Orientational dynamics of a ferronematic liquid crystal in a rotating magnetic field
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TICAL, NONLINEAR, AND SOFT MATTER PHYSICS
Orientational Dynamics of a Ferronematic Liquid Crystal in a Rotating Magnetic Field A. N. Boychuk, A. N. Zakhlevnykh, and D. V. Makarov Perm State University, Perm, 614990 Russia e-mail: [email protected] Received April 1, 2015
Abstract—The behavior of the orientational structure of a ferronematic in a rotating uniform magnetic field is investigated using the continual theory. The time-dependent system of equations describing the dynamics of the ferronematic is derived. The dependences of the angles of rotation of the director and of the magnetization of the ferronematic on the velocity of field rotation are determined for various values of the material parameters. Two regimes (synchronous and asynchronous) of rotation of the ferronematic structure are detected. In the synchronous regime, the director rotates with the frequency of the magnetic field and a constant phase delay. The asynchronous regime is characterized by a time-dependent phase delay. The dependence of the critical angular velocity of magnetic field rotation, which determines the boundary between the synchronous and asynchronous regimes, on the magnetic field strength is derived. DOI: 10.1134/S1063776115090046
1. INTRODUCTION Liquid crystals (LCs), which are anisotropic soft materials with spontaneous orientational order, are attractive media for dispersion of colloidal particles of various origins (ferromagnetic, ferroelectric, carbon nanotubes, etc.) [1]. The liquid-crystal (LC) matrix determines the ordering of anisotropic particles implanted into it, which considerably changes the response of the composite system to external effects and opens new possibilities for using LC materials in information display devices and optoelectronics. Such suspensions are very sensitive to external effects and possess peculiar electric, magnetic, and optical properties that differ from those of initial components and that vary under the action of external fields. New applications for such materials considerably depend on the possibility of controlling the orientational response and the spatial distribution of particles in the LC matrix. Ferronematics, viz., colloidal suspensions of magnetic nanoparticles in nematic liquid crystals (NLCs), are examples of such systems. These systems were predicted theoretically in [2], which laid the foundations continual description of ferronematics. Beginning from this pioneering publication, wide applications of ferronematics have become obvious, because their magnetic susceptibility is several orders of magnitude higher than the susceptibility of pure LCs. The first experimental attempts at synthesizing LC suspensions were not quite successful. However, with the development of new methods for stabilizing nanoparticles in thermotropic LCs in the last decade, it has become
possible to experimentally implement ferronematics successfully, which has stimulated numerous experimental and theoretical investigations of their physical properties and phase transitions induced by external fields (see r
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