Spontaneous liquid crystal and ferromagnetic ordering of colloidal magnetic nanoplates.

Ferrofluids are familiar as colloidal suspensions of ferromagnetic nanoparticles in aqueous or organic solvents. The dispersed particles are randomly oriented but their moments become aligned if a magnetic field is applied, producing a variety of exotic and useful magnetomechanical effects. A longstanding interest and challenge has been to make such suspensions macroscopically ferromagnetic, that is having uniform magnetic alignment in the absence of a field. Here we report a fluid suspension of magnetic nanoplates that spontaneously aligns into an equilibrium nematic liquid crystal phase that is also macroscopically ferromagnetic. Its zero-field magnetization produces distinctive magnetic self-interaction effects, including liquid crystal textures of fluid block domains arranged in closed flux loops, and makes this phase highly sensitive, with it dramatically changing shape even in the Earth's magnetic field.

Chemically induced twist-bend nematic liquid crystals, liquid crystal dimers, and negative elastic constants.

Here we report the chemical induction of the twist-bend nematic phase in a nematic mixture of ether-linked liquid crystal dimers by the addition of a dimer with methylene links; all dimers have an odd number of groups in the spacer connecting the two mesogenic groups. The twist-bend phase has been identified from its optical texture and x-ray scattering pattern as well as NMR spectroscopy, which demonstrates the phase chirality. Theory predicts that the key macroscopic property required for the stability of this chiral phase formed from achiral molecules is for the bend elastic constant to tend to be negative; in addition the twist elastic constant should be smaller than half the splay elastic constant. To test these important aspects of the prediction we have measured the bend and splay elastic constants in the nematic phase preceding the twist-bend nematic using the classic Frederiks methodology and all three elastic constants employing the dynamic light scattering approach. Our results show that, unlike the splay, the bend elastic constant is small and decreases significantly as the transition to the induced twist-bend nematic phase is approached, but then exhibits unexpected behavior prior to the phase transition.

Test of clock model in ellipsometric study of thin and thick free-standing films of an antiferroelectric liquid crystal.

The temperature dependences of the ellipsometric parameters in a weak dc external field are studied in thin and thick free-standing films of MHPOBC. The results for thin films consisting of two, three, and four layers are analyzed within the discrete phenomenological model. We find very good quantitative agreement between the theory and experiment, which indicates an odd-even effect. We find that the XY structures are stable for an odd number of layers, whereas planar, Ising-like structures are stable for an even number of layers. The experiments on thick (several tens of layers) films show a combination of bulklike and free-surface behavior. This is most pronounced at high temperatures, where the interior of the film is not tilted, whereas the layers at the air interfaces show qualitatively similar temperature dependance of the ellipsometric parameters as in the four-layer film.

Structures and phase transitions in thin free standing films of an antiferroelectric liquid crystal.

The premises of a discrete mean-field model for polar smectic liquid crystals are tested by analyzing the ellipsometric experiments on two, three, and four-layer freestanding films of MHPOBC. The measured temperature dependences of the ellipsometric parameters in a weak dc external field are compared to the predictions of a simple clock model. A very good quantitative agreement is found indicating an odd-even effect: XY structures are stable for odd and Ising-like structures for an even number of layers.

Structural transitions in holographic polymer-dispersed liquid crystals.

Dynamic light scattering was used to analyze the structural and dynamic properties of nematic director field within liquid crystal domains formed in holographic polymer-dispersed liquid crystal transmission gratings. Samples prepared from two different types of prepolymer mixture: one curable with visible (VIS) and another curable with UV light were investigated. In both formulations a critical slowing down of thermal director fluctuations, signifying the second-order structural transition of the nematic director field was observed in the vicinity of some critical external electric field as well as close to some critical temperature. For VIS samples also the size and the shape of phase separated droplets and viscoelastic and surface anchoring parameters of the liquid crystalline (LC) material forming the droplets were deduced. The viscoelastic constants were found to significantly deviate from the viscoelastic parameters of the pure LC mixture.

Observation of thermal fluctuations of disclination lines in a nematic liquid crystal.

We present measurements of the line tension and the viscous drag of a disclination line in nematic liquid crystal via observation of thermal fluctuations of the line position. Fourier analysis of the displacement of a disclination line due to thermal fluctuations was used to obtain the amplitudes and the relaxation rates of eigenmodes of the fluctuations. From them the line tension and the viscous drag were deduced. Our measurements show the coupling between flow and nematic orientation affects the viscous drag considerably, while the values of the line tension agree with a simple theory already predicted by de Gennes.

Dynamic light scattering and 31P NMR study of the self-assembly of deoxyguanosine 5'-monophosphate: the effect of added salt.

