Pretransitional Anomalies Research Articles

Distortion-sensitive insight into the pretransitional behavior of 4-n-octyloxy-4′-cyanobiphenyl (8OCB)

Results of studies of the static and dynamic dielectric properties in rod-like 4-n-octyloxy-4′-cyanobiphenyl (8OCB) with isotropic (I)–nematic (N)–smectic A (SmA)–crystal (Cr) mesomorphism, combined with measurements of the low-frequency nonlinear dielectric effect and heat capacity are presented. The analysis is supported by the derivative-based and distortion-sensitive transformation of experimental data. Evidence for the I–N and N–SmA pretransitional anomalies, indicating the influence of tricritical behavior, is shown. It has also been found that neither the N phase nor the SmA phase are uniform and hallmarks of fluid–fluid crossovers can be detected. The dynamics, tested via the evolution of the primary relaxation time, is clearly non-Arrhenius and described via τ(T) = τc(T−TC)−ϕ. In the immediate vicinity of the I–N transition a novel anomaly has been found: Δτ ∝ 1/(T − T*), where T* is the temperature of the virtual continuous transition and Δτ is the excess over the ‘background behavior’. Experimental results are confronted with the comprehensive Landau–de Gennes theory based modeling.

Pretransitional anomalies in the orientational dynamics induced by temperature gradient in nematic hybrid-oriented cells

We have carried out a numerical study of the system of hydrodynamic equations including director reorientation, fluid flow, and temperature redistribution across a hybrid-oriented liquid crystal cell (HOLCC), where the nematic sample is confined by two parallel horizontal surfaces, a temperature gradient is applied and directed normal to the boundaries, and the temperature on the lower cooler boundary is reduced towards T(NA). Calculations show that the temperature gradient causes the investigated system to settle down to a stationary flow regime in the horizontal direction. Presmectic anomalies in the vicinity of the lower boundary have been investigated as well, taking into account the fluctuations of the local smectic order parameter above T(NA). It has been shown that these fluctuations influence both magnitude and direction of the hydrodynamic flow, produced by the temperature gradient. We have also investigated the role of hydrodynamic flow in the resulting relaxation processes, for a number of dynamic regimes in a HOLCC containing 4-cyano-4'-octylbiphenyl.

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.

Pretransitional anomalies in the shear flow near a second-order nematic-smectic- a phase change.

Pretransitional anomalies in a shear flow near a second order nematic-smectic- A ( NA ) phase transition temperature T(NA), for liquid crystals (LC's), taking into account the fluctuations of the local smectic order parameter above the T(NA), are investigated. It is shown that the tumbling instability of the Couette shear flow for polar LC compounds, such as 4-n-octyl- 4(') -cyanobiphenyl (8CB), in the vicinity of T(NA), e.g., at a few tens of mK from T(NA) in the nematic phase, occurs at any shear rate gamma;.

Critical Behavior of the Dielectric Modulus in Nitrobenzene-Dodecane Mixture

The dielectric properties of a homogenous critical mixture of nitrobenzene-decane were studied in the range 1 Hz< f1 Mz. The temperature dependences of the “static” dielectric permittivity ε’ (1 MHz) and the electric conductivity σ(1 Hz) exhibit pretransitional anomalies which may be associated with the same critical exponent ø = 1−α ≈ 0.88, where α is the critical exponent of the specific heat. The same data were analyzed using the dielectric modulus representation. They show loss curves for the imaginary part of the modulus M ̋( f ). It was found that the temperature evolutions of the peak frequency τ = 1/2π fp and the peak maximum of M ̋ =M ̋( fp) also exhibit critical anomalies. Their forms resemble anomalies obtained for the imaginary part of the dielectric permittivity ε̋( f ), carried out for 40 MHz < f <1 GHz, in an ethanol-dodecane critical mixture [S. J. Rzoska, K. Orzechowski, and A. Drozd-Rzoska, Phys. Rev. E 65, 042501 (2002)]. - PACS: 64.70.Ja, 77.20.+y , 64.60.Fr

Non-linear dielectric effect in superpressed chiral isopentylcyanobiphenyl (5 [formula omitted]CB)

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].

Temperature studies of dielectric permittivity and mass density pretransitional anomalies in binary mixtures

Results of temperature studies of dielectric permittivity and mass density in binary mixtures are presented. The anomaly of dielectric permittivity as a function of temperature in nitrobenzene-n-alkanes binary mixtures has been studied. Molar dielectric permittivity which takes into account the anomaly of dielectric permittivity and the mass density anomaly has been introduced.

