Supercritical behavior of magnetic and liquid model systems

The supercritical transitions are widely occurring. They include the supercritical transitions in the liquid-vapor system, ferromagnetic transitions, transitions in polymers, many transitions in liquid crystals, and some structural transitions. In the paper it is emphasized that the nature of the critical and supercritical transitions is the same – these are continuous fluctuation transitions. Above the critical temperature the system passes through a region of lowered stability, which leads to increase of fluctuations of energy and external parameters of the system. From the point of view of thermodynamic stability this indicates the existence of a continuous supercritical transition between supercritical mesophases. Knowing the basic stability characteristics of a system, we derive the equation of these mesophase transitions. Depending on a thermal equation type, we can get one or several such equations, which may not coincide. This approves the fact that a supercritical transition occurs in a certain interval of thermodynamic forces. In the paper the relations between the critical exponents of thermodynamic parameters of the system are obtained and the conditions of continuous conjugation of the lowered stability line to subcritical coexistence line are investigated. The results are applied to the Curie–Weiss and van der Waals models: we obtain the quasi-spinodal equation for these systems and analyze the critical and supercritical behavior of the stability characteristics.

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

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.