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 ( <10Oe) normal to the unperturbed director.

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