Relationship between dielectric properties and critical behavior of the electric birefringence in binary liquid mixtures

Pretransitional Effects of the Isotropic Liquid-Plastic Crystal Transition

We report on strong pretransitional effects across the isotropic liquid-plastic crystal melting temperature in linear and nonlinear dielectric response. Studies were carried out for cyclooctanol (C8H16O) in the unprecedented range of temperatures 120 K < T < 345 K. Such pretransitional effects have not yet been reported in any plastic crystals. Results include the discovery of the experimental manifestation of the Mossotti Catastrophe behavior, so far considered only as a hypothetical paradox. The model interpretations of experimental findings are proposed. We compare the observed pretransitional behavior with the one observed in octyloxycyanobiphenyl (8OCB), typical liquid crystal (LC), displaying a reversed sequence of phase transitions in orientational and translational degrees of order on varying temperature. Furthermore, in its nematic phase, we demonstrate first-ever observed temperature-driven crossover between regions dominated by isotropic liquid and smectic A pretransitional fluctuations. We propose a pioneering minimal model describing plastic crystal phase behavior where we mimic derivation of classical Landau-de Gennes-Ginzburg modeling of Isotropic-Nematic-Smectic A LC phase behavior.

Pretransitional behavior of the nonlinear dielectric effect for the liquid-solid transition in nitrobenzene

The nonlinear dielectric effect describes changes of dielectric permittivity induced by the strong electric field. This report shows the evidence for the critical-like pretransitional behavior for the liquid-solid transition in the supercooled nitrobenzene. Hallmarks of such behavior extend even above the melting temperature. A method for the analysis of pretransitional effects, avoiding the biasing impact of the noncritical background contribution, is proposed.

Nonlinear dielectric effect in supercritical diethyl ether

Nonlinear dielectric effect (NDE) describes changes of dielectric permittivity induced by a strong electric field in a liquid dielectric. The most classical finding related to this magnitude is the negative sign of NDE in liquid diethyl ether (DEE), recalled by Peter Debye in his Nobel Prize lecture. This article shows that the positive sign of NDE in DEE is also possible, in the supercritical domain. Moreover, NDE on approaching the gas-liquid critical point exhibits a unique critical effect described by the critical exponent ψ ≈ 0.4 close to critical temperature (T(C)) and ψ ≈ 0.6 remote from T(C). This can be linked to the emergence of the mean-field behavior in the immediate vicinity of T(C), contrary to the typical pattern observed for critical phenomena. The multi-frequency mode of NDE measurements made it possible to estimate the evolution of lifetime of critical fluctuations. The new way of data analysis made it possible to describe the critical effect without a knowledge of the non-critical background contribution in prior.

Dual field nonlinear dielectric spectroscopy in a glass forming EPON 828 epoxy resin

Results of the dual field nonlinear dielectric spectroscopy (NDS) studies in supercooled glass forming epoxy resin EPON 828 are presented. For the NDS, changes of dielectric permittivity induced by DC (rectangular) or AC (sine-wave) pulses of a strong electric field were probed by a weak radio frequency electric field. A clear stretched exponential (x < 1) decay after switching off the DC pulse and a single exponential decay (x = 1) after switching off the AC pulse were found. The same results are presented for preliminary studies in superpressed low molecular glass former di-isobutyl phthalate. This observation may be considered as an argument for the heterogeneous picture of supercooled glass forming materials. The temperature dependences of the stationary responses related to DC and AC strong electric field excitations are also shown. The sensitivity of the applied set up made it possible to detect NDS outputs even for electric fields E(strong) < 10 kV cm(-1), qualitatively weaker than in similar 'nonlinear, dielectric' experimental studies on glass forming materials carried out so far.

Anomalous temperature behavior of nonlinear dielectric effect in supercooled nitrobenzene

Studies of the nonlinear dielectric effect (NDE), describing changes of the dielectric permittivity induced by a strong electric field, in normal and supercooled liquid nitrobenzene are presented. An unusual increase of the stationary NDE, portrayed by the critical-like relation approximately (T-T+)(-1), was obtained. Nitrobenzene samples solidified at TS approximately T+ +2 K , approximately 10 K below the reported melting temperature Tm approximately 278 K . The anomalous increase of the NDE coincided with a slow relaxation process, detected in time-resolved measurements. The results presented offer a reinterpretation of the classical description by Piekara and Piekara [A. Piekara and B. Piekara, C. R. Hebd. Seances Acad. Sci. 203, 852 (1936)] of a positive sign contribution to the NDE in liquids. It is suggested that the obtained anomaly may be associated with the appearance of local quasinematic structures. This is supported by a speculative link to a general model for liquid-liquid transitions [H. Tanaka, Phys. Rev. E 62, 6968 (2000)] and a phenomenological model originally developed for the self-focusing of laser beams [J. Hanus, Phys. Rev. 178, 420 (1969)]. The case of the isotropic-nematic transition in liquid crystalline materials is also recalled. The NDE results reported here are related neither to the glass transition phenomenon nor to the recently developed concept of a second liquid-liquid transition.

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

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)].

Shear viscosity studies above and below the critical consolute point in a nitrobenzene-decane mixture

The shear viscosity has been studied in a nitrobenzene-decane critical mixture above the critical consolute temperature T(C), in the homogeneous phase, and below T(C), in coexisting phases. The form of background viscosity for coexisting phases has been postulated. The same value of the critical exponent straight phi has been obtained in the lower (L), upper (U), and homogeneous (H) phases. The pretransitional amplitudes (A(L,U)) in coexisting phases are approximately the same, whereas A(L,U)/A(H) approximately 0.965. In the homogeneous phase the possibility of the appearance of the quasinematic, field-induced structure of critical fluctuations has been discussed.

Dynamics of critical fluctuations in a binary mixture of limited miscibility under a strong electric field

Experimental investigations of relaxation after switching off the strong electric field in a nitrobenzene-dodecane mixture are presented. Studies were conducted for mixtures of critical and noncritical concentrations using the time-resolved nonlinear dielectric effect. The decays obtained can be portrayed by means of the stretched exponential function with the value of the exponent in agreement with the dynamic droplet model predictions. It has been shown that experimental decays exhibit a universal scaling behavior. The relaxation time (scaling factor) shows a power behavior with the exponent y approximately 1.2 for the critical mixture and y-->1 for the noncritical one. These values are much smaller than theoretically predicted y=1.8-1.9. Based on the assumption that a strong electric field induces in the mixture a quasinematic structure with semiclassical critical properties, a quantitative explanation of this difference is proposed.

Classical-nonclassical crossover behavior of critical and noncritical liquid binary solutions in a strong electric field

A nitrobenzene-dodecane binary mixture has been studied with the use of the nonlinear dielectric effect for one critical and two noncritical concentrations in a series of frequencies (f(m)) in a weak measuring electric field. It has been found that for the temperature region where 1/f(m)>tau (tau, the lifetime of fluctuations), the critical exponent psi=gamma-2beta approximately equal to 0.59 and consequently exponents gamma (susceptibility) and beta (order parameter) exhibit nonclassical values. Close to the phase-transition temperature where 1/f(m)<tau, different patterns appear for critical and noncritical concentrations. For the latter the exponent psi tends to zero that may point to the classical behavior, with gamma approximately equal to 1 and beta-->0.5. For the critical concentration psi approximately equal to 0.4, this may be the consequence of the semiclassical behavior with gamma approximately equal to 1 and beta approximately equal to 0.325.