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The structural transitions in an initially homeotropic bent-rod nematic liquid crystal excited by ac fields of frequency f well above the dielectric inversion point [f.sub.i] are characterized. The studies have shown many new features associated with the secondary instabilities that develop at umbilical sites of the bend Freedericksz (BF) state.

The electric-field-generated effects in the nematic phase of a twin mesogen formed of bent-core and calamatic units, aligned homeotropically in the initial ground state, are examined beyond the dielectric inversion point. The sustained propagative instability is shown to be a novel aspect common to the patterned states.

The characterization of the sequence of bifurcations generated by ac fields in a nematic layer held between unidirectionally rubbed ITO electrodes is presented.

A novel dc field-driven propagative instability associated with the thermally induced layer undulations of the smectic C phase in a phenyl benzoate is reported. The mechanism responsible for the drift has involved a coupling between transverse field gradients due to the conductivity anisotropy and the transverse component of the flexoelectric polarization.

We report on transientstructures, formed by thermotropic smectic-Aliquid crystals, resembling the myelin figures of lyotropic lamellarliquid crystals. The thermotropic myelin structures form during thesolubilization of a smectic-A droplet in an aqueous phase containinga cationic surfactant at concentrations above the critical micelleconcentration. Similar to the lyotropic myelin figures, the thermotropicmyelins appear in an optical microscope as flexible tubelike structuresgrowing at the smectic/aqueous interface. Polarizing microscopy andconfocal fluorescence microscopy show that the smectic layers areparallel to the tube surface and form a cylindrically bent arrangementaround a central line defect in the tube. We study the growth behaviorof this new type of myelins and discuss similarities to and differencesfrom the classical lyotropic myelin figures. [ABSTRACT FROM AUTHOR]

Experimental findings on the electric Freedericksz instability are presented for an achiral, dielectrically slightly negative, 90°-twisted smectic C liquid crystal. In the base state, the smectic layers are perpendicular to the confining electrodes, with the c director sweeping through 180° across the sample thickness. Thresholdless reorientation is inferred from a continuous variation of cell capacitance with voltage. Above a few volts, in the first instance of its kind, the periodic nature of the equilibrium Freedericksz state reveals itself in the stripe morphology. The pattern period increases with applied voltage, and decreases with increasing field frequency. These changes are brought about through creation and movement of edge dislocations. Homogeneous reorientational state is not recovered at increased fields even up to dielectric break down. A model involving an undulatory band, which has the twist localized within it and is flanked by two uniformly and transversely aligned regions, accounts for the optical features of the periodic state. [ABSTRACT FROM AUTHOR]

We report, for a rodlike nematic liquid crystal with small positive dielectric and conductivity anisotropies, and in the 90°-twisted configuration, low frequency (<2 Hz) square wave electric field generated Carr-Helfrich director modulation appearing transiently over a few seconds at each polarity reversal and vanishing almost completely under steady field conditions. Significantly, the instability is polarity sensitive, with the maximum distortion localized in the vicinity of the negative electrode, rather than in the midplane of the layer. This is revealed by the wave vector alternating in the two halves of the driving cycle between the alignment directions at the two substrates. Besides the Carr-Helfrich mechanism, quadrupolar flexoelectric polarization arising under electric field gradient is strongly indicated as being involved in the development of the transient periodic order. Similar transient instability is also observed in other nematic compounds with varying combinations of dielectric and conductivity anisotropies, showing its general nature. The study also deals with various characteristics of the electro-optic effect that emerge from the temporal variation of optical response for different driving voltages, frequencies, and temperatures. [ABSTRACT FROM AUTHOR]

We study the micellar solubilization of three thermotropicliquidcrystal compounds by immersing single drops in aqueous solutions ofthe ionic surfactant tetradecyltrimethylammonium bromide. For bothnematic and isotropic drops, we observe a linear decrease of the dropsize with time as well as convective flows and self-propelled motions.The solubilization is accompanied by the appearance of small aqueousdroplets within the nematic or isotropic drop. At low temperatures,nematic drops expell small nematic droplets into the aqueous environment.Smectic drops show the spontaneous formation of filament-like structureswhich resemble the myelin figures observed in lyotropic lamellar systems.In all cases, the liquid crystal drops become completely solubilized,provided the weight fraction of the liquid crystal in the system isnot larger than a few percent. The solubilization of the liquid crystaldrops is compared with earlier studies of the solubilization of alkanesin ionic surfactant solutions. [ABSTRACT FROM AUTHOR]

