Showing papers by "Sylwester J. Rzoska published in 2002"

Complex dynamics of isotropic 4-cyano-4-n-pentylbiphenyl (5CB) in linear and nonlinear dielectric relaxation studies.

Abstract: A comprehensive presentation of the temperature evolution of ``linear'' and ``nonlinear'' dielectric relaxation in the isotropic phase of nematic liquid crystalline compound 5CB (4-cyano-4-n-pentylbiphenyl) is given. The ``nonlinear'' relaxation is related to the strong pretransitional rise in the lifetime of prenematic fluctuations. The ``linear'' relaxation has a clear non-Debye and non-Arrhenius form. In the immediate vicinity of the nematic clearing point it shows a weak pretransitional anomaly. Results obtained coincide with the complex liquid relaxation pattern found in transient grating optical Kerr effect studies [A. Sengupta and M. D. Fayer, J. Chem. Phys. 102, 4193 (1995); R. Torre et al., Philos. Mag. A 77, 645 (1997)]. The striking similarity to the behavior found in critical, binary mixtures suggests the extension of the ``fluidlike'' hypothesis for the isotropic-nematic transition to dynamic phenomena in the isotropic phase [P. K. Mukherjee. J. Phys.: Condens. Matter 10, 9191 (1998)]. The presence of both glassy and fluidlike features in isotropic 5CB coincides with the recent results of simulation analysis for the system of hard ellipsoids by Latz et al. [Phys. Rev. E 62, 5173 (2000)] and with the novel general picture for liquid-liquid transitions proposed by Tanaka [Phys. Rev. E 62, 6968 (2000)].

Pressure effect on the smectic-A-isotropic phase transition.

Abstract: We examine the effect of pressure on the smectic-A-isotropic phase transition within the Landau phenomenological theory. The influence of pressure on the smectic-A-isotropic phase transition is discussed by varying the coupling between the orientational and the translational order parameter. The transition is found to be of first order even at elevated pressure. The pressure dependence nonlinear dielectric effect in the isotropic phase of the smectic-A-isotropic transition is calculated. The theoretical results are in good qualitative agreement with available experimental results.

Glassy and fluidlike behavior of the isotropic phase of n-cyanobiphenyls in broad-band dielectric relaxation studies

Abstract: It is shown that the temperature behavior of peaks f p, of dielectric loss curves in the isotropic phase of n-cyanobiphenyls n = 8, 9, 10 with isotropic-nematic and isotropic-smectic A transitions exhibits features characterisic for both supercooled, glass-forming liquids and critical, binary mixtures. The behavior of f p T can be portrayed by the Vogel-Fulcher-Tamman relation and the “critical-like”, mode-coupling theory (MCT) equation. The latter is supported by the novel analysis of electric conductivity σ T . The obtained f p T and σT dependencies can be related by using the fractional Debye-Einstein-Stokes law. For all tested mesogens the static dielectric permittivities ɛ′ T and T are described by dependencies resembling those applied in the homogeneous phase of critical mixturesbut with specific-heat critical exponent α≈ 0.5. This behavior agrees with the novel fluidlike description for the isotropic-nematic transition (P.K. Mukherjee, Phys. Rev. E 51, 5745 (1995); A. Drozd-Rzoska, Phys. Rev. E 59, 5556 (1999)). The obtained glassy features of dielectric relaxation support the recent simulation analysis carried out by M. Letz et al.Phys. Rev. E 62, 5173 (2000)).

Critical behavior in broad-band dielectric relaxation on approaching the critical consolute point in ethanol-dodecane mixture.

Abstract: We present a quantitative experimental evidence for the influence of critical fluctuations on the high frequency dielectric relaxation in a binary critical mixture. The analysis clearly shows the presence of critical anomaly both for the maxima (epsilon(")(p)) and frequencies (f(p)) of loss curves peaks. For the temperature evolution of epsilon(")(p) similar relation as for the static dielectric permittivity was obtained. The temperature dependence of relaxation times can be portrayed by using the Eyring-type equation remote from the critical consolute point and shows the unusual precritical speeding up. The obtained behavior resembles that found recently in the isotropic phase of nematic liquid crystals [A. Drozd-Rzoska and S. J. Rzoska, Phys. Rev. E 65, 041701 (2002)]. Results presented agree also with the recent theoretical suggestion [S. Goodyear and S. C. Tucker, J. Chem. Phys. 111, 9673 (1999)] that relaxation in supercritical fluids may exhibit glasslike features.

On the Tricritical Point of the Isotropic — Nematic Transition in a Rod-Like Mesogen Hidden in the Negative Pressure Region

Abstract: A presentation of experimental results suggesting fluidlike, spinodal and tricritical nature of the isotropic — nematic transition are given. Results presented based on broad-band dielectric spectroscopy and strong electric field induced changes of dielectric permittivity. Results above as well the obtained systematic changes of the temperature discontinuity of the isotropic — nematic transition suggest the possible existence of a tricritical point which may be experimentally reached due the application of negative pressures. The presented picture for the I-N transition correlates with the recent general model for liquid-liquid transition proposed by Tanaka [Phys. Rev. E68, 6968 (2000)].

Non-linear dielectric effect in superpressed chiral isopentylcyanobiphenyl (5∗CB)

Abstract: Results are presented of linear and non-linear dielectric studies in pressurized isopentylcyanobiphenyl (5 ∗ CB). Temperature studies of linear dielectric relaxation in 5 ∗ CB showed the Vogel–Fulcher–Tammann evolution of relaxation times and the broadening of loss curves on cooling [J. Chem. Soc. Farad. Trans. 94 (1998) 387; Phys. Chem. Chem. Phys. 1 (1999) 4843]. The behavior characteristic for glassy liquids was also found in the presented preliminary tests of dielectric relaxation in pressurized 5 ∗ CB. For pressure dependencies for maxima of loss curves and for the static dielectric permittivity additional pretransitional anomalies was found. They may be associated with premesomorphic fluctuation (inhomogeneities) in the isotropic (fluid-like) surrounding. The stationary response of strong electric field induced changes of dielectric permittivity (NDE) also exhibited a singular behavior. It was found that NDE decreased strongly towards negative values on pressuring. This is a novel behavior which can be hardly explained by classic models developed for normal liquids [Dielectric Physics, PWN–Elsevier, Warsaw, 1980 and 1993].

Preliminary Studies on the Dielectric Permittivity in the Isotropic and Mesophase of Cholesteryl Oleyl Carbonate

Abstract: The phase detection in cholesteryl oleyl carbonate was studied over a broad temperature range by dielectric spectroscopy. Tests were conducted in the static and the low frequency ionic regions. They point to the possibility of relatively simple detection of phase transition. Some manifestations of pretransitional behaviours were also found. - Pacs: 64.70Md, 77.22Ch, 64.30+t.

Linear and Non-linear Dielectric Pretransitional Behavior Near the Isotropic-nematic Phase Transition for 4-cyano-4-n-pentylbiphenyl (5CB)

Abstract: Linear and non-linear dielectric permittivity measurements for n-pentylcyanobiphenyl 5CB are presented. By two different experimental methods the same value of the temperature discontinuity for the isotropic-nematic transition was obtained. Broadband dielectric relaxation tests showed a significant influence of the pretransitional behavior on the dynamic properties above and below the nematic clearing temperature (T 1 - N ). The form of the loss curves is clearly non-Debye'an and can be portrayed within the Cole-Davidson approximation (CD). In the isotropic and nematic phases the CD parameter β C D drops on approaching the clearing temperature. This is connected with the broadening of the dielectric absorption width.