Nematic director response in ferronematic cells

Abstract

This paper examines ferronematic switching in a homeotropic cell in the presence of a magnetic field normal to the cell plane. At low fields we find thresholdless switching of the nematic director, consistent with experimental data. At higher fields, there are three regimes, depending on the strength of the director-ferroparticle coupling. For low coupling, there is an inverse Frederiks effect: the nematic reorientation increases and then reduces, disappearing at a critical field. At intermediate coupling, the reorientation reduces at high fields but remains finite. For high coupling, however, the director switching saturates. There is a dimensionless temperature scale t involving the temperature, the mean nematic elastic constant, the colloidal density and the cell dimension. For low t, high magnetic fields can cause the ferroparticles to segregate. The segregation is coupled to the director distortion, and this can drive the inverse Frederiks transition first order, causing bistability for intermediate fields. These features are perturbed but not changed structurally by the effect of a small bias magnetic field (< 10 Oe) normal to the unperturbed director.

Fredericksz Transition Threshold in Nematic Liquid Crystals Filled with Ferroelectric Nano-Particles

“…As early as 1970, Brochard and de Gennes [2] pointed out that if the colloidal particles possess a permanent magnetic moment, then the orientation of the local moments and the nematic director would be coupled, and this would give rise to some new and interesting physical effects. These systems are known as ferronematics, and further work, both experimental and theoretical, seems to confirm the picture predicted by Brochard and de Gennes [3][4][5].…”

“…The theory is an extension of the basic ideas used by de Gennes and coworkers [2], and is related to an analogous theory for ferronematics [5], with an additional complication of local field effects.…”

Enhanced Magneto-Optical Properties of Suspensions of Spindle Type Mono-Dispersed Hematite Nano-Particles in Liquid Crystal

“…Due to the anchoring, the reorientation of the nano-particles in magnetic field resulted in a non-threshold reorientation of LC, and monitoring of magnetic field !10 Oe could be easily achieved. At low anchoring between LC and particles' surface, an inverse Fréedericksz effect was predicted, when the nematic reorientation reduced and then disappeared continuously at a critical magnetic field [10,11].…”

“…Taking into account the theoretical prediction [10,11] that even low concentrations of anisometric magnetic particles can affect the magneto-optical properties of LC, and to solve the particles aggregation problem, one can study diluted suspensions of nano-particles in liquid crystals as proposed in [12] for ferroelectric suspensions (which has been realized in Refs. [12][13][14][15][16]).…”

The Frederiks Effect and Related Phenomena in Ferronematic Materials

“…Finally, in section 7 we draw some conclusions and place our work in a larger context. A brief preliminary report of our work has been published elsewhere [15]. ψ θ…”