We report on the inverse flexoelectric effects observed in a nematic liquid crystal with a small positive dielectric anisotropy subject to static and very low frequency (<1 Hz) a.c. fields. The Bobylev–Pikin flexobands appear at a temperature-dependent d.c. threshold. Under square wave excitation, a new type of transient optical response occurs soon after each polarity reversal, and we ascribe it to the gradient flexoelectric distortion explicable on the basis of the presence of intrinsic double layers. This instability is characterized by a threshold voltage that decreases with temperature. Its maximum amplitude increases linearly with voltage close to threshold, and occurs after polarity reversal at a time τm that scales inversely as the voltage; τm decreases exponentially with frequency and temperature. After each polarity change, the ionic current following the charging current decreases almost exponentially to a non-zero value; the residual current increases monotonically with the applied bias. [ABSTRACT FROM AUTHOR]

Electric field experiments have been carried out on +1 and -1 defects formed in alignment inversion walls, in a planarly aligned nematic phenyl benzoate. The results show that the defects are non-singular in the core and exhibit a flexoelectric response to an applied d.c. or low frequency a.c. field. When the c -director flux lines are circular, as in a +1 defect in a wall parallel to the easy axis, flexoelectro-optic switching characterized by an azimuthal angle variation is observed. When the c -director flux is radial, the response is seemingly through polar angle variations involving no rotation of the extinction brushes due to crossed polarizers. This conclusion follows from the field-induced structural distortions observed at a -1 defect having a combination of radial and tangential c -director fields. [ABSTRACT FROM AUTHOR]

This paper concerns the instability behavior of a nematic liquid crystal in the region of dielectric inversion frequency for different waveforms of the exciting electric field. The critical frequency separating the regimes of dielectric and electroconvective primary bifurcation states shows a notable dependence on the waveform. In particular, it is found to undergo a large downshift for square-wave and sawtooth-wave fields as compared to sine-wave and triangle-wave fields. This seems to underscore the significance of harmonics in nonsinusoidal fields for the evolution of patterned electroconvective states. The study also deals with the flow pattern associated with the periodic state and the sequence of secondary instabilities occurring at higher fields to emphasize the role of the Carr-Helfrich mechanism for the instabilities in this region. The relevance of dielectric heating to the formation of transient structures is also pointed out. [ABSTRACT FROM AUTHOR]

Two electroconvection (EC) pattern morphologies--a cellular and a subsequent roll pattern--have been detected in the same frequency range in a nematic with positive permittivity and conductivity anisotropies. The frequency dependences of the onset voltages and critical wave numbers have been determined both for homeotropic and planar alignments. It has been proven that both pattern morphologies have a dielectric time symmetry. We also discuss possible sources for the pattern formation in the frame of both the isotropic (Felici-Benard) mechanism, as well as the standard model of EC.

We report, for a nematic liquid crystal with a low conductivity anisotropy, an ac field generated transition from a uniformly planar to a periodically modulated director configuration with the wave vector parallel to the initial director. Significantly, with unblocked electrodes, this instability is not excited by dc fields. Additionally, in very low frequency square wave fields, it occurs transiently after each polarity reversal, vanishing completely during field constancy. The time of occurrence of maximum distortion after polarity reversal decreases exponentially with voltage. The time dependence of optical phase change during transient distortion is nearly Gaussian. The pattern threshold Vc is linear in sqrt[f], f denoting the frequency; the critical wave number qc of the modulation scales nearly linearly as sqrt[f] to a peak at approximately 50 Hz before falling slightly thereafter. The observed Vc(f) and qc(f) characteristics differ from the predictions of the standard model (SM). The instability may be interpreted as a special case of the Carr-Helfrich distortion suppressed in static fields due to weak charge focusing and strong charge injection. Its transient nature in the low frequency regime is suggestive of the possible role of gradient flexoelectric effect in its occurrence. The study includes measurement of certain elastic and viscosity parameters relevant to the application of the SM.

Competing modes of electric-field-driven bifurcation have been examined in a planarly aligned nematic liquid crystal with a small positive dielectric anisotropy and large electrical conductivity. The line of bifurcation into the electroconvective state is linear in frequency not only up to the codimension-2 (C2) point but well beyond. This contrasts with the recent theoretical prediction of the absence of further bifurcations within the Freedericksz state, above the C2 frequency. Further, in the frequency regime beyond C2, within the space-filling electroconvective state, a strongly localized dynamical state (SLDS) appears at a voltage threshold which again is linear in frequency. Occurrence of the SLDS through a secondary forward bifurcation is significant in view of the reported observations on the so-called worms, an instability belonging to the SLDS, as the first form of convection seen at the primary subcritical bifurcation.