Showing papers by "Jan Jadżyn published in 2001"

Fluidlike behavior of dielectric permittivity in a wide range of temperature above and below the nematic-isotropic transition.

Abstract: Singular behavior of the static dielectric permittivity of n-alkyloxycyanobiphenyls $({\mathrm{C}}_{n}{\mathrm{H}}_{2n+1}\mathrm{O}\ensuremath{-}\mathrm{P}\mathrm{h}\ensuremath{-}\mathrm{P}\mathrm{h}\ensuremath{-}\mathrm{C}\ensuremath{\equiv}N,$ $n=6,$ 7) was studied above and below the nematic clearing point ${(T}_{I\ensuremath{-}N}).$ On approaching the clearing point, the evolution of principal components of the nematic permittivity tensor, ${\ensuremath{\varepsilon}}_{\ensuremath{\parallel}}$ and ${\ensuremath{\varepsilon}}_{\ensuremath{\perp}},$ is described by the order parameter exponent $\ensuremath{\beta}\ensuremath{\approx}0.25.$ The mean value of the nematic permittivity ${\ensuremath{\varepsilon}}_{\mathrm{mean}}=({\ensuremath{\varepsilon}}_{\ensuremath{\parallel}}+2{\ensuremath{\varepsilon}}_{\ensuremath{\perp}})/3$ exhibits a singular behavior similar to that observed in the isotropic phase and that for the diameter of the coexistence curve in binary mixtures. The derivative of experimental data $d{\ensuremath{\varepsilon}}_{\mathrm{iso}}(T)/dT$ and $d{\ensuremath{\varepsilon}}_{\mathrm{mean}}(T)/dT$ shows the specific-heat-like behavior with universal exponents $\ensuremath{\alpha}={\ensuremath{\alpha}}^{\ensuremath{'}}\ensuremath{\approx}0.5.$ Results obtained confirm the hypothesis of the fluidlike, pseudospinodal, and tricritical behavior of the isotropic to nematic phase transition. [A. Drozd-Rzoska, Phys. Rev. E 59, 5556 (1999)].

The shear viscosity minimum of freely flowing nematic liquid crystals

Abstract: The analysis of temperature dependence of the shear viscosity of freely flowing nematic liquid crystals shows that the anisotropic fluid adopts such a manner of flow, which corresponds to the minimum of its viscosity at given conditions. We postulate that the principle is of general significance, i.e. it concerns any fluid in the flow.

Viscosity of the Series of 1,n-Alkanediols

Abstract: The paper presents the shear viscosity data for the series of 1,n-alkanediols, HO(CH2)nOH, for n = 2−10.

Viscosity of the Homologous Series of n-Alkylcyanobiphenyls

Abstract: The paper presents the shear viscosity data for the isotropic phase of the homologous series of mesogenic n-alkylcyanobiphenyls (CnH2n+1PhPhC⋮N, nCB) for n = 2−12.

Quasicritical behavior of the low-frequency dielectric permittivity in the isotropic phase of liquid crystalline materials.

Abstract: Results presented give evidence of the existence of quasicritical, fluidlike behavior in the isotropic phase of 4-cyano-4-pentyl-biphenyl (5CB) for frequencies ranging from the static to the ionic-dominated [low-frequency (LF)] region. Despite the boost of dielectric permittivity on lowering the frequency below 1 kHz, values of the isotropic-nematic transition discontinuity (approximately 1.1 K) and the critical exponent alpha (approximately 0.5) remain constant. It is shown that the contribution from residual ionic impurities is a linear function of temperature in the critical, prenematic fluctuation-dominated region. The validity of the fluidlike and critical behavior for LF dielectric permittivity confirmed results of a derivative analysis of the experimental data: d(epsilon)/dT proportional to (T-T*)(-alpha), originally proposed for critical mixtures. Results of a preliminary test in the isotropic phase of 4-decyl-4'-isothiocyanatobiphenyl (10BT), on approaching the smectic-E phase, may indicate a general validity of results obtained.

Shear viscosity of nematic liquid crystals in the vicinity of the smectic-A phase in alkyloxycyanobiphenyl mixtures

Abstract: The paper presents a singular temperature behavior of the shear viscosity measured for freely flowing nematic liquid crystals in vicinity to smectic-A phase in n-octyloxycyanobiphenyl and n-hexyloxycyanobiphenyl mixtures--a system exhibiting the reentrant nematic phase.

Critical anomaly of dielectric permittivity for the temperature and pressure paths on approaching the critical consolute point.

Abstract: The experimental results of isothermal pressure dielectric permittivity « studies in a critical mixture characterized by a negative shift of critical temperature induced by pressure ( dTC / dP ,0) are presented. The critical effect is portrayed by the same relation as in previous «(T) and «( P) studies, with the critical exponent a50.1260.03. The advantage of pressure studies is the negligible influence of the correction-to-scaling term and the low-frequency Maxwell-Wagner effect. This conclusion is supported by the distortion-sensitive derivative analysis of the experimental data. In contrast to previous «( P) studies, carried out in mixtures with dTC / dP .0, the critical effect manifests by the bending-up behavior near the critical point. It is suggested that signs of the critical amplitudes of «( P) and «(T) anomalies may be related to the excess volume V E and the excess enthalpy H E , respectively.

