V. Popa-Nita

Alignment of carbon nanotubes in nematic liquid crystals.

The self-organizing properties of nematic liquid crystals can be used to align carbon nanotubes dispersed in them. Because the nanotubes are so much thinner than the elastic penetration length, the alignment is caused by the coupling of the unperturbed director field to the anisotropic interfacial tension of the nanotubes in the nematic host fluid. In order to relate the degree of alignment of the nanotubes to the properties of the nematic liquid crystal, we treat the two components on the same footing and combine Landau-de Gennes free energies for the thermotropic ordering of the liquid crystal and for the lyotropic nematic ordering of carbon nanotubes caused by their mutually excluded volumes. The phase ordering of the binary mixture is analyzed as a function of the volume fraction of the carbon nanotubes, the strength of the coupling and the temperature. We find that the degree of ordering of the nanorods is enslaved by the properties of the host liquid and that it can be tuned by raising or lowering the temperature or by increasing or decreasing their concentration. By comparing the theory to recent experiments, we find the anchoring energy of multiwalled carbon nanotubes to be in the range from 10(-10) to 10(-7) N m(-1).

Liquid crystal-carbon nanotubes mixtures

The self-organizing properties of nematic liquid crystals (LCs) can be used to align carbon nanotubes (CNTs) dispersed in them. In the previous paper [P. van der Schoot, V. Popa-Nita, and S. Kralj, J. Phys. Chem. B 112, 4512 (2008)], we have considered the weak anchoring limit of the nematic LC molecules at the nanotubes surface, where the CNT alignment is caused by the anisotropic interfacial tension of the nanotubes in the nematic host fluid. In this paper, we present the theoretical results obtained for strong enough anchoring at the CNT-LC interface for which the nematic ordering around nanotube is apparently distorted. Consequently, relatively strong long-range and anisotropic interactions can emerge within the system. In order to get insight into the impact of LC ordering on the alignment of nanotubes we treat the two mixture components on the same footing and combine Landau-de Gennes free energy for the thermotropic ordering of the liquid crystal and Doi free energy for lyotropic nematic ordering of carbon nanotubes caused by their mutually excluded volume. The phase ordering of the binary mixture is analyzed as a function of the volume fraction of the carbon nanotubes, the strength of coupling, and the temperature. We find that the degree of ordering of the nanorods can be tuned by raising or lowering the temperature or by increasing or decreasing their concentration.

The influence of nanoparticles on the phase and structural ordering for nematic liquid crystals

We study the influence of nanoparticles (NPs) on liquid crystal (LC) ordering. As regards the structural ordering we consider NPs as a source of a quenched random field. Roughly such a situation is encountered in mixtures of LCs and aerosil NPs (aerosil NPs are spherular ones). Using the semi-microscopic lattice model and Brownian molecular simulation we show that after a quench from the isotropic phase a quasi-stable domain pattern forms. The characteristic size of an average domain is inversely proportional to the concentration of NPs, and domain patterns exhibit memory effects. In the study of the phase behaviour we limit consideration to NPs resembling LC molecules. A Landau-type free energy expression is derived for the mixture, originating from the Maier‐Saupe molecular approach. We show that the resulting phase behaviour exhibits the slave–master behaviour as the temperature or pressure is varied.

Nematic–smectic-A phase transition in porous media

Abstract We study the effects of both orientational and translational disorder on the nematic–smectic-A (NA) phase transition, using the Landau–de Gennes treatment. The only influence of random-field orientational disorder is to slightly increase the NA transition temperature, whereas the translational randomness has four important effects. (i) The NA transition temperature is decreased by the randomness. (ii) The bulk second-order NA phase transition is transformed into a first-order one. (iii) There is a small degree of translational order even in the high-temperature phase. (iv) The strict long-range smectic-A order is converted into a quasi-long-range order by any finite amount of disorder.

