Impurities in nematic liquid crystal samples inducing changes in the DSM1-DSM2 transition phase diagram

In this paper we report, for the first time, the experimental phase diagram for the DSM1-DSM2 transition. We show the effects due to changes in the conductivity anisotropy resulting from impurities introduced in the cases of both homeotropic and homogeneously planar aligned nematic liquid crystal cells.     

Electro-optic behaviour of a non-polar nematic liquid crystal and its mixtures

In this work we report preliminary results on the properties of a non-polar bicyclohexane nematic liquid crystal. Moreover, its binary mixtures both with a low viscosity phenylcyclohexane and with a normal polar nematic liquid have been investigated. The elastic, viscous and electro-optical properties of these compounds are presented. The non-polar compound, and its mixture with a low percentage of the well known liquid crystal MBBA, exhibit an electrohydrodynamic behaviour in which the conducting regime is absent, while the dielectric regime spreads to low frequencies. Additionally, at higher frequencies of the applied electric field, a regime whose thresholds are linear in frequency is observed. On the other hand, the mixture formed by 50 wt % of the non-polar compound with MBBA exhibits at low frequencies the usual behaviour, followed at higher frequencies by the linear regime.     

Preliminary Results on the Elastooptic Behavior of a Cholesteric Liquid Crystal

We report preliminary results on the optical properties of a cholesteric liquid crystal when it is submitted to a normal stress. We see for small deformations the sample reacts elastically and the pitch decreases or increases following if we press or dilate the sample, without hysteresis. For higher stresses other physical processes take place giving also an hysteresis of the behavior.     

A possible helical model for sodium glycocholate micellar aggregates

Sodium glycocholate crystallizes in the tetragonal space group14 witha =b = 27.793(4),c = 7.937(1) Å andZ = 8. Refinement based on 2290 observed reflections led to a conventionalR = 0.10. The bile salt molecules are arranged in a helix with 2₁ symmetry stabilized mainly by polar interactions. Four helices are held together by hydrogen bonds involving water molecules, giving rise to hydrophilic channels, with a small cross section, which can include water molecules. The packing of these tetramers form hydrophobic channels containing some disordered acetone and water molecules. The helices will be checked as a model for the micellar aggregates of this important conjugated bile salt, following the same strategy successfully applied to sodium deoxycholate. Supplementary Data relating to this article are deposited with the British Library as Supplementary Publication No. SUP 82107 (20 pages).     

Nonmonotone curvilinear line search methods for unconstrained optimization

We present a new algorithmic framework for solving unconstrained minimization problems that incorporates a curvilinear linesearch. The search direction used in our framework is a combination of an approximate Newton direction and a direction of negative curvature. Global convergence to a stationary point where the Hessian matrix is positive semidefinite is exhibited for this class of algorithms by means of a nonmonotone stabilization strategy. An implementation using the Bunch-Parlett decomposition is shown to outperform several other techniques on a large class of test problems.The work of this author was based on research supported by the National Science Foundation Grant CCR-9157632, the Air Force Office of Scientific Research Grant F49620-94-1-0036 and the Department of Energy Grant DE-FG03-94ER61915.These authors were partially supported by Agenzia Spaziale Italiana, Roma, Italy.     

Crystal structures of bile salts: Sodium taurocholate

Crystals of sodium taurocholate (NaC₂₆H₄₄NO₇S · 2.5 H₂O) belonging to the triclinic space groupP1 have unit cell parametersa = 12.731 (2),b = 16.104 (2),c = 7.628 (1) A, =83.40 (1), = 101.20 (1), = 105.35 (1)°, and two molecules in the asymmetric unit. The refinement, carried out on 4424 observed reflections, gaveR = 0.059 andR _w = 0.066. The packing is characterized by bilayers, formed by antiparallel monolayers and with nonpolar outermost surfaces, held together by van der Waals interactions. Inside the bilayers there are channels, lined with polar groups, and filled by sodium ions and water molecules. A structural unit has been identified that could provide a reasonable model for the micellar aggregates of this bile salt. Supplementary Data relevant to this article have been deposited with the British Library under the reference number SUP 82125 (38 pages).     

