Thermodynamic potential of ferroelectric nematic liquid crystals and consequences for polarization switching

The ferroelectric nematic (Nf ) liquid crystal phase is a highly polar fluid, with spontaneous polarization (PS) values of the order of µC cm−2 and viscosities of around 10 Pas. The combination of high polarity and fluidity makes these materials unique polar dielectrics. We consider the free energy of the ferroelectric nematic phase and derive its thermodynamic potentials. This allows us to predict that the spontaneous polarization will saturate as a function of applied voltage, rather than field. Further, we determine that the inclusion of an alignment layer, which is usual in liquid crystal devices, could provide a significantly enhanced energy barrier to switching. Indeed, an insulating alignment layer introduces a polar anchoring energy in addition to the orientational anchoring energy usually considered in liquid crystal devices. We confirm experimentally that measurements of the spontaneous polarization depend very slightly on the thickness of the Nf layer and more dramatically on the polar interactions of the phase with the confining surfaces. In relatively thin devices (∼10 µm) with an alignment layer present, we demonstrate that this effect can be so pronounced that polarization switching is completely suppressed. We also explore the influence of the preparation conditions of a thin film of ferroelectric nematic material on the stability and lifetime of the sample.