Self-assembling and dynamical properties of deoxyguanosine 5'-monophosphate in isotropic aqueous solutions were studied by (31)P NMR spectroscopy and dynamic light scattering (DLS). All solutions had the same c = 4 wt% guanosine concentration, while the added KCl molarity ranged from 0 to 1.5 M. (31)P NMR measurements show that potassium ions strongly enhance the stacking process of guanosine tetramers until a saturation is reached at 0.1 M KCl with more than 70% of the molecules aggregated. Polarized light scattering reveals the presence of a fast relaxation mode that arises from the translational dynamics of the self-assembled stacks. The diffusion coefficient of this mode shows a strong dependence on molarity of added salt, which can be described in terms of the coupled mode and counterion condensation theories for polyelectrolyte solutions. Depolarized light scattering reveals the rotational dynamics of the self-assembled stacks which exhibits a pronounced slowing down with increasing the added salt content.

Glasslike character of molecular ordering in discotic lyomesophases.

We studied dynamic light scattering from isotropic solutions of triphenylene derivative in dodecane at concentrations close to the isotropic-cholesteric phase transition that takes place at c=25 wt. % at room temperature. The correlation decay g((2))(t) reveals the presence of three dynamic modes, from which two appear in polarized (VV) and one in depolarized (VH) scattering. The VH mode, which is independent of the scattering wave vector q, exhibits an Arrhenius type temperature dependence and shows a strong deviation from the exponential relaxation associated with the stretching-exponent parameter beta(VH) approximately 0.5 for all the investigated concentrations. The VV modes both exhibit a quadratic dispersion of the inverse relaxation time 1/tau on the scattering wave vector q. For concentrations 5 wt. %

Structural properties of the ferrielectric phases of a chiral liquid crystal revealed by dynamic light scattering.

Dynamic light scattering is used to study the antiferro- and ferrielectric phases in a bulk sample of the chiral liquid crystal R-10OTBBB1M7. Comparison of the detected fluctuation modes in the two ferriphases to calculated selection rules for their coupling to light are consistent with the distorted clock structures recently reported by an optical ellipsometry study, but indicate that the distortion has a polar component in both ferriphases. We also find a surprisingly large amplitude and dispersion associated with scattering from fluctuations in the tilt angle magnitude. In terms of the discrete interlayer interaction models currently being used to account for the ferriphases, this finding suggests that the interlayer interaction coefficients are of comparable magnitude to the intralayer coefficients that stabilize the tilt angle.

Aging of surface anchoring and surface viscosity of a nematic liquid crystal on photoaligning poly-(vinyl-cinnamate).

Dynamic light scattering was used to measure the azimuthal anchoring energy coefficient W(straight phi) of nematic liquid crystal (5CB) on photoaligning poly-(vinyl-cinnamate) layer. Measurements were repeated several times within two months. The results show that W(straight phi) increases in the first few days after filling the cell with liquid crystal. Then it remains approximately constant at W(straight phi)=5 x 10(-6) J/m(2) for at least two months. Also, presence of very large effective surface viscosity is observed. This phenomenon is of transient nature and attributed to swelling and dissolving of photosensitive polymer into the liquid crystal, which gives rise to an inhomogeneity of viscoelastic properties. Numerical modeling of the fluctuation spectrum shows that an inhomogeneous surface layer can account for the observed effective surface viscosity.

Influence of ions on the "V-shaped" electro-optic response of ferroelectric liquid crystals.

It has recently been shown that in surface-stabilized ferroelectric liquid crystal cells with high spontaneous polarization, the polarization charge self-interaction leads to a "V-shaped" optical response to an applied voltage. The presence of ionic free charges in the liquid crystal changes the internal electric field, and therefore also the spatial dependence of the polarization and optic axis orientation. We have numerically solved the nonlinear ion diffusion equation in the electric field due to external voltage, spontaneous polarization, and ions, and calculated the electro-optic response to a triangular applied voltage. When the period of the driving voltage is smaller than the zero-field diffusion time of the ions across the cell, and larger than the ion transit time in the applied field, an inverse hysteresis in the electro-optic response is obtained. At these time scales ions also cause characteristic changes in the shape of the electro-optic response.

Influence of an external electric field on structure in surface-stabilized smectic-C chevron cells

We use a Landau-de Gennes model to study the effect of static external electric fields on the director and layer structure and switching in surface stabilized smectic-C cells with uniform chevron structure. We consider uniform switching of the whole cell and find the threshold electric field needed to switch the director from one stable state to the other stable state. The effect of temperature and cell thickness on the value of the threshold electric field is studied. We also find the switching time as a function of the strength of the applied electric field and study the director and layer structure during the switch.

Dynamic light scattering as a probe of orientational dynamics in confined liquid crystals

The eigenmodes of director orientational fluctuations in nematic liquid crystals in confined geometries were studied both theoretically and experimentally by dynamic light-scattering tehnique. The fundamental mode of the orientational fluctuations shows a crossover from bulk behavior, dominated by bulk elastic constant K, to surface dominated one, in which the relaxation rate is determined by the ratio of surface anchoring strength W and viscosity eta. The contribution of surface viscosity zeta is also significant when its characteristic length zeta/eta becomes comparable to the size of the confined system. It was measured in nematic liquid crystal in cylindrical pores of polycarbonate (Nuclepore) membranes to be of the order of 10 nm.