Deformations induced by d.c. electric field in cholesteric liquid crystals

Summary We report results of coupled capacitance and optical measurements made on a cholesteric mixture submitted to a d.c. electric field parallel to the helical axis. A definite hierarchy for the onset of different deformations is found if the sample thickness is much larger than the helical pitch. Three distinct thresholds are measured for sample thickness in the range 6 μm≤d≤100 μm.

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Riassunto Si riportano i risultati di misure accoppiate di capacità e di riflessione selettiva compiute su una miscela colesterica sottoposta ad un campo elettrico statico orientato parallelamente all'asse dell'elica. Si trova una ben definita sequenza dell'insorgere delle diverse deformazioni se lo spessore del campione è molto piú grande del passo dell'elica. Tre distinte soglie sono misurate per spessori compresi tra 6 μm e 100 μm.

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Резюме Мы приводим результаты связанных измерений емкости и селективного оптического отражения, проведенных на холестеричной смеси, помещенной в статическое злектрическое поле, параллельно спиральной оси. Получается определенная иерархия для появления различных деформаций, если толщина образца много больше, чем винтовой шаг. ъыли обнаружены три резко выраженных порога для толщин образцов в диапазоне 6 μm≤d≤100 μm.

     

Stochastic ferroelectric switching of lead zirconate titanate thin films

We investigate the repolarization phenomenon in a ferroelectric film. Our ferroelectric sample was lead zirconate titanate (PZT) obtained by sol-gel synthesis and deposited by spin coating on ITO/glass substrate. A series of repolarizations were induced in the ferroelectric film by applying a triangular wave and the current peaks related to the switchings of the ferroelectric domains were acquired for statistical analyses. It is shown that the dynamics and statistics of polarization switchings are well simulated by a simple mean-field model in which a double-well, asymmetric potential is included to describe the asymmetry at the PZT-ITO interface.     

Time-dependent behaviour of mechanical shear deformations in a cholesteric liquid crystal

Summary In this paper we present the experimental results of time-resolved variations of the apparent pitch of a cholesteric mixture after a steplike shearing deformation is applied to it. We show that the relative variation of the wave-length of the maximum reflection ΔP/P _o varies from initial positive values (P>P ₀) to negative ones (P<P ₀ after a few seconds and finally it tends to a saturation value with ΔP/P ₀<0 in a long time. A qualitative explanation based on the mechanism of the unwinding of the twisted helix structure and a possible tilt of the molecular is also given.

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Riassunto In questo lavoro sono presentati i risultati sperimentali della variazione risolta nel tempo del passo apparente di un CLC sottoposto ad una deformazione di taglio a gradino. Si mostra che la variazione relativa della lunghezza d’onda del massimo di riflessione ΔP/P ₀ varia da un valore iniziale positivo (P>P ₀) ad un valore negativo (P<P ₀) dopo pochi secondi, e tende ad un valore di stturazione con ΔP/P ₀<0 all’infinito. Noi diamo una spiegazione qualitativa basata sul meccanismo dello srotolamento della struttura elicoidale e sul possibile tilt degli strati molecolari.

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Резюме В этой статье приводятся экспериментальные результаты для разрешенных во времени изменений видимого шага холестеричиой смеси после приложения ступенчатой деформации сдвига. Мы показываем, что относительное изменение длины волны максимального отражения ΔP/P ₀ изменяется от начальных ппложительных знаьений (P<P ₀) до отрицательных значениь (P<P ₀ после нескольких секунд и окончательно стремится к величине насьшения с ΔP/P ₀<0 в течение длительного времени. Предлагается качественное объяснение, основанное на механизме структуры закрученной спирали и возможного изменния наклона молекул.