters. [ABSTRACT FROM AUTHOR]

Using a magnetic Frederiks transition technique, we measure the temperature and concentration dependences of splay K1, twist K2, and bend K3 elastic constants for the lyotropic chromonic liquid crystal sunset yellow formed through noncovalent reversible aggregation of organic molecules in water. K1 and K3 are comparable to each other and are an order of magnitude higher than K2. At higher concentrations and lower temperatures, K1 and the ratios K1/K3 and K1/K2 increase, which is attributed to elongation of self-assembled lyotropic chromonic liquid crystal aggregates, a feature not found in conventional thermotropic and lyotropic liquid crystals formed by covalently bound units of a fixed length. [ABSTRACT FROM AUTHOR]

We report on homodyne dynamic light scattering measurements of orientational fluctuation modes in both calamitic and bent-core nematic liquid crystals, carried out in the new split-helix resistive magnet at the National High Magnetic Field Laboratory. The relaxation rate and inverse scattered intensity of director fluctuations exhibit a linear dependence on field-squared up to 25 tesla, which is consistent with strictly lowest order coupling of the tensor order parameter Q to field (QαβBαBβ) in the nematic free energy. However, we also observe evidence of field dependence of certain nematic material parameters, an effect which may be expected from the mean field scaling of these quantities with the magnitude of Q and the predicted variation of Q with field. [ABSTRACT FROM AUTHOR]

The effect of director pretilt on the twist magnetic Fréedericksz transition of nematics was investigated in a planar cell. The director configuration was calculated as a function of magnetic inductance. The dielectric and optical response of the nematic liquid crystal was numerically modeled. A dielectric measurement method for determining the elastic constant K22 is presented. The influence of the conditions for the Mauguin effect is discussed. The theoretical predictions were confirmed by our experiments. Experimental data for all elastic constants of a bent-core nematic material are presented and discussed. [ABSTRACT FROM AUTHOR]

The existence of the elusive biaxial phase has been the subject of much discussion since it was predicted by Freiser in 1970. More recently, there have been numerous attempts to find a thermotropic liquid crystal that exhibits a biaxial phase and with this, conflicting reports about whether such a phase has been positively identified in bent-core liquid crystals. One reason for the discrepancy is that there is currently no way to role out surface effects or anchoring transitions, both of which may give a false positive identification of a uniaxial-biaxial nematic transition. We have developed a technique that uses a magnetic field to align the uniaxial director, thus widening its application to any bent-core nematic material. [ABSTRACT FROM AUTHOR]

Using a combination of dynamic light scattering and Freedencksz transitions induced in applied magnetic and electric fields, we have determined the absolute magnitudes of the Frank elastic constants and effective orientational viscosities of the bent-core nematic liquid crystal, 4-chloro-1,3-phenylene bis 4-[4'- (9-decenyloxy)benzoyloxy] benzoate. At a fixed temperature 2 °C below the isotropic-nematic transition, we find K11 = 3.1 × 10-12 N, K22 = 0.31 × l0-12 N, K33 = 0.88 × 10-12 N, ηsplay = 1.1 Pa s, ηtwist = 0.37 Pa s, and ηbend = 1.2 Pa s. The unusual anisotropies of these parameters are discussed in terms of short-range, smectic-C-like correlations among molecules in the nematic phase. [ABSTRACT FROM AUTHOR]

We present studies of chiral nematic liquid crystals composed of flexible dimer molecules subject to large dc magnetic fields between 0 and 31 T. We observe that these fields lead to selective reflection of light depending on temperature and magnetic field. The band of reflected wavelengths can be tuned from ultraviolet to beyond the IR-C band. A similar effect induced by electric fields has been presented previously, and was explained by a field-induced oblique-heliconical director deformation in accordance with early theoretical predictions. The use of magnetic field here instead of electric field allows precise measurements of some material constants and holds promise for wireless tuning of selective reflection. [ABSTRACT FROM AUTHOR]

We have determined the nematic-isotropic transition temperature as a function of an applied magnetic field in three different thermotropic liquid crystalline dimers. These molecules are comprised of two rigid calamitic moieties joined end to end by flexible spacers with odd numbers of methylene groups. They show an unprecedented magnetic field enhancement of nematic order in that the transition temperature is increased by up to 15 K when subjected to a 22 T magnetic field. The increase is conjectured to be caused by a magnetic-field-induced decrease of the average bend angle in the aliphatic spacers connecting the rigid mesogenic units of the dimers. [ABSTRACT FROM AUTHOR]

Second-harmonic generation (SHG) is studied in the magnetically aligned nematic phase of a bent-core liquid crystal (BCN) and compared to similar measurements made on a conventional rodlike (calamitic) nematic compound. The second-harmonic (SH) light detected from both materials is predominantly due to scattering and therefore incoherent. Results on the calamitic are consistent with a polarization induced by ordinary director fluctuations in the nematic phase. However, the SH scattering collected in the BCN exhibits a different temperature and angular dependence. We discuss how these differences could arise from the effects of short-range correlated, smectic-C-type molecular clusters, which have been detected in recent studies on various BCN materials.

