Biomedical subjects

V Popa-Nita

Publications and source records attributed to V Popa-Nita.

6 recordsLinked to original sources

PubMed2004

Random anisotropy nematic model: connection with experimental systems.

We study theoretically the phase behavior of the continuum Random Anisotropy Nematic model. A domain-type pattern is assumed to appear in a distorted nematic liquid crystal (LC) phase. We map the model parameters to physical quantities characterizing LCs confined to Controlled-Pore Glasses and LC-aerosil dispersions. The domain size dependence on the disorder strength is obtained in accordance with the Imry-Ma prediction. The model estimates for temperature shifts of the paranematic-nematic phase transition and for the critical point, where this transition ceases to exist, are compared to the available experimental results.

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PubMed2003

Waves at the nematic-isotropic interface: the role of surface tension anisotropy, curvature elasticity, and backflow effects.

Recently, a theoretical description of waves at the nematic-isotropic interface has been proposed using a generalized dynamical Landau-Ginzburg-de Gennes theory [V. Popa-Nita and T. J. Sluckin, Phys. Rev. E 66, 041703 (2002)]. This calculation assumed an isotropic surface tension, i.e., independent of the director orientation at the interface and neglected all coupling between the director and the hydrodynamic flow. As a consequence, the director was assumed to keep a fixed orientation and do not couple with the oscillations of the interface. These assumptions are rather crude in real nematics where surface tension anisotropy may be as large as 20% and where hydrodynamic coupling with the director is known to be important. In this paper we propose to take into account these two effects: as a result, interface oscillations couple with the director field via hydrodynamic flows and backflow effects. We analyze how these phenomena change the dispersion relation. Finally, we review experiments on the nematic-isotropic interface and discuss how to measure experimentally the dispersion relation.

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PubMed2002

Phase-field model for front propagation in a temperature gradient: selection and competition between the correlation and the thermal lengths.

A phase-field model is presented to study the propagation and the selection of a front in directional growth. The phase transition can be first or second order and is described by a nonconserved order parameter. In general, the thermal length l(u) (inversely proportional to the temperature gradient) is much larger than the correlation length l(phi), which gives the width of the front, and there is no direct competition between them (epsilon =l(phi)/l(u)<<1). In this paper, we consider a situation where these two lengths can be of the same order of magnitude (epsilon =l(phi)/l(u) close to 1). This happens in liquid crystals at the nematic-cholesteric phase transition. The problem of the front selection is solved theoretically by first performing an asymptotic analysis of the governing equations in the limit epsilon-->0, and then by solving the equations numerically. The main result is that the front is selected in a single way (no continuum of solutions) as long as epsilon not equal 0, whatever the velocity and the order of the phase transition. Finally, we show that the order parameter profile and the front temperature can change significantly when epsilon approaches 1.

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PubMed2002

Surface modes at the nematic-isotropic interface.

We examine surface modes at the nematic-isotropic interface using the generalized dynamical Landau-de Gennes theory. We assume an isothermal, infinite, unbounded nematic-isotropic system characterized by a scalar order parameter, both phases having the same density and viscosity, respectively. The generalized dispersion relation is obtained and analyzed in particular cases. Order parameter relaxation dominates in the short wavelength limit, while in the long wavelength limit viscous damping becomes important. We study the crossover between the two regimes and estimate the extent of this region for the liquid crystal 8CB.

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PubMed2000

Model for the planar-homeotropic anchoring transition induced by trans-cis isomerization.

We present a model to explain the planar-homeotropic anchoring transition of azobenzene induced by UV illumination via trans-cis isomerization. We consider bulk and surface as two different phases (separated by an infinitely sharp interface) which are in equilibrium. We obtain a relation for the exposure time after which the transition takes place.

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PubMed1999

Kinetics of phase ordering of nematic liquid crystals confined in porous media.

Employing a time-dependent Ginzburg-Landau model, we investigate the influence of a random field on the phase ordering kinetics of nematic liquid crystals. We find that in the scaling regime the effect of random field (slowing down the growth of nematic) dominates over initial conditions for spatial dimensionality d< or =2, whereas for d>2 the random field has all its effect in the "initial-growth" regime. In this last case the mere confinement of liquid crystals is insufficient to produce slow growth of the nematic order.

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