The Structural Transitions in Liquid Crystals Doped with Fine Magnetic Particles

Abstract: The stable ferronematics and ferrosmectics (combination of liquid crystals with fine magnetic particles) were prepared for 8CB liquid crystal and magnetc particles 11 nm in diameter. The structural transitions i.e. magnetic Fredericksz transition were indicated by means of dielectric and conductivity measurements.The experimental results i.e. the proof of initial condition \(\overrightarrow n _0 \bot \overrightarrow m _0\) (\(\overrightarrow n _0\) is director of LC molecules and \(\overrightarrow m _0\) is magnetic moment of particles) and the increase of the threshold field of Fredericksz transition vs volume conduction of magnetic particles are in qualitative agreement with the Burylov and Raikher theory of thermotropic ferronematics.

The Elastic Constants of Nematic n-Hexylcyanobiphenyl Determined with the Capacitance Method

Abstract: Abstract The splay (K 11) and bend (K 33) elastic constants of n-hexylcyanobiphenyl (C6H13-Ph-Ph-C=N) were determined from the voltage (U) dependence of the capacitance (C) of the planar nematic cell with a small molecular pretilt angle. The capacitance changes are due to distortion of the director n driven by the applied electric field. K 11 was obtained from the Freedericksz threshold voltage (Uth) and K 33 from the C(U) dependence above the threshold voltage by means of the method proposed by Gruler et al. and Uchida et al. The significance of the pretilt angle in the determination of K 33 is discussed.

Determination of the order parameter of 4-(trans-4'-n-heptyl-cyclohexyl)isothiocyanato-benzene by means of dielectric relaxation spectroscopy

Abstract: Studies of the dielectric relaxation were made for the isotropic and nematic phases of 4-(trans-4'-n-heptylcyclohexyl)isothiocyanatobenzene, (C/sub 7/H/sub 15/-C/sub y/H/sub x/-Ph-N=C=S) (7CHBT) in the frequency range from 100 kHz to 1 GHz. In the nematic phase two relaxation processes were recorded for the dielectric permittivity component measured parallel to the molecular orientation (director n/spl I.oarr/) and three relaxation processes for the perpendicular permittivity component. The elementary contributions to the total dielectric absorption are found to be of the Debye-type. The strength of the nematic potential and the nematic order parameter as a function of temperature were estimated. The results obtained were compared with the predictions of the Maier-Saupe theory.

Universal Scaling of Alpha Relaxation in Complex Liquids

Abstract: A plot is given, showing the result of a scaling analysis of dielectric loss curves containing, apart from low molecular glass former data (glycerol, dibutyl phtalate), also loss curves of the following liquid crystallinematerials, mostly in the isotropic phase: 4-(2-methylbutyl)-4'cyanobiphenyl (5*CB, supercooled isotropic phase), 4-cyano-4-n-alkyl biphenyls (nematogens 5CB and 8CB, isotropic phase), 4-(4-cyano-4-butylcyclohexyl)-4'-octylbiphenyl (laterally substituted nematogen, isotropic phase), and 4-n-alkyl-4'-isothiocyanatobiphenyl (5 and 10 BT, isotropic and SmE phases). The plot applies the scaling formula originally proposed forglassforming, supercooled liquids [Dendzik et al. 7 ]. The result supports the recent suggestion that dielectric relaxation in the isotropic phase of nematogens may show some features typical for "glassy" materials.

Nonlinear dielectric relaxation in solutions of hydrogen bonded liquid crystalline carboxylic acid

Abstract: This paper presents the results of the nonlinear dielectric studies performed on solutions of 4-butyl-1-cyclohexanecarboxylic acid in cyclohexane at 25/spl deg/C. Two molecules of the acid, linked by two hydrogen bonds O-H...O, form a cyclic dimer of a rigid and linear structure. The increase of the number of dimers per unit volume finally leads to the nematic liquid crystalline phase. The isotropic to nematic pre-transitional effects were detected with nonlinear dielectric spectroscopy.

Nonlinear dielectric relaxation spectra of mesogenic molecules in benzene solutions

Abstract: The frequency dependence of the nonlinear dielectric increment was studied for benzene solutions of 4,4′-n-hexylcyanobiphenyl (6CB), 4-(trans-4′-n-hexylcyclohexyl)-isothiocyanatobenzene (6CHBT) and 1-[3-n-decylbiphenyl]-2-[4-isothiocyanatophenyl]-ethane (10TPEB) molecules. In dilute solutions of these strongly dipolar nematogenic molecules (c 1M) of 6CB and 6CHBT in benzene large positive field-induced changes of permittivity are observed. Such a large nonlinearity in polarization is expected in the vicinity of the isotropic-nematic phase transition because the molecular correlation length of the nematic domains becomes unusually large.