We have found that bent-core liquid crystalline materials show exceptionally large intrinsic flow birefringence in their isotropic liquid phase. This effect is more than 100 times larger than typical values measured for low molecular weight liquid crystals. The specific flow birefringence (i.e., normalized by the flow viscosity) is an order of magnitude larger than in both side-chain polymeric as well as low molecular weight liquid crystals. We propose that this large enhancement for bent-core compounds may be attributed to nanoscale smecticlike clusters that persist above the nematic-isotropic transition temperature, and shear align under shear flow; however, this mechanism has not yet been definitively confirmed.

In this paper we report and analyze complex spatiotemporal dynamics recorded in electroconvection in the nematic liquid crystal I52, driven by an ac voltage slightly above the onset value. The instability mechanism creating the pattern is an oscillatory (Hopf) instability, giving rise to two pairs of counterpropagating rolls traveling in oblique directions relative to the unperturbed director axis. If a system of nonlinear partial differential equations shows the same set of unstable modes, the pattern above the onset is represented in a weakly nonlinear analysis as a superposition of the traveling rolls in terms of wave envelopes varying slowly in space and time. Motivated by this procedure, we extract slowly varying envelopes from the space-time data of the pattern, using a four-wave demodulation based on Fourier analysis. In order to characterize the spatiotemporal dynamics, we apply a variety of diagnostic methods to the envelopes, including the calculation of mean intensities and correlation lengths, global and local Karhunen-Loève decompositions in Fourier space and physical space, the location of holes, the identification of coherent vertical structures, and estimates of Lyapunov exponents. The results of this analysis provide strong evidence that our pattern exhibits extensive spatiotemporal chaos. One of its main characteristics is the presence of coherent structures of low and high intensities extended in the vertical (parallel to the director) direction.

We report on measurements of magnetic-field induced nematic order in the bent-core liquid crystal 4-chlororesorcinol bis[4-(4-n-dodecyloxybenzoyloxy) benzoate]. Using the 31 T solenoid at the National High Magnetic Field Laboratory, we have observed, at temperatures less than 1 degrees above the clearing point, a first-order transition to the nematic phase. The critical magnetic field at which this occurs increases with temperature. We discuss these results within the context of both Maier-Saupe and Landau-de Gennes mean-field models for the nematic-isotropic transition. The implications of possible tetrahedratic order are also discussed. To our knowledge, this is the first observation of such a magnetic-field induced transition in a thermotropic liquid crystal; the reasons for which this behavior is now attainable are discussed.

Flexoelectricity is a unique property of liquid crystals; it is a linear coupling between electric polarizations and bend and/or splay distortions of the direction of average molecular orientation. Recently it was shown [J. Harden, Phys. Rev. Lett. 97, 157802 (2006)] that the bend flexoelectric coefficient in bent-core nematic liquid crystals can be three orders of magnitude higher than the effect with calamitic (rod-shaped) molecular shape. Here we report the converse of the flexoelectric effect: An electric field applied across a bent-core liquid crystal sandwiched between thin flexible substrates produces a director distortion which is manifested as a polarity-dependent flexing of the substrates. The flex magnitude is shown to be consistent with predictions based upon both the measured value of the bend flexoelectric constant and the elastic properties of the substrates. Converse flexoelectricity makes possible a new class of microactuators with no internal moving parts, which offers applications as diverse as optical beam steering to artificial muscles.

An experimental study of the heat capacity, mass density, magnetic-field-induced optical birefringence, linewidth and intensity of scattered light, and the viscosities associated with nematic order parameter fluctuations and fluid flow has been performed on an achiral bent-core liquid crystal above its clearing point temperature. The measurements reveal a transition between two optically isotropic phases that is consistent with recent theoretical predictions of a "tetrahedratic" form of orientational order.

