Isotropic-nematic phase transitions in liquid crystals

Journal de Physique II, 1993

Physical Review E, 2001

The orientational properties of an isotropic dense liquid composed by anisotropic molecules, such as a liquid crystal in an isotropic phase, is studied. Using a Langevin-like equation it will be shown that the rotational motion of each molecule can be divided in two elements describing two kinds of physical motion. The first describes the Brownian rotational motion and another the coherent rotation induced by the external fields. It will be shown that, even at the isotropic phase, an order parameter describing the mean degree of alignment of the molecules around a given point can be defined. This order parameter also separates the order coming from the coherent motion from the order generated by the anisotropy in the thermal fluctuations. At the end the proposed model is compared with an experiment and it is shown that the coherent motion is enough to explain the experimental results.

Archive for Rational Mechanics and Analysis, 2012

We introduce a new class of non-isothermal models describing the evolution of nematic liquid crystals and prove their consistency with the fundamental laws of classical Thermodynamics. The resulting system of equations captures all essential features of physically relevant models, in particular, the effect of stretching of the director field is taken into account. In addition, the associated initial-boundary value problem admits global-in-time weak solutions without any essential restrictions on the size of the initial data.

The European Physical Journal E, 2007

We report the experimental high electric field phase diagram of a nematic liquid crystal which exhibits a large negative dielectric anisotropy. We measure simultaneously the birefringence (∆n) and the dielectric constant (⊥) at various applied fields as functions of the local temperature of an aligned sample. We also measure the higher harmonics of the electrical response of the medium. The following experimental results are noted: (i) enhancement of orientational order parameter S in the nematic phase due to both the Kerr effect and quenching of director fluctuations; (ii) enhancement in the paranematic to nematic transition temperature (TPN) with field; (iii) divergence of the order parameter susceptibility beyond the tricritical point as measured by third harmonic electrical signal; (iv) a small second harmonic electrical signal which also diverges near TPN, indicating the presence of polarised domains. Our measurements show that ∆TPN(= TPN(E) − TNI (0)) varies linearly with |E| whereas the Landau de Gennes theory predicts a dependence on E 2. It is argued that the quenching of director fluctuations by the field makes the dominant contribution to all the observations, including the thermodynamics of the transition. PACS. 64.70.Md Transitions in liquid crystals-61.30.Gd Orientational order of liquid crystals; electric and magnetic field effects on order-61.30.Dk Continuum models and theories of liquid crystal structure 140

2015

Hamiltonian model describing general quadratic interactions among liquid crystal molecules (with positive dielectric anisotropy, and D2h symmetry) is investigated in the presence of an external field Ea (coupling to their long axes), through Monte Carlo simulations. While the isotropic-to-uniaxial nematic (I-NU) transition temperature is enhanced with increasing field (attributed as due to the quenching of the primary director), the uniaxial-to-biaxial nematic transition on the other hand, seemingly concurrent with I-NU transition at Ea =0, is split, and occurs at progressively low temperatures with increasing field. By comparing our recent detailed investigations on this (zero-field) transition via a sampling procedure facilitating the computation of the density of states of the system [Latha et. al., Phys. Rev. E, 89 050501(R) (2014)], we infer that the applied field is effectively decreasing the degree of the mesoscopic inhomogeneity of the medium (with respect to the distributio...

Brazilian Journal of Physics, 2010

Thermotropic, thermo-morphologic and thermo-optical properties of the planar oriented nematic liquid crystals have been investigated for large temperature interval and especially for the direct nematic-isotropic liquid and the reverse isotropic liquid-nematic phase transition regions. Temperature dependences of the optical transmission, absorption coefficient and optical birefringence for both heating and cooling processes were obtained. Nonlinear thermotropic and thermo-optical behaviour and temperature hysteresis for the optical transmission, absorption coefficient and optical birefringence at the phase transitions has been found.

The European Physical Journal E, 2011

We report simultaneous measurements of shear viscosity (η) and dielectric constant ( ) of octyloxy cyanobiphenyl (8OCB) in the nematic (N) and smectic-A (SmA) phases as functions of temperature and electric field. With increasing electric field η increases in the N phase whereas it decreases in the SmA phase and saturates beyond a particular field in both the phases. The flow curves in the intermediate-field range show two Newtonian regimes in the N phase. The temperature-dependent behavior of η and at zero or at small electric field suggests the occurrence of several structures that results from precessional motion of the director along the neutral direction as reported in similar other system. We show that the precessional motions are gradually suppressed with increasing electric field and the effective viscosity resembles with the Miesowicz viscosity η 1 at high enough electric field. In the intermediate field range the temperature-dependent η exhibits anomalous behavior across the N-SmA phase transition which is attributed to the large contribution of Leslie coefficient α 1.

Europhysics Letters (EPL), 2004

We report the first experimental high-electric-field phase diagram of a nematic liquid crystal with negative dielectric anisotropy. The variation of the paranematic-nematic transition temperature (TPN) is essentially linear with |E|, whereas the Landau-de Gennes theory predicts a quadratic variation. It is argued that the quenching of director fluctuations by the field contributes significantly to the thermodynamics of the transition.

2008

We derive the reorienting dielectric torque acting on the director, considering the frequency dependence of the dielectric tensor. The model takes account into the effects of multiple relaxations in both parallel and perpendicular components of the dielectric tensor and predicts the ``dielectric memory effect'' (DME), i.e., dependence of the dielectric torque on both the ``present'' and ``past'' values of the electric field and the director. In a sharply rising electric field, the DME slows down director reorientation for the materials whose dielectric anisotropy is positive at low frequencies, but speeds up the response for the dielectrically negative materials. We also demonstrate, both theoretically and experimentally that an induced ``memory'' polarization leads to a dielectric torque in the switch-off phase which has an opposite sign to that of the LC's dielectric anisotropy, when a specific switching-off profile is used; this reverse torque accelerates the director relaxation back to the equilibrium state.

The first theoretical observation of the tricritical point for the isotropic to biaxial nematic phase transition of biaxial nematic liquid crystals in the presence of an external field is reported. The influence of an external magnetic field on the isotropic to biaxial nematic phase transition have been studied using Landau phenomenological theory. Topological classification of phase diagrams in the field -temperature coordinates is performed. It is shown that for a particular value of the magnetic field, the first order isotropic to biaxial nematic phase transition becomes second order phase transition at a tricritical point.