Biomedical subjects

V Popa-Nita

Publications and source records attributed to V Popa-Nita.

10 recordsLinked to original sources

PubMed2006

Random anisotropy nematic model: nematic-non-nematic mixture.

The influence of a random-anisotropy- (RA-) type disorder on the phase separation of the nematogen-non-nematogen mixture is studied. A combination of the phenomenological Landau-de Gennes and Flory-Huggins theories is used. We assume that the non-nematogen component (i.e., impurity) enforces the RA disorder to the enclosing thermotropic liquid-crystal (LC) phase. The Imry-Ma argument is used according to which the lower-temperature phase exhibits a domain-type pattern. The disorder strength is measured in terms of the dimensionless parameter Lambda. We consider the case in which the LC molecules and impurities mix in the isotropic phase for Lambda=0. The impurities enforce a finite degree of orientational ordering even in the high-temperature paranematic phase. In the low-temperature phase they give rise to a domain-type structure, resulting in the distorted nematic (speronematic) phase. We show that the onset of orientational ordering increases the phase separation tendency. The RA field, however, opposes this tendency. With increasing value of Lambda the difference between the paranematic and speronematic ordering decreases. Consequently the structure of the phase-separated pattern can be much more complex in comparison to the Lambda=0 case.

Journal Article↗

PubMed2006

Influence of a random field on particle fractionation and solidification in liquid-crystal colloid mixtures.

The influence of a random-anisotropy (RA) type disorder on the phase separation of nematogen-colloid mixtures is studied theoretically by combining the phenomenological Landau-de Gennes, Carnahan-Starling, and hard-sphere crystal theories. We assume that the colloids enforce the RA disorder on the surrounding thermotropic liquid-crystal (LC) molecules. We adopt the Imry-Ma argument according to which the lower-temperature phase exhibits a domain-type pattern. The colloids impose a finite degree of orientational ordering even in the isotropic (paranematic) phase. In the ordered phase they give rise to a domain-type structure, resulting in the distorted nematic (speronematic) phase. The RA field opposes the phase separation tendency. With increasing disorder the difference between the paranematic and speronematic ordering decreases. Consequently there is a critical disorder, above which both phases become identical from the orientation point of view, but have different concentrations of colloids. We have also estimated another characteristic value of disorder above which the isotropic phase can exist only in a liquid state, the crystal phase being suppressed completely.

Anisotropy↗

PubMed2005

Waves at the nematic-isotropic interface: thermotropic nematogen-non-nematogen mixtures.

We develop a theory for surface modes at the nematic-isotropic interface in thermotropic nematogen-non-nematogen mixtures. We employ the dynamical generalization of the Landau-de Gennes model for the orientational (nonconserved) order parameter, coupled with the Cahn-Hilliard equation for concentration (conserved parameter), and include hydrodynamic degrees of freedom. The theory uses a generalized form of the Landau-de Gennes free-energy density to include the coupling between the concentration of the non-nematogen fluid and the orientational order parameter. Two representative phase diagrams are shown. The method of matched asymptotic expansions is used to obtain a generalized dispersion relation. Further analysis is made in particular cases. Orientational order parameter relaxation dominates in the short-wavelength limit, while in the long-wavelength limit viscous damping processes become important. There is an intermediate region (depending on the temperature) in which the interaction between conserved parameter dynamics and hydrodynamics is important.

Journal Article↗

PubMed2005

Transformation of phase transitions driven by an anisotropic random field.

We carry out a comparative study of the influence of a random anisotropy field on continuous and discontinuous phase transitions. The ordered phase, which is reached via a continuous symmetry breaking phase transition, is characterized by an order parameter and by a corresponding hydrodynamic continuum field. We assume that the response of the hydrodynamic field to the imposed disorder results in a domainlike pattern of the system. For a strong enough disorder both transitions become gradual. For weaker disorder strengths the disorder converts a second order transition into a discontinuous one.

Journal Article↗

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.

Journal Article↗

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.

Journal Article↗

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.

Journal Article↗

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.

Journal Article↗

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.

Journal Article↗

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.

Journal Article↗