We demonstrate that the classical Kramers' escape problem can be extended to describe a bistable system under the influence of noise consisting of the superposition of a white Gaussian noise with the same noise delayed by time tau . The distribution of times between two consecutive switches decays piecewise exponentially, and the switching rates for 0

Flexoelectricity is a coupling between orientational deformation and electric polarization. We present a direct method for measuring the flexoelectric coefficients of nematic liquid crystals (NLCs) via the electric current produced by periodic mechanical flexing of the NLC's bounding surfaces. This method is suitable for measuring the response of bent-core liquid crystals, which are expected to demonstrate a much larger flexoelectric effect than traditional, calamitic liquid crystals. Our results reveal that not only is the bend flexoelectric coefficient of bent-core NLCs gigantic (more than 3 orders of magnitude larger than in calamitics) but also it is much larger than would be expected from microscopic models based on molecular geometry. Thus, bent-core nematic materials can form the basis of a technological breakthrough for conversion between mechanical and electrical energy.

The transition from surface to bulk normal dielectric rolls in a nematic liquid crystal is imaged by fluorescence confocal polarizing microscopy. The three-dimensional director structure and the liquid flow are scanned in both the layer plane and the transverse plane. Two systems of small-scale convective flow are formed, one at each electrode. Strong anchoring makes director oscillations difficult and charges accumulate by the Carr-Helfrich mechanism. The middle region is a structureless convection where the director oscillates with the frequency of the applied voltage. The small-scale flow eventually fills the cell from one electrode to the other as one system of thin and elongated rolls. The true dielectric mode is not a director pattern, rather a surface flow instability.

Magnetic birefringence and dynamic light scattering measurements of orientational order parameter fluctuations at the isotropic-nematic phase transition of a bent-core liquid crystal reveal a pretransitional temperature dependence consistent with the standard Landau-deGennes mean field theory. However, as follows: the transition in the bent-core compound is more weakly first order (TNI-T* approximately 0.4 degrees C), the leading Landau coefficient is approximately 30 times lower, the viscosity associated with nematic order fluctuations is approximately 10 times higher, and the density change is approximately 10 times lower, than typically observed in calamitic (rod-shaped) liquid crystals. One consistent explanation for these anomalies is an optically isotropic phase composed of microscopic complexes or "clusters" of bent-core molecules.

We characterize three nonstandard electrohydrodynamic instabilities in nematic liquid crystals composed of bent-core molecules. In addition to their shape, another important attribute of this material is that the anisotropy in the electrical conductivity changes sign as the frequency of the applied electric field changes. These instabilities do not appear to fit within the standard model for electroconvection. The first instability creates a pattern with stripes parallel to the initial director orientation, with a wavelength about equal to the separation of the cell plates. The next is the previously reported prewavy instability. The third instability is optically and dynamically identical to the prewavy instability, but is distinguished by different threshold behavior.

We report on the distribution spectra of the fluctations in the amount of power injected into a liquid crystal undergoing electroconvective flow. The probability distribution functions (PDFs) of the fluc-tuations as well as the magnitude of the fluctuations have been determined in a wide range of imposed stress both for unconfined and confined flow geometries. These spectra are compared to those found in other systems held far from equilibrium, and find that in certain conditions we obtain the universal PDF form reported by Phys. Rev. Lett. 84, 3744 (2000)]. Moreover, the PDF approaches this universal form via an interesting mechanism whereby the distribution's negative tail evolves towards form in a different manner than the positive tail.

We present results on dendritic growth of electrohydrodynamic convection in a nematic liquid crystal subject to parallel magnetic and electric fields. Previous work found that these dendrites have many properties in common with crystalline dendrites. Nevertheless, crystalline dendrites are significantly different from the system studied here. Specifically, the length selection mechanism for these dendrites is substantially richer than that which controls crystalline dendritic growth. In contrast with the sharp selection mechanism operating in the case of crystalline dendrites, these dendrites show only partial selection. As the separation between electrodes and the magnetic field becomes larger, the selection becomes even less sharp. We quantify the selection by measuring two important characteristics of these dendrites, their length scale, as reflected by the tip radius of curvature, and their growth speed. We measure these quantities as functions of the most important control parameters: the spacing of the liquid crystal cell, the magnetic field, and the applied voltage. A nontrivial scaling relationship is found for the tip radius of curvature. These dendrites occur in a system containing only one state of matter, and they are defined not by an abrupt boundary but by a diffuse interface. We find that the width of that interface is determined solely by the applied magnetic field.

We report measurements of the energy dissipation during electroconvection of a nematic liquid crystal in the fully turbulent regime. The energy dissipation is quantified by measuring the electric current at constant potential difference. In certain cases, we observe the surprising result that the dimensionless energy dissipation ceases to increase with increasing potential difference. In some circumstances, a power-law scaling relationship between the dimensionless excess energy dissipation and the reduced voltage is observed, but the exponent in this relationship varies dramatically with both the time scale of the driving electric field and the separation between the electrodes.