Twist, splay, and uniform domains in ferroelectric nematic liquid crystals

Abstract: The recently-discovered ferroelectric nematic ($\mathrm{N}\mathrm{F}$) liquid crystal presents a host of defect phenomena due to its unique polar nature and long-ranged electrostatic interactions. Like the solid state ferroelectrics, the depolarization field in the material favors a spontaneous spatial variation of the polarization $\mathbf{P}$, manifesting in myriad ways including a twist in the bulk and different arrangements of alternating polarization domains. Unlike the solid-state ferroelectrics with a bulk crystalline structure, the configuration of the $\mathrm{N}\mathrm{F}$ fluids is determined not only by the reduction of depolarization fields but also by the alignment of molecules at interfaces. In this work, we consider an $\mathrm{N}\mathrm{F}$ confined to a thin cell, pre-patterned with various types of apolar surface anchoring produced by photoalignment. For uniform planar alignment, we find that the sample forms a series of striped domains. For a cell pre-patterned with a radial +1 defect pattern, the $\mathrm{N}\mathrm{F}$ breaks up into "pie-slice" polarization domains. We calculate the elastic and electrostatic energy balance which determines the observed configurations and demonstrate that the electrostatic interactions tend to decrease the characteristic domain size $\lambda$ while the elastic and surface anchoring interactions facilitate a larger $\lambda$. We also demonstrate that ionic screening mitigates electrostatic interactions, increasing $\lambda$ and, above some critical concentration, eliminating the domains altogether.