The pretransitional anomaly of dielectric permittivity in the isotropicphase of nematogens and in the homogeneous phase of criticalsolutions

The pretransitional behaviour of static dielectric permittivity in theisotropic phase of nematogens and in the homogeneous phase of a weaklyoff-critical binary solution is compared. The pretransitional anomalies aredescribed by analogous relations, with critical exponentsϕ = 1-α≈0.50 and ϕ = 1-α≈0.89, respectively. Testswere conducted for pressure paths approaching the virtual critical point. Thespecific-heat-like dependences of derivatives of dielectric permittivities inboth systems have been shown. This result agrees with the dependence suggestedfor critical mixtures by Mistura (1973 J. Chem. Phys. 59 4563). Results obtainedpoint to the validity of the recently formulated hypothesis on the fluid-likecritical behaviour of the isotropic-nematic transition (Mukherjee P K et al 1995 Phys. Rev. E 51 4570, Mukherjee P K 1998 J. Phys.: Condens. Matter 10 9191).

Quasi-Critical Behaviour of “Linear” and “Nonlinear” Dielectric Permittivity in the Isotropic Phase of Nematogens

Results are presented of dielectric permittivity and nonlinear dielectric effect (NDE) studies in the isotropic phase of n-pentylcyanobiphenyl (5CB), Both properties can be described by relations identical to those applied in critical binary solutions. For the isothermal, pressure and isobaric, temperature paths approaching the nematic clearing point pretransitional anomalies of dielectric permittivity and NDE are described by the same relations with critical exponents α ≈ 0.5 and γ = 1, respectively This substantiates the postulate of the isomorphism of critical phenomena in the isotropic phase of nematogens. For tested “linear” and “nonlinear” changes of dielectric permittivity the same values of the discontinuity of the isotropic — nematic phase transition was obtained.

An analogy between superconductors and smectics A

The conformation of a smectic A can be described by a phase function φ( R), the n-th layer corresponding to φ(R) = 2 πn. The role of the phase in smectics A and in superfluids is similar. This analogy leads to the following predictions for a second order smectic A ↔ nematic transition: (1) the transition temperature is lowered if twist, or bend distortions are imposed: these distortions correspond to a magnetic field in superconductors. (2) the Frank coefficients K 2 and K 3 of the nematic phase must show pretransitional anomalies.

Phase transitions in K1-xRbxAlF4: II - brillouin and ultrasonic studies

Abstract The concentration dependence of the phase transitions features has been studied in K1-xRbxAlF4 for x varying between 0 and 0.09 through Brillouin and ultrasonic measurements. For x larger than 0.02, a new phase (IIIa) appears in the sequence phase II-phase IV of pure KAlF4. The elastic constants show pretransitional and transitional anomalies around the II-IIIa first order transition at T0(x) which are supported by a Landau development of the free energy up to the 6th order using the appropriate order parameter (X3 symmetry soft mode) adequately coupled to the strains, where only the second order term varies with x and is written as α(T-Tc(x)). This model also agrees with birefringence measurements (see paper III).

Some Remarks on the Polymorphism of Smectics

Abstract We discuss the nature of the order in the main smectic phases (A, C, and B) together with the related phase transitions. (1) The B phase may correspond to a plastic crystal, or to a system of weakly coupled solid layers: to discriminate between these two models, acoustical studies using shear waves appear to be most promising, (2) the transitions A  N, C  N, C  A (where N stands for nematic) may be of second order in certain cases, which will allow for a number of interesting experiments. The C  A transition should be very similar to the λ transition of helium, and some critical exponents which are not accessible in helium could be measured here. The N  A transition is similar to the onset of superconductivity in a metal. A bend (or twist) deformation tends to suppress the A phase just as a magnetic field destroys superconductivity. In the N phase the twist and bend elastic constants should show pretransitional anomalies. The C  N transition is more complex. A tentative calculation suggests...

An analogy between superconductors and smectics A

The conformation of a smectic A can be described by a phase function φ( R), the n-th layer corresponding to φ( R) = 2 πn. The role of the phase in smectics A and in superfluids is similar. This analogy leads to the following predictions for a second order smectic A↔ nematic transition: (1) the transition temperature is lowered if twist, or bend distortions are imposed: these distortions correspond to a magnetic field in superconductors. (2) the Frank coefficients K 2 and K 3 of the nematic phase must show pretransitional anomalies.