Magnetic-field-induced isotropic-nematic phase transition in porous media

Abstract In the framework of the random anisotropy nematic spin model we investigate the influence of an external magnetic ordering field on the properties of isotropic-nematic phase transition in porous media. Depending on the values of relative randomness three types of phase diagrams are predicted. In the relative low randomness regime the magnetic field dramatically changes the phase diagram due to possibility of existence of two speronematic phases. At relative intermediary and large randomness the influence of the ordering field is only to increase the speronematic - paranematic phase transition temperature and to diminish the strength of the transition.

Phase Ordering in Mixtures of Liquid Crystals and Nanoparticles

We have studied the coupling interaction between liquid crystal (LC) molecules and nanoparticles (NPs) in LC=NPs mixtures. Using a simple phenomenological approach, possible structures of the coupling term are derived for strongly anisotropic NPs. The coupling terms include (i) an interaction term promoting the mutual ordering of the LC molecules and the NPs, and (ii) the Flory-Huggins-type term enforcing the phase separation. Both contributions exhibit the same scaling dependence on the diameter of the NPs. However, these terms only exist for a finite degree of nematic LC ordering. The magnetic response due to the LC-NPs coupling is probed experimentally for a mixture of weakly anisotropic magnetic NPs and a ferroelectric LC. A finite coupling effect was observed in the ferroelectric LC phase, suggesting such systems can be used as soft magnetoelectrics.

Surface tension and capillary waves at the nematic-isotropic interface in ternary mixtures of liquid crystal, colloids, and impurities.

In mixtures of thermotropic liquid crystals with spherical poly(methyl methacrylate) particles, self-supporting networklike structures are formed during slow cooling past the isotropic-to-nematic phase transition. Experimental results support the hypothesis that a third component, alkane remnants slowly liberated from the particles, plays a crucial role. A theoretical model, based on the phenomenological Landau-de Gennes, Carnahan-Starling, and hard-sphere crystal theories, is developed to describe the continuous phase separation in a ternary nematic-impurity-colloid mixture. The interfacial tension and the dispersion relation of the surface modes of the nematic-isotropic interface are determined. The colloids decrease the interfacial tension and the damping rate of surface waves, whereas impurities act in an opposite way. This should strongly influence the formation of abovementioned networklike structures and could help explain some of their rheological properties.

Waves at the Nematic-Isotropic Interface

ABSTRACT We examine surface modes at the nematic-isotropic interface in polymer-liquid-crystal mixtures using the dynamical Landau-de Gennes model for the orientational (non-conserved) order parameter coupled with Cahn-Hilliard equation for concentration (conserved order parameter) and with hydrodynamic degrees of freedom. The generalized dispersion relation is obtained and analyzed in particular cases. Orientational order parameter relaxation dominates in the short wavelength limit, while in the long wavelength limit viscous damping becomes important. The effect of polymer is to increase the surface tension and as a consequence the relaxation rate in the hydrodynamic limit. Finally, we discuss how to measure experimentally the dispersion relation.

The phase behavior of rigid rods in an anisotropic mean field with applications to carbon nanotubes in nematic liquid crystals

The phenomenological model [van der Schoot et al., J. Phys. Chem. B 112, 4512 (2008)] for predicting the alignment of carbon nanotube (CNT) dispersions in thermotropic liquid crystals is extended to include the attractive interactions between CNTs. The influence of the attractive forces (described by a spatially uniform mean field) on the phase behavior and orientational properties of the mixture are analyzed.

Mixtures Composed of Liquid Crystals and Nanoparticles

The past decade has witnessed an increased interest in the study of mixtures [1–3] of various soft materials and nanoparticles. A characteristic feature of a nanoparticle is that at least one of its dimensions is limited to between 1 and 100 nm. It is of interest to find combinations where each component introduces a qualitatively different behavior into the system. Such systems are expected to play an important role in the emerging field of nanotechnology and also in composites with extraordinary material properties.