Showing papers by "Jan Jadżyn published in 2005"

Anomalous rotational diffusion in the vicinity of the isotropic to nematic phase transition

Abstract: Dielectric relaxation measurements are performed with very high accuracy on a liquid crystalline compound n-octylcyanobiphenyl (8CB) in the isotropic (I), nematic (N) and smectic A (SA) phases. The data obtained display an essential difference in the rotational diffusion process in the vicinity of the I–N phase transition in comparison to that taking place in the vicinity of the N–SA phase transition. Thus, for the I–N transition, anomalously slow diffusion (subdiffusion), characterized by an anomalous coefficient α<1, is observed, while normal Brownian rotational diffusion with is found for the N–SA transition. It is also shown how the fractal parameter α is temperature dependent with an extremely sharp variation at the I–N transition point in the form of a lambda-like profile.

Anomalous dielectric relaxation in strong ac external fields.

Abstract: Dielectric relaxation of complex polar fluids is considered in the context of the anomalous diffusion characterized by a fractional parameter α⩽1 (subdiffusion). An infinite hierarchy of three-term differential-recurrence equations governing the time evolution of the electric polarization is established by following a purely phenomenological procedure. The matrix-continued fraction method is used to derive the exact numerical solution of the stationary regime for an assembly of nonelectrically interacting, polar symmetric-top molecules in presence of a strong ac electric field. The results so obtained are valid to any order in the field strength parameter γ1, thus extending previous theories applicable to fields of very small amplitudes only. This is illustrated by Cole-Cole diagrams and three-dimensional relaxation spectra for the first- and third-harmonic components of the electric polarization as a function of α,γ1, and the angular frequency.

Anomalous Kerr effect relaxation in an alternating field.

Abstract: Perturbation theory is used to derive the complex harmonic components (stationary regime) arising in Kerr effect relaxation for an assembly of nonelectrically interacting, polar, and polarizable symmetric-top molecules acted on by a strong dc bias electric field superimposed on a weak ac electric field. The approach starts from a fractional kinetic equation written in configuration space and represents an extension of the Smoluchowski equation to fractional dynamics. This equation is solved in the context of a subdiffusive process characterized by an anomalous exponent α ranging from 0 to 1, the Brownian limit. By using a perturbation procedure restricted to the second order in the ac field strength, analytic expressions for the electric birefringence spectra representing the frequency dependence of the first (in ω) and the second (in 2ω) harmonic components are obtained. Various Cole–Cole-like diagrams are presented in order to illustrate the results so obtained and to emphasize the role played by the fr...

Pretransitional critical-like behavior of dielectric permittivity in mixtures of mesomorphic and nonmesomorphic compounds.

Abstract: Static dielectric measurements are performed in the temperature range of the isotropic-nematic phase transition of binary mixtures of mesomorphic n-heptylcyanobiphenyl (7CB) and nonmesomorphic: (a) n-heptylcyanophenyl (7CP) and (b) carbon tetrachloride (CCl4) , the solutes of different molecular shape and polarity. In the whole studied range of the solutes mole fraction (x) (x(max) approximately equal to 0.17) , the critical-like temperature behavior of the permittivity in the vicinity of the transition from the isotropic phase to the two-phase (nematic+isotropic) region can be well described with a critical exponent close to 0.5, as in a pure 7CB, indicating the tricritical nature of the transitions. It seems to be important that the fitting-determined temperatures of the virtual second-order transition in the solutions, T*, i.e., the temperature limit of the thermodynamic stability of supercooled isotropic phase, correspond well to the experimentally observed low-temperature limits of the two-phase isotropic+nematic region.

Anomalous Dielectric Relaxation in Binary Mixtures of Mesogenic Solvent/Non-Mesogenic Solute

Abstract: J. Jadzyn, D. Bauman, J.-L. Dejardin, M. Ginovska and G. Czechowski Institute of Molecular Physics, Polish Academy of Sciences Smoluchowskiego 17, 60-179 Poznan, Poland Faculty of Technical Physics, Poznan University of Technology Nieszawska 13a, 60-965 Poznan, Poland MEPS, Groupe de Physique Statistique et Moleculaire, Universite de Perpignan 52, Av. Paul Alduy, 66860 Perpignan, France University of Sts. Cyril & Methodius, Faculty of Electrical Engineering P.O. Box 574, 91000 Skopje, Macedonia

Static and dynamic dielectric effects in the vicinity of the isotropic to nematic phase transition in 7CB

Abstract: Static and dynamic dielectric measurements were performed with very high accuracy on a mesogenic compound n‐heptylcyanobiphenyl (7CB) in the isotropic (I) and nematic (N) phases. The critical‐like temperature behaviour of the static permittivity of isotropic 7CB in the vicinity of the I–N phase transition can be described with the critical exponent close to 0.5, indicating the tricritical nature of the transition. Anomalously slow rotational diffusion (subdiffusion), characterized by a fractal value of the diffusion exponent α, is observed in the vicinity of the I–N transition with a lambda‐like profile of the exponent temperature dependence.

Dipolar interactions in liquids and linear dielectric relaxation spectroscopy

Abstract: Linear dielectric relaxation studies performed on two isotropic liquids composed of the molecules of the same moment of inertia and a quite different polarity: C10H21-O-Ph-COO-Ph-CN (the dipole moment of about 5 D) and C10H21-O-Ph-OOC-Ph-CN (2.5 D) showed that, at given temperature, the relaxation times corresponding to the rotation around the short axis of the two kinds of molecules coincide to each other, regardless the polarity of the molecules and their abilities to accomplish dipolar aggregation. The studies allow one to estimate the lifetime of the intermolecular aggregates due to the dipolar interactions in liquids as no longer than 0.1 ns.

Gelation Process of Toluene-Based bis-Urea in Cyclohexane Studied with Magnetic Resonance Imaging

Abstract: Magnetic resonance imaging was used for studies of the gelation and swelling processes of toluene-based bis-urea prepared in a form of cylindrical tablet and immersed in cyclohexane. The processes were investigated with the use of Bruker AVANCE (300 MHz) spectrometer equipped with a micro imaging probe head. The images were taken for cyclohexane protons within the bis-urea gel formed around the sample dry core at different intervals of the immersion. The mobility of the solvent molecules was estimated from the spatially resolved distribution of the spin–spin relaxation time T2 and the spin densities ρ, calculated on the basis of the images obtained. It was shown that the time-evolution of the thickness of the gel layer can be well described by the power equation with an exponent equal to 0.47 (±0.04), indicating the Fickian nature of the diffusion of cyclohexane molecules.