Magnetic field induced orientational transitions in liquid crystals doped with carbon nanotubes

Abstract

We propose a continuum theory of orientational phase transitions induced by an external magnetic field in a suspension of carbon nanotubes in a nematic liquid crystal. It is shown that in a magnetic field a non-uniform and two different uniform phases are possible in the suspension. The uniform phases of the suspension differ by the type of orientational coupling of nanotubes with the liquid crystal matrix (the planar type when the nanotubes are oriented along the matrix director, and the homeotropic type when the nanotubes are perpendicular to the director). The possibility of a redistribution of the nanotube concentration (segregation effect) is shown. The fields of orientational transitions between uniform and non-uniform phases of the suspension are found analytically. It is shown that, when the nanotubes are weakly coupled to the matrix, the magnetic field induces reentrant transitions (uniform planar phase–non-uniform phase–uniform homeotropic phase–non-uniform phase). These transitions can be of first or of second order depending on the carbon nanotubes segregation intensity.

Liquid crystallinity of carbon nanotubes

“…53,54 On the other hand, CNTs, viewed as highly anisometric rigid rod-like particles, can be manipulated by electric or magnetic elds due to the dielectric and magnetic anisotropy, which is the basis for their use in modern display devices. [55][56][57][58] For all of these reasons, the interest in liquid crystallinity of CNTs is quite active. This paper is a review of the literature that deals with liquid crystallinity of CNTs and the relevant technological applications.…”

Magnetic segregation effect in liquid crystals doped with carbon nanotubes

“…1). In the framework of the continuum theory [39], the directions of the preferred orientation of the NLC molecules and the CNTs are determined using the unit vectors n and m , respectively. The anchoring of NLC molecules on the surface of the plates will be considered absolutely rigid and planar, so the director of the NLC n = n 0 at z = ± L /2, where the vector n 0 sets the direction of the easy orientation axis on the layer boundaries.…”

“…Recently, a continuum theory was proposed in [39] to describe the magneto-orientation response of an NLC–CNT suspension. It is based on the generalization of the free energy of an LC, taking into account the fact that a small amount of CNT impurities has been added to the NLC matrix.…”

“…Experiments show [28,32–34] that in nematic liquid crystal–carbon nanotube (NLC-CNT) mixtures additionally functionalized with ferromagnetic particles, the magneto-optical response increases in comparison with pure LC. Existing theoretical models that describe NLC-CNT suspensions are related to the mean-field theories [35–36], generalizations of the Landau–de Gennes phenomenological theory [37–38] or modifications of the free-energy functional of the LC, which take into account the presence of the CNT impurity [39].…”

“…In the present work, on the basis of the free energy functional [39] magnetic-field-induced equilibrium transitions in NLC-CNT suspension have been studied. The influence of segregation effects on the nature of the magnetic Fréedericksz transition and the magneto-optical response of a flat layer of the mixture are analyzed.…”