Biomedical subjects

Sylwester J Rzoska

Publications and source records attributed to Sylwester J Rzoska.

4 recordsLinked to original sources

PubMed2006

Pretransitional behavior in the isotropic phase of a nematic liquid crystal with the transverse permanent dipole moment.

The results presented give the evidence for the quasicritical, pretransitional behavior of dielectric properties in the isotropic phase of a rodlike nematic liquid crystal with the transverse permanent dipole moment. Studies were conducted in 2-cyano-4-pentylbiphenyl 4-(trans-4-pentylcyclohexyl) benzoate, focusing on the static-and ionic-dominated low-frequency (LF) regions. For the static dielectric permittivity [epsilon(')(T)] the application of the derivative analysis revealed the pretransitional anomaly associated with the specific heat exponent alpha approximately 0.5. For the LF domain the contribution to epsilon(')(T) from residual ionic impurities follows a linear temperature dependence on approaching the isotropic-nematic (I-N) transition. This dependence and pretransitional anomalies of electric conductivity and dielectric modulus can be associated with the influence of prenematic fluctuations. "Linear" dielectric studies were supported by the static nonlinear dielectric effect measurements, which delivered reliable estimations of the temperature of the hypothetical continuous phase transition T(*) and the discontinuity of the I-N transition DeltaT approximately 1.7 K.

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PubMed2006

Derivative-based analysis for temperature and pressure evolution of dielectric relaxation times in vitrifying liquids.

The derivative-based analysis for detecting regions of the validity of the Vogel-Fulcher-Tammann (VFT) dependence for superpressed and supercooled liquids is discussed. For the temperature (T) path the analysis introduced by Stickel [J. Chem. Phys. 104, 2043 (1996); J. Chem. Phys. 107, 1086 (1997)] is recalled. For the pressure (P) path the derivation based on the counterpart of the VFT dependence proposed in Paluch [J. Phys.: Condens. Mater 10, 4131 (1998)] is presented. The appearance of two ideal glass temperatures (T(0)) or pressures (P(0)), fragility strength coefficients (D(T), D(p)), and prefactors [formula--see text] for VFT equations in following dynamical domains, i.e., high-temperature (DeltaT(high)) and low-temperature (DeltaT(low)) or low-pressure (DeltaP(low)), and high-pressure (DeltaP(high)), is stressed. It is noteworthy that the values of T(0)(DeltaT(high)) > T(0)(DeltaT(low)), D(T)(DeltaT(high)) << D(T)(DeltaT(low)), and [formula--see text]. Analogous behavior was noted for isothermal DeltaP(L) and DeltaP(H) dynamic domains. A similar derivative-based approach is also applied to test the validity of the mode coupling theory (MCT) critical-like equation [formula--see text]. It yields the temperature T(X) and the MCT power ("critical") exponent [formula--see text] exclusively from the simple linear regression. The extension of such an analysis for the pressure path is also given. The hardly discussed question of the error of estimations of [formula--see text] and T(X) is emphasized. The relation between the derivative based behavior mentioned above and the apparent activation enthalpy (temperature path) or the apparent activation volume (pressure path) is indicated. The presented analysis was applied to discuss the dynamic crossovers in supercooled and superpressed diethyl phthalate, based on experimental data supplemented by those given in Pawlus [Phys. Rev. E 68, 021503 (2003)].

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PubMed2005

Mode coupling behavior in glass-forming liquid crystalline isopentylcyanobiphenyl.

Linear and nonlinear dielectric measurements of liquid crystalline chiral isopentylcyanobiphenyl (5*CB) and n -pentylcyanobiphenyl (5CB), combined with viscosity eta (T) data, are presented. The 5*CB compound glassifies on cooling in the cholesteric phase whereas 5CB crystallizes in the nematic phase. In both compounds the temperature evolution of dielectric relaxation times, the dc conductivity, and the viscosity are well described by the "critical-like" description from mode coupling theory (MCT). However, for 5*CB a unique coincidence of the MCT "critical" temperature and extrapolated temperature of the hypothetical continuous isotropic-cholesteric (T*) phase transition was found. The temperature dependence of the strong electric-field-induced changes of the dielectric permittivity exhibits a strong anomaly in the direction of negative values on approaching T* , not observed up to now. The anomaly is described by the susceptibility-related critical exponent gamma=1 . The divergence of the "nonlinear" dielectric relaxation follows a power dependence described by the exponent y=1 . This paper recalls the recent discussions on the glassy dynamics of a "hard-ellipsoid" liquid and the possible relationship between the glass transition, critical phenomena, and isotropic-nematic transition.

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PubMed2